Internet DRAFT - draft-dnoveck-nfsv4-migration-issues
draft-dnoveck-nfsv4-migration-issues
NFSv4 D. Noveck, Ed.
Internet-Draft EMC
Intended status: Informational P. Shivam
Expires: July 19, 2012 C. Lever
B. Baker
ORACLE
January 16, 2012
NFSv4.0 migration: Implementation experience and spec issues to resolve
draft-dnoveck-nfsv4-migration-issues-02
Abstract
The migration feature of NFSv4 provides for moving responsibility for
a single filesystem from one server to another, without disruption to
clients. Recent implementation experience has shown problems in the
existing specification for this feature. This document discusses the
issues which have arisen and explores the options available for
curing the issues via clarification and correction of the NFSv4.0
specification.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 19, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Implementation Experience . . . . . . . . . . . . . . . . . . 6
3.1. Implementation issues . . . . . . . . . . . . . . . . . . 6
3.1.1. Failure to free migrated state on client reboot . . . 6
3.1.2. Server reboots resulting in a confused lease
situation . . . . . . . . . . . . . . . . . . . . . . 7
3.1.3. Client complexity issues . . . . . . . . . . . . . . . 8
3.2. Sources of Protocol difficulties . . . . . . . . . . . . . 9
3.2.1. Issues with nfs_client_id4 generation and use . . . . 9
3.2.2. Issues with lease proliferation . . . . . . . . . . . 11
4. Issues to be resolved . . . . . . . . . . . . . . . . . . . . 12
4.1. Possible changes to nfs_client_id4 client-string . . . . . 12
4.2. Possible changes to handle differing nfs_client_id4
string values . . . . . . . . . . . . . . . . . . . . . . 13
4.3. Other issues within migration-state sections . . . . . . . 13
4.4. Issues within other sections . . . . . . . . . . . . . . . 14
5. Proposed resolution of protocol difficulties . . . . . . . . . 14
5.1. Proposed changes: nfs_client_id4 client-string . . . . . . 14
5.2. Client-string Models (AS PROPOSED) . . . . . . . . . . . . 15
5.2.1. Non-Uniform Client-string Model . . . . . . . . . . . 16
5.2.2. Uniform Client-string Model . . . . . . . . . . . . . 17
5.3. Proposed changes: merged (vs. synchronized) leases . . . . 21
5.4. Other proposed changes to migration-state sections . . . . 22
5.4.1. Proposed changes: Client ID migration . . . . . . . . 22
5.4.2. Proposed changes: Callback re-establishment . . . . . 23
5.4.3. Proposed changes: NFS4ERR_LEASE_MOVED rework . . . . . 23
5.5. Proposed changes to other sections . . . . . . . . . . . . 24
5.5.1. Proposed changes: callback update . . . . . . . . . . 24
5.5.2. Proposed changes: clientid4 handling . . . . . . . . . 24
5.6. Migration, Replication and State (AS PROPOSED) . . . . . . 26
5.6.1. Migration and State . . . . . . . . . . . . . . . . . 26
5.6.2. Replication and State . . . . . . . . . . . . . . . . 28
5.6.3. Notification of Migrated Lease . . . . . . . . . . . . 29
5.6.4. Migration and the Lease_time Attribute . . . . . . . . 31
6. Results of proposed changes . . . . . . . . . . . . . . . . . 32
6.1. Results: Failure to free migrated state on client
reboot . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.2. Results: Server reboots resulting in confused lease
situation . . . . . . . . . . . . . . . . . . . . . . . . 33
6.3. Results: Client complexity issues . . . . . . . . . . . . 34
6.4. Result summary . . . . . . . . . . . . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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10.1. Normative References . . . . . . . . . . . . . . . . . . . 36
10.2. Informative References . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
This document is in the informational category, and while the facts
it reports may have normative implications, any such normative
significance reflects the readers' preferences. For example, we may
report that the reboot of a client with migrated state results in
state not being promptly cleared and that this will prevent granting
of conflicting lock requests at least for the lease time, which is a
fact. While it is to be expected that client and server implementers
will judge this to be a situation that is best avoided, the judgment
as to how pressing this issue should be considered is a judgment for
the reader, and eventually the nfsv4 working group to make.
We do explore possible ways in which such issues can be avoided, with
minimal negative effects, in the expectation that the working group
will choose to address these issues, but the choice of exactly how to
address this is best given effect in a working group document.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In the context of this informational document, these normative
keywords will always occur in the context of a quotation, most often
direct but sometimes indirect. The context will make it clear
whether the quotation is from:
o The current definitive definition of the NFSv4.0 protocol, whether
that is the original NFSv4.0 specification [RFC3530], the current
pending draft of RFC3530bis expected to become the definitive
definition of NFSv4.0 once certain procedural steps are taken
[cur-v4.0-bis], or an eventual RFC3530bis RFC, taking over the
role of definitive definition of NFSv4.0 from RFC3530.
As the identity of that document may change during the lifetime of
this document, we will often refer to the current or pending
definition of NFSv4.0 and quote from portions of the documents
that are identical among all existing drafts. Given that RFC3530
and all RFC3530bis drafts agree as to the issues under discussion,
this should not cause undue difficulty. Note that to simplify
document maintenance, section names rather than section numbers
are used when referring to sections in existing documents so that
only minimal changes will be necessary as the identity of the
document defining NFSv4.0 changes.
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o A proposed or possible text to serve as a replacement for the
current definitive document text. Sometimes, a number of possible
alternative texts may be listed and benefits and detriments of
each examined in turn.
3. Implementation Experience
3.1. Implementation issues
Note that the examples below reflect current experience which arises
from clients implementing the recommendation to use different
nfs_client_id4 id strings for different server addresses, i.e. using
what is later referred to herein as the "non-uniform client-string
model"
This is simply because that is the experience implementers have had.
The reader should not assume that in all cases, this practice is the
source of the difficulty. It may be so in some cases but clearly it
is not in all cases.
3.1.1. Failure to free migrated state on client reboot
The following sort of situation has proved troublesome:
o A client C establishes a clientid4 C1 with server ABC specifying
an nfs_client_id4 with "id" value "C-ABC" and verifier 0x111.
o The client begins to access files in filesystem F on server ABC,
resulting in generating stateids S1, S2, etc. under the lease for
clientid C1. It may also access files on other filesystems on the
same server.
o The filesystem is migrated from ABC to server XYZ. When
transparent state migration is in effect, stateids S1 and S2 and
clientid4 C1 are now available for use by client C at server XYZ.
So far, so good.
o Client C reboots and attempts to access data on server XYZ,
whether in filesystem F or another. It does a SETCLIENTID with an
nfs_client_id4 with "id" value "C-XYZ" and verifier 0x112. There
is thus no occasion to free stateids S1 and S2 since they are
associated with a different client name and so lease expiration is
the only way that they can be gotten rid of.
Note here that while it seems clear to us in this example that C-XYZ
and C-ABC are from the same client, the server has no way to
determine the structure of the "opaque" id. In the protocol, it
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really is opaque. Only the client knows which nfs_client_id4 values
designate the same client on a different server.
3.1.2. Server reboots resulting in a confused lease situation
Further problems arise from scenarios like the following.
o Client C talks to server ABC using an nfs_client_id4 id like
"C-ABC" and verifier v1. As a result a lease with clientid4 c.i
is established: {v1, "C-ABC", c.i}.
o fs_a1 migrates from server ABC to server XYZ along with its state.
Now server XYZ also has a lease: {v1, "C-ABC", c.i}.
o Server ABC reboots.
o Client C talks to server ABC using an nfs_client_id4 id like
"C-ABC" and verifier v1. As a result a lease with clientid4 c.j
is established: {v1, "C-ABC", c.j}.
o fs_a2 migrates from server ABC to server XYZ. Now server XYZ also
has a lease: {v1, "C-ABC", c.j}.
o Now server XYZ has two leases that match {v1, "C-ABC", *}, when
the protocol clearly assumes there can be only one.
Note that if the client used "C" (rather than "C-ABC") as the
nfs_client_id4 id string, the exact same situation would arise.
One of the first cases in which this sort of situation has resulted
in difficulties is in connection with doing a SETCLIENTID for
callback update.
The SETCLIENTID for callback update only includes the nfs_client_id4,
assuming there can only be one such with a given nfs_client_id4
value. If there are multiple, confirmed client records with
identical nfs_client_id4 values, there is no way to map the callback
update request to the correct client record.
One possible accommodation for this particular issue that has been
used is to add a RENEW operation along with SETCLIENTID (on a
callback update) to disambiguate the client.
When the client updates the callback info to the destination, the
client would, by convention, send a compound like this:
{ RENEW clientid4, SETCLIENTID nfs_client_id4,verf,cb }
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The presence of the clientid4 in the compound would allow the server
to differentiate among the various leases that it knows of, all with
the same nfs_client_id4 value.
While this would be a reasonable patch for an isolated protocol
weakness, interoperable clients and servers would require that the
protocol truly be updated to allow such a situation, specifically
that of multiple clientid4's with the same nfs_client_id4 value. The
protocol is currently designed and implemented assuming this can't
happen. We need to either prevent the situation from happening, or
fully adapt to the possibilities which can arise. See Section 4 for
a discussion of such issues.
3.1.3. Client complexity issues
Consider the following situation:
o There are a set of clients C1 through Cn accessing servers S1
through Sm. Each server manages some significant number of
filesystems with the filesystem count L being significantly
greater than m.
o Each client Cx will access a subset of the servers and so will
have up to m clientid's, which we will call Cxy for server Sy.
o Now assume that for load-balancing or other operational reasons,
numbers of filesystems are migrated among the servers. As a
result, each client-server pair will have up to m clientid's and
each client will have up to m**2 clientids. If we add the
possibility of server reboot, the only bound on a client's
clientid count is L.
Now, instead of a clientid4 identifying a client-server pair, we have
many more entities for the client to deal with. In addition, it
isn't clear how new state is to be incorporated in this structure.
The limitations of the migrated state (inability to be freed on
reboot) would argue against adding more such state but trying to
avoid that would run into its own difficulties. For example, a
single lockowner string presented under two different clientids would
appear as two different entities.
Thus we have to choose between:
o indefinite prolongation of foreign clientid's even after all
transferred state is gone.
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o having multiple requests for the same lockowner-string-named
entity carried on in parallel by separate identically named
lockowners under different clientid4's
o Adding serialization at the lock-owner string level, in addition
to that at the lockowner level.
In any case, we have gone (in adding migration as it was described)
from a situation in which
o Each client has a single clientid4/lease or each server it talks
to.
o Each client has a single nfs_client_id4 for each server it talks
to.
o Every state id can be mapped to an associated lease based on the
server it was obtained from.
To one in which
o Each client may have multiple clientid4's for a single server.
o For each stateid, the client must separately record the clientid4
that it is assigned to, or it must manage separate "state blobs"
for each fsid and map those to clientid4's.
o Before doing an operation that can result in a stateid, the client
must either find a "state blob" based on fsid or create a new one,
possibly with a new clinetid4.
o There may be multiple clientid4's all connected to the same server
and using the same nfs_clientid4.
This sort of additional client complexity is troublesome and needs to
be eliminated.
3.2. Sources of Protocol difficulties
3.2.1. Issues with nfs_client_id4 generation and use
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
agree. The section entitled "Client ID" says:
The second field, id is a variable length string that uniquely
defines the client.
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There are two possible interpretations of the phrase "uniquely
defines" in the above:
o The relation between strings and clients is a function from such
strings to clients so that each string designates a single client.
o The relation between strings and clients is a bijection between
such strings and clients so that each string designates a single
client and each client is named by a single string.
The first interpretation would make these client-strings like phone
numbers (a single person can have several) while the second would
make them like social security numbers.
Endless debate about the true meaning of "uniquely defines" in this
context is quite possible but not very helpful. The following points
should be noted though:
o The second interpretation is more consistent with the way
"uniquely defines" is used elsewhere in the spec.
o The spec as now written intends the first interpretation (or is
internally inconsistent). In fact, it recommends, although it
doesn't "RECOMMEND" that a single client have at least as many
client-strings as server addresses that it interacts with. It
says, in the third bullet point regarding construction of the
string (which we shall henceforth refer to as client-string-BP3):
The string should be different for each server network address
that the client accesses, rather than common to all server
network addresses.
o If internode interactions are limited to those between a client
and its servers, there is no occasion for servers to be concerned
with the question of whether two client-strings designate the same
client, so that there is no occasion for the difference in
interpretation to matter.
o When transparent migration of client state occurs between two
servers, it becomes important to determine when state on two
different servers is for the same client or not, and this
distinction becomes very important.
Given the need for the server to be aware of client identity with
regard to migrated state, either client-string construction rules
will have to change or there will be need to get around current
issues, or perhaps a combination of these two will be required.
Later sections will examine the options and propose a solution.
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One consideration that may indicate that this cannot remain exactly
as it is today has to do with the fact that the current explanation
for this behavior is not correct. The current definitive definition
of the NFSv4.0 protocol [RFC3530], and the current pending draft of
RFC3530bis [cur-v4.0-bis] both agree. The section entitled "Client
ID" says:
The reason is that it may not be possible for the client to tell
if the same server is listening on multiple network addresses. If
the client issues SETCLIENTID with the same id string to each
network address of such a server, the server will think it is the
same client, and each successive SETCLIENTID will cause the server
to begin the process of removing the client's previous leased
state.
In point of fact, a "SETCLIENTID with the same id string" sent to
multiple network addresses will be treated as all from the same
client but will not "cause the server to begin the process of
removing the client's previous leased state" unless the server
believes it is a newer instance of the same client, i.e. if the id is
the same and there is a different verifier. If the client does not
reboot, the verifier should not change. If it does reboot, the
verifier will change, and the server should "begin the process of
removing the client's previous leased state.
The situation of multiple SETCLIENTID requests received by a server
on multiple network addresses is exactly the same, from the protocol
design point of view, as when multiple (i.e. duplicate) SETCLIENTID
requests are received by the server on a single network address. The
same protocol mechanisms that prevent erroneous state deletion in the
latter case prevent it in the former case. There is no reason for
special handling of the multiple-network-appearance case, in this
regard.
3.2.2. Issues with lease proliferation
It is often felt that this is a consequence of the client-string
construction issues, and it is certainly the case that the two are
closely connected in that non-uniform client-strings make it
impossible for the server to appropriately combine leases from the
same client. See Section 5.2.1 for a discussion of non-uniform
client-strings.
However, even where the server could combine leases from the same
client, it needs to be clear how and when it will do so, so that the
client will be prepared. These issues will have to be addressed at
various places in the spec.
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This could be enough only if we are prepared to do away with the
"should" recommending non-uniform client-strings and replace it with
a "should not" or even a "SHOULD NOT". Current client implementation
patterns make this an unpalatable choice for use as a general
solution, but it is reasonable to "RECOMMEND" this choice for a well-
defined subset of clients. One alternative would be to create a way
for the server to infer from client behavior which leases are held by
the same client and use this information to do appropriate lease
mergers. Prototyping and detailed specification work has shown that
this could be done but the resulting complexity is such that a better
choice is to "RECOMMEND" use of the uniform model for clients
supporting the migration feature.
4. Issues to be resolved
4.1. Possible changes to nfs_client_id4 client-string
The fact that the reason given in client-string-BP3 is not valid
makes the existing "should" insupportable. We can't either
o Keep a reason we know is invalid.
o Keep saying "should" without giving a reason.
What are often presented as reasons that motivate use of the non-
uniform model always turn out to be cases in which, if the uniform
model were used, the server will treat a client which accesses that
server via two different IP addresses as part of a single client, as
it in fact is. This may be disconcerting to a client unaware that
the two IP addresses connect to the same server. This is thus not a
reason to use the non-uniform model but rather an illustration of the
fact that those using the uniform model must use server behavior to
determine whether any trunking of IP addresses exists, as is
described in Section 5.2.2.
It is always possible that a valid new reason will be found, but so
far none has been proposed. Given the history, the burden of proof
should be on those asserting the validity of a proposed new reason.
So we will assume for now that the "should" will have to go. The
question is what to replace it with.
o We can't say "MUST NOT", despite the problems this raises for
migration since this is pretty late in the day for such a change.
Many currently operating clients obey the existing "should".
Similar considerations would apply for "SHOULD NOT" or "should
not".
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o Dropping client-string-BP3 entirely is a possibility but, given
the context and history, it would just be a confusing version of
"SHOULD NOT".
o Using "MAY" would clearly specify that both ways of doing this are
valid choices for clients and that servers will have to deal with
clients that make either choice.
o This might be modified by a "SHOULD" (or even a "MUST") for
particular groups of clients.
o There will have to be some text explaining why a client might make
either choice but, except for the particular cases referred to
above, we will have to make sure that it is truly descriptive, and
not slanted in either direction.
4.2. Possible changes to handle differing nfs_client_id4 string values
Given the difficulties caused by having different nfs_client_id4
client-string values for the same client, we have two choices:
o Deprecate the existing treatment and basically say the client is
on its own doing migration, if it follows it.
o Introduce a way of having the client provide client identity
information to the server, if it can be done compatibly while
staying within the bounds of v4.0.
4.3. Other issues within migration-state sections
There are a number of issues where the existing text is unclear
and/or wrong and needs to be fixed in some way.
o Lack of clarity in the discussion of moving clientids (as well as
stateids) as part of moving state for migration.
o The discussion of synchronized leases is wrong in that there is no
way to determine (in the current spec) when leases are for the
same client and also wrong in suggesting a benefit from leases
synchronized at the point of transfer. What is needed is merger
of leases, which is necessary to keep client complexity
requirements from getting out of hand.
o Lack of clarity in the discussion of LEASE_MOVED handling.
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4.4. Issues within other sections
There are a number of cases in which certain sections, not
specifically related to migration require additional clarification.
This is generally because text that is clear in a context in which
leases and clientids are created in one place and live there forever
may need further refinement in the more dynamic environment that
arises as part of migration.
Some examples:
o Some people are under the impression that updating callback
endpoint information for an existing client, which is part of the
client's handling of migration, may cause the destination server
to free existing state. There needs to be additions to clarify
the situation.
o The handling of the sets of clientid4's maintained by each server
needs to be clarified. In particular, the issue of how the client
adapts to the presumably independent and uncoordinated clientid4
sets needs to be clearly addressed
o Statements regarding handling of invalid clientid4's need to be
clarified and/or refined in light of the possibilities that arise
due to lease motion and merger.
5. Proposed resolution of protocol difficulties
5.1. Proposed changes: nfs_client_id4 client-string
We propose replacing client-string-BP3 with the following text and
adding the following proposed Section 5.2 to provide implementation
guidance.
o The string MAY be different for each server network address that
the client accesses, rather than common to all server network
addresses. The considerations that might influence a client to
use different strings for each are explained in Section 5.2.
o Despite the use of the word "string" for this identifier, and the
fact that using strings will often be convenient, it should be
understood that the protocol defines this as opaque data. In
particular, those receiving such an id should not assume that it
will be in UTF-8 format nor should they reject it if it is not.
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5.2. Client-string Models (AS PROPOSED)
One particular aspect of the construction of the nfs4_client_id4
string has proved recurrently troublesome. The client has a choice
of:
o Presenting the same id string to each server address accessed.
This is referred to as the "uniform client-string model" and is
discussed in Section 5.2.2.
o Presenting a different id string to each server address accessed.
This is referred to as the "non-uniform client-string model" and
is discussed in Section 5.2.1.
Construction of the client-string has been a troublesome issue
because of the way in which the NFS protocols have evolved.
o NFSv3 as a stateless protocol had no need to identify the state
shared by a particular client-server pair. Thus there was no
occasion to consider the question of whether a set of requests
come from the same client, or whether two server IP addresses are
connected to the same server. As the environment was one in which
the user supplied the target server IP address as part of
incorporating the remote filesystem in the client's file name
space, there was no occasion to take note of server trunking.
Within a stateless protocol, the situation was symmetrical. The
client has no server identity information and the server has no
client identity information.
o NFSv4.1 is a stateful protocol with full support for client and
server identity determination. This enables the server to be
aware when two requests come from the same client (they are on
sessions sharing a clientid4) and the client to be aware when two
server IP addresses are connected to the same server (they return
the same server name in responding to an EXCHANGE_ID).
NFSv4.0 is unfortunately halfway between these two. The two client-
string models have arisen in attempts to deal with the changing
requirements of the protocol as implementation has proceeded and
features that were not very substantial in [RFC3530], got more
substantial.
o In the absence of any implementation of the fs_locations-related
features (replication, referral, and migration), the situation is
very similar to that of NFSv3, with the addition of state but with
no concern to provide accurate client and server identity
determination. This is the situation that gave rise to the non-
uniform client-string model.
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o In the presence of replication and referrals, the client may have
occasion to take advantage of knowledge of server trunking
information. Even more important, migration, by transferring
state among servers, causes difficulties for the non-uniform
client-string model, in that the two different client-strings sent
to different IP addresses may wind up on the same IP address,
adding confusion.
Both models have to deal with the asymmetry in client and server
identity information between client and server. Each seeks to make
the client's and the server's views match. In the process, each
encounters some combination of inelegant protocol features and/or
implementation difficulties. The choice of which to use is up to the
client implementer and the sections below try to give some useful
guidance.
5.2.1. Non-Uniform Client-string Model
The non-uniform client-string model is an attempt to handle these
matters in NFSv4.0 client implementations in as NFSv3-like a way as
possible.
For a client using the non-uniform model, all internal recording of
clientid4 values is to include, whether explicitly or implicitly, the
server IP address so that one always has an (IP-address, clientid4)
pair. Two such pairs from different servers are always distinct even
when the clientid4 values are the same, as they may occasionally be.
In this model, such equality is always treated as simple
happenstance.
Making the client-string different on different servers means that a
server has no way of tying together information from the same client
and so will treat a single client as multiple clients with multiple
leases for each server network address. Since there is no way in the
protocol for the client to determine if two network addresses are
connected to the same server, the resulting lack of knowledge is
symmetrical and can result in simpler client implementations in which
there is a single clientid/lease per server network addresses.
Support for migration, particularly with transparent state migration,
is more complex in the case of non-uniform client-strings. For
example, migration of a lease can result in multiple leases for the
same client accessing the same server addresses, vitiating many of
the advantages of this approach. Therefore, client implementations
that support migration with transparent state migration SHOULD NOT
use the non-uniform client-string model.
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5.2.2. Uniform Client-string Model
When the client-string is kept uniform, the server has the basis to
have a single clientid4/lease for each distinct client. The problem
that has to be addressed is the lack of explicit server identity
information, which is made available in NFSv4.1.
When the same client-string is given to multiple IP addresses, the
client can determine whether two IP addresses correspond to a single
server, based on the server's behavior. This is the inverse of the
strategy adopted for the non-uniform model in which different server
IP addresses are told about different clients, simply to prevent a
server from manifesting behavior that is inconsistent with there
being a single server for each IP address, in line with the
traditions of NFS. So, to compare:
o In the non-uniform model, servers are told about different clients
because, if the server were to use accurate information as to
client identity, two IP addresses on the same server would behave
as if they were talking to the same client, which might prove
disconcerting to a client not expecting such behavior.
o In the uniform model, the servers are told about there being a
single client, which is, after all, the truth. Then, when the
server uses this information, two IP addresses on the same server
will behave as if they are talking to the same client, and this
difference in behavior allows the client to infer the server IP
address trunking configuration, even though NFSv4.0 does not
explicitly provide this information.
The approach given below shows one example of how this might be
done.
For a client using the uniform model, clientid4 values are treated as
important information in determining server trunking patterns. For
two different IP addresses to return the same clientid4 value is a
necessary, though not a sufficient condition for them to be
considered as connected to the same server. As a result, when two
different IP addresses return the same clientid4, the client needs to
determine, using the procedure given below or otherwise, whether the
IP addresses are connected to the same server. For such clients, all
internal recording of clientid4 values needs to include, whether
explicitly or implicitly, identification of the server from which the
clientid4 was received so that one always has a (server clientid4)
pair. Two such pairs from different servers are always considered
distinct even when the clientid4 values are the same, as they may
occasionally be.
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In order to make this approach work, the client must have accessible,
for each nfs4_client_id4 used (only one in the uniform model) a list
of all server IP addresses, together with the associated clientid4
values. As a part of the associated data structures, there should be
the ability to mark a server IP structure as having the same server
as another and to mark an IP-address as currently unresolved. One
way to do this is to a allow each such entry to point to another with
the pointer value being one of:
o A pointer to another entry for an IP address associated with the
same server, where that IP address is the first one referenced to
access that server.
o A pointer to the current entry if there is no earlier IP address
associated with the same server, i.e. where the current IP address
is the first one referenced to access that server. We'll refer to
such an IP address as the lead IP address for a given server.
o The value NULL if the address's server identity is currently
unresolved.
When a SETCLIENTID is done and a clientid4 returned, the data
structure is searched for a matching clientid4 and processing depends
on what is found. We will refer to the IP address on which this
SETCLIENTID is done as X. The SETCLIENTID will use the common
nfs_client_id4 and specify X as part of the callback parameters. We
call the clientid4 and verifier returned by this operation XC and XV.
Note that at this point no SETCLIENTID_CONFIRM has yet been done.
This is because we have either established a new clientid4 on a
previously unknown server or changed the callback parameters on a
clientid4 associated with some already known server. We don't want
to confirm something that we are not sure we want to happen.
o If no matching clientid4 is found, the IP address X and clientid4
XC are added to the list and considered as having no existing
known IP addresses trunked with it. The IP address is marked as a
lead IP address for a new server. A SETCLIENTID_CONFIRM is done
using XC and XV.
o If a matching clientid4 is found which is marked unresolved,
processing on the new IP address is suspended. In order to
simplify processing, there can only be one unresolved IP address
for any given clientid4.
o If one or more matching clientid4's is found, none of which is
marked unresolved, the new IP address in entered and marked
unresolved. After applying the steps below to each of the lead IP
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addresses with a matching clientid4, the address will have been
resolved: either it will be part of the same server as a new IP
address to be added to an existing set of IP addresses for a
server, or it will be recognized as a new server. At the point at
which this determination is made, the unresolved indication is
cleared and any suspended SETCLIENTID processing is restarted
So for each lead IP address IPn with a clientid4 matching XC, the
following steps are done.
o If the server has an associated stateid S, S is used in a request
issued on the address X with the fact of whether it is recognized
on X giving definitive information of X's server identity.
o If S is not recognized as valid on X, then X and IPn are
recognized as distinct and we go on to the next IPn, until we run
out of them.
o If S is recognized as valid on X, then X and IPn are recognized as
connected to the same server and the entry for X is marked as
associated with IPn. The entry is now resolved and processing can
be restarted for IP addresses whose clientid4 matched XC and whose
resolution had been deferred.
o If there is no such S for IPn, a different procedure is used. a
SETCLIENTID is done to update the callback parameters to reflect
the possibility that X will be marked as associated with the
server whose lead IP address is IPn. So assume that we do that
SETCLIENTID and get back verifier Vn.
o Note that we don't want this to happen if address X is not
associated with this server. So we do a SETCLIENTID_CONFIRM on
address IPn using verifier Vn.
o If the verifier generated on X is accepted on IPn, then X and IPn
are recognized as connected to the same server and the entry for X
is marked as associated with IPn. The entry is now resolved and
processing can be restarted for IP addresses whose clientid4
matched XC but whose resolution had been deferred.
o If the verifier generated on X is not accepted on IPn, then X and
IPn are distinct and the callback update will not be confirmed.
So we go on to the next IPn, until we run out of them.
The procedure above has made no explicit mention of the possibility
that server reboot can occur at any time. To address this
possibility the client should periodically use the clientid4 XC in
RENEW operations, directed to both the IP address X and the current
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lead IP address that is currently being tested for identity.
o When XC becomes invalid on X, the resolution process should be
terminated, subject to being redone later. Before redoing the
resolution, XC should be checked on all the lead IP addresses on
which it was valid. Once a new clientid4 is established on any
servers on which XC became invalid, a new clientid4 can be
established on X and the resolution process for X can be
restarted.
o When XC does not becomes invalid on X, but becomes invalid on the
current IPn being tested, it should be concluded that X and IPn do
not match and that it is time to advance to the next IPn, if any.
o In the event of a reboot detected on any server lead IP, the set
of IP addresses associated with the server should not change and
state should be re-established for the lease as a whole, using all
available connected server IP addresses. It is prudent to verify
connectivity by doing a RENEW using the new clientid4 on each such
server address before using it, however.
If we have run out of IPn's without finding a matching server, X is
considered as having no existing known IP addresses trunked with it.
The IP address is marked as a lead IP address for a new server. A
SETCLIENTID_CONFIRM is done using XC and XV.
The following are advantages for the implementation of using the
uniform client-string model:
o Clients can take advantage of server trunking (and clustering with
single-server-equivalent semantics) to increase bandwidth or
reliability.
o There are advantages in state management so that, for example, we
never have a delegation under one clientid revoked because of a
reference to the same file from the same client under a different
clientid.
o The uniform client-string model allows the server to do any
necessary automatic lease merger in connection with migration,
without requiring any client involvement. This consideration is
of sufficient weight to cause us RECOMMEND use of the uniform
client-string model for clients supporting transparent state
migration.
The following implementation considerations might cause issues for
client implementations.
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o This model is considerably different from the non-uniform model,
which most client implementations have been following. Until
substantial implementation experience is obtained with this model,
reluctance to embrace something so new is to be expected.
o Mapping between server network addresses and leases is more
complicated in that it is no longer a one-to-one mapping.
How to balance these considerations depends on implementation goals.
5.3. Proposed changes: merged (vs. synchronized) leases
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
agree. The section entitled "Migration and State" says:
As part of the transfer of information between servers, leases
would be transferred as well. The leases being transferred to the
new server will typically have a different expiration time from
those for the same client, previously on the old server. To
maintain the property that all leases on a given server for a
given client expire at the same time, the server should advance
the expiration time to the later of the leases being transferred
or the leases already present. This allows the client to maintain
lease renewal of both classes without special effort:
There are a number of problems with this and any resolution of our
difficulties must address them somehow.
o The current v4.0 spec recommends that the client make it
essentially impossible to determine when two leases are from "the
same client".
o It is not appropriate to speak of "maintain[ing] the property that
all leases on a given server for a given client expire at the same
time", since this is not a property that holds even in the absence
of migration. A server listening on multiple network addresses
may have the same client appear as multiple clients with no way to
recognize the client as the same.
o Even if the client identity issue could be resolved, advancing the
lease time at the point of migration would not maintain the
desired synchronization property. The leases would be
synchronized until one of them was renewed, after which they would
be unsynchronized again.
To avoid client complexity, we need to have no more than one lease
between a single client and a single server. This requires merger of
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leases since there is no real help from synchronizing them at a
single instant.
For the uniform model, the destination server would simply merge
leases as part of state transfer, since two leases with the same
nfs_client_id4 values must be for the same client.
We have made the following decisions as far as proposed normative
statements regarding for state merger. They reflect the facts that
we want to support fully migration support in the simplest way
possible and that we can't say MUST since we have older clients and
servers to deal with.
o Clients SHOULD use the uniform client-string model in order to get
good migration support.
o Servers SHOULD provide automatic lease merger during state
migration so that clients using the uniform id model get the
support automatically.
If the clients and the servers obey the SHOULD's, having more than a
single lease for a given client-server pair will be a transient
situation, cleaned up as part of adapting to use of migrated state.
Since clients and servers will be a mixture of old and new and
because nothing is a MUST we have to ensure that no combination will
show worse behavior than is exhibited by current (i.e. old) clients
and servers.
5.4. Other proposed changes to migration-state sections
5.4.1. Proposed changes: Client ID migration
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
agree. The section entitled "Migration and State" says:
In the case of migration, the servers involved in the migration of
a filesystem SHOULD transfer all server state from the original to
the new server. This must be done in a way that is transparent to
the client. This state transfer will ease the client's transition
when a filesystem migration occurs. If the servers are successful
in transferring all state, the client will continue to use
stateids assigned by the original server. Therefore the new
server must recognize these stateids as valid. This holds true
for the client ID as well. Since responsibility for an entire
filesystem is transferred with a migration event, there is no
possibility that conflicts will arise on the new server as a
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result of the transfer of locks.
This poses some difficulties, mostly because the part about "client
ID" is not clear:
o It isn't clear what part of the paragraph the "this" in the
statement "this holds true ..." is meant to signify.
o The phrase "the client ID" is ambiguous, possibly indicating the
clientid4 and possibly indicating the nfs_client_id4.
o If the text means to suggest that the same clientid4 must be used,
the logic is not clear since the issue is not the same as for
stateids of which there might be many. Adapting to the change of
a single clientid, as might happen as a part of lease migration,
is relatively easy for the client.
We have decided to address this issue as follows, with the relevant
changes all reflected in Section 5.6.
o Make it clear that both clientid4 and nfs_client_id4 are to be
transferred.
o Indicate that the initial transfer will result in the same
clientid4 after transfer but this is not guaranteed since there
may conflict with an existing clientid4 on the destination server
and because lease merger can result in a change of the clientid4.
5.4.2. Proposed changes: Callback re-establishment
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
agree. The section entitled "Migration and State" says:
A client SHOULD re-establish new callback information with the new
server as soon as possible, according to sequences described in
sections "Operation 35: SETCLIENTID - Negotiate Client ID" and
"Operation 36: SETCLIENTID_CONFIRM - Confirm Client ID". This
ensures that server operations are not blocked by the inability to
recall delegations.
The above will need to be fixed to reflect the possibility of merging
of leases and the text to do this appears as part of Section 5.6.
5.4.3. Proposed changes: NFS4ERR_LEASE_MOVED rework
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
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agree. The section entitled "Notification of Migrated Lease" says:
Upon receiving the NFS4ERR_LEASE_MOVED error, a client that
supports filesystem migration MUST probe all filesystems from that
server on which it holds open state. Once the client has
successfully probed all those filesystems which are migrated, the
server MUST resume normal handling of stateful requests from that
client.
There is a lack of clarity that is prompted by ambiguity about what
exactly probing is and what the interlock between client and server
must be. This has led to some worry about the scalability of the
probing process, and although the time required does scale linearly
with the number of fs's that the client may have state for with
respect to a given server, the actual process can be done
efficiently.
To address these issues we propose replacing the above with the text
addressing NFS4RR_LEASE_MOVED as given in Section 5.6.3.
5.5. Proposed changes to other sections
5.5.1. Proposed changes: callback update
Some changes are necessary to reduce confusion about the process of
callback information update and in particular to make it clear that
no state is freed as a result:
o Make it clear that after migration there are confirmed entries for
transferred clientid4/nfs_client_id4 pairs.
o Be explicit in the sections headed "otherwise," in the
descriptions of SETCLIENTID and SETCLIENTID_CONFIRM, that these
don't apply in the cases we are concerned about.
5.5.2. Proposed changes: clientid4 handling
To address both of the clientid4-related issues mentioned in
Section 4.4, we propose replacing the last three paragraphs of the
section entitled "Client ID" with the following:
Once a SETCLIENTID and SETCLIENTID_CONFIRM sequence has
successfully completed, the client uses the shorthand client
identifier, of type clientid4, instead of the longer and less
compact nfs_client_id4 structure. This shorthand client
identifier (a client ID) is assigned by the server and should be
chosen so that it will not conflict with a client ID previously
assigned by same server. This applies across server restarts or
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reboots.
Distinct servers MAY assign clientid4's independently, and will
generally do so. Therefore, a client has to be prepared to deal
with multiple instances of the same clientid4 value received on
distinct IP addresses, denoting separate entities. When trunking
of server IP addresses is not a consideration, a client should
keep track of (IP-address, clientid4) pairs, so that each pair is
distinct. For a discussion of how to address the issue in the
face of possible trunking of server IP addresses, see Section 5.2.
When a clientid4 is presented to a server and that clientid4 is
not recognized, the server will reject the request with the error
NFS4ERR_STALE_CLIENTID. This can occur for a number of reasons:
* A server reboot causing loss of the server's knowledge of
client
* Client error sending an incorrect clientid4 or valid clientid4
to the wrong server.
* Loss of lease state due to lease expiration.
* Client or server error causing the server to believe that the
client has rebooted (i.e. receiving a SETCLIENTID with an
nfs_client_id4 which has a matching id and a non-matching
verifier.
* Migration of all state under the associated lease causes its
non-existence to be recognized on the source server.
* Merger of state under the associated lease with another lease
under a different clientid causes the clientid4 serving as the
source of the merge to cease being recognized on its server.
In the event of a server reboot, or loss of lease state due to
lease expiration, the client must obtain a new clientid4 by use of
the SETCLIENTID operation and then proceed to any other necessary
recovery for the server reboot case (See the section entitled
"Server Failure and Recovery"). In cases of server or client
error resulting in this error, use of SETCLIENTID to establish a
new lease is desirable as well.
In the last two cases, different recovery procedures are required.
See Section 5.6 for details. Note that in cases in which there is
any uncertainty about which sort of handling is applicable, the
distinguishing characteristic is that in reboot-like cases, the
clientid4 and all associated stateid cease to exist while in
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migration-related cases, the clientid4 ceases to exist while the
stateids are still valid.
The client must also employ the SETCLIENTID operation when it
receives a NFS4ERR_STALE_STATEID error using a stateid derived
from its current clientid4, since this indicates a situation, such
as server reboot which has invalidated the existing clientid4 and
associated stateids (see the section entitled "lock-owner" for
details).
See the detailed descriptions of SETCLIENTID and
SETCLIENTID_CONFIRM for a complete specification of the
operations.
5.6. Migration, Replication and State (AS PROPOSED)
When responsibility for handling a given filesystem is transferred to
a new server (migration) or the client chooses to use an alternate
server (e.g., in response to server unresponsiveness) in the context
of filesystem replication, the appropriate handling of state shared
between the client and server (i.e., locks, leases, stateids, and
client IDs) is as described below. The handling differs between
migration and replication.
If a server replica or a server immigrating a filesystem agrees to,
or is expected to, accept opaque values from the client that
originated from another server, then it is a wise implementation
practice for the servers to encode the "opaque" values in network
byte order. When doing so, servers acting as replicas or immigrating
filesystems will be able to parse values like stateids, directory
cookies, filehandles, etc. even if their native byte order is
different from that of other servers cooperating in the replication
and migration of the filesystem.
5.6.1. Migration and State
In the case of migration, the servers involved in the migration of a
filesystem SHOULD transfer all server state from the original to the
new server. This must be done in a way that is transparent to the
client. This state transfer will ease the client's transition when a
filesystem migration occurs. If the servers are successful in
transferring all state, the client will continue to use stateids
assigned by the original server. Therefore the new server must
recognize these stateids as valid.
If transferring stateids from server to server would result in a
conflict for an existing stateid for the destination server with the
existing client, transparent state migration MUST NOT happen for that
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client. Servers participating in using transparent state migration
should co-ordinate their stateid assignment policies to make this
situation unlikely or impossible. The means by which this might be
done, like all of the inter-server interactions for migration, are
not specified by the NFS version 4.0 protocol.
Handling of clientid values is similar but not identical. The
clientid4 and nfs_client_id4 information (id and verifier) will be
transferred with the rest of the state information and the
destination server should use that information to determine
appropriate clientid4 handling. Although the destination server may
make state stored under an existing lease available under the
clientid4 used on the source server, the client should not assume
that this is always so. In particular,
o If there is an existing lease with an nfs_client_id4 that matches
a migrated lease (same id and verifier), the server SHOULD merge
the two, making the union of the sets of stateids available under
the clientid4 for the existing lease. As part of the lease
merger, the expiration time of the lease will reflect renewal done
within either of the ancestor leases (and so will reflect the
latest of the renewals).
o If there is an existing lease with an nfs_client_id4 that
partially matches a migrated lease (same id and a different
verifier), the server MUST eliminate one of the two, possibly
invalidating one of the ancestor clientid4's. Since verifiers are
not ordered, the later lease renewal time will prevail.
When leases are not merged, the transfer of state should result in
creation of a confirmed client record with empty callback information
but matching the {v, x, c} for the transferred client information.
This should enable establishment of new callback information using
SETCLIENTID and SETCLIENTID_CONFIRM.
A client may determine the disposition of migrated state by using a
stateid associated with the migrated state and in an operation on the
new server and using the associated clientid4 in a RENEW on the new
server.
o If the stateid is not valid and an error NFS4ERR_BAD_STATEID is
received, either transparent state migration has not occurred or
the state was purged due to verifier mismatch.
o If the stateid is valid and an error NFS4ERR_STALE_CLIENTID is
received on the RENEW, transparent state migration has occurred
and the lease has been merged with an existing lease on the
destination server.
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o If the stateid is valid and the clientid4 is valid, the lease has
been transferred intact.
Since responsibility for an entire filesystem is transferred with a
migration event, there is no possibility that conflicts will arise on
the new server as a result of the transfer of locks.
The servers may choose not to transfer the state information upon
migration. However, this choice is discouraged, except where
specific issues such as stateid conflicts make it necessary. In the
case of migration without state transfer, when the client presents
state information from the original server (e.g. in a RENEW op or a
READ op of zero length), the client must be prepared to receive
either NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from the new
server. The client should then recover its state information as it
normally would in response to a server failure. The new server must
take care to allow for the recovery of state information as it would
in the event of server restart.
When a lease is transferred to a new server (as opposed to being
merged with a lease already on the new server), a client SHOULD re-
establish new callback information with the new server as soon as
possible, according to sequences described in sections "Operation 35:
SETCLIENTID - Negotiate Client ID" and "Operation 36:
SETCLIENTID_CONFIRM - Confirm Client ID". This ensures that server
operations are not blocked by the inability to recall delegations.
5.6.2. Replication and State
Since client switch-over in the case of replication is not under
server control, the handling of state is different. In this case,
leases, stateids and client IDs do not have validity across a
transition from one server to another. The client must re-establish
its locks on the new server. This can be compared to the re-
establishment of locks by means of reclaim-type requests after a
server reboot. The difference is that the server has no provision to
distinguish requests reclaiming locks from those obtaining new locks
or to defer the latter. Thus, a client re-establishing a lock on the
new server (by means of a LOCK or OPEN request), may have the
requests denied due to a conflicting lock. Since replication is
intended for read-only use of filesystems, such denial of locks
should not pose large difficulties in practice. When an attempt to
re-establish a lock on a new server is denied, the client should
treat the situation as if its original lock had been revoked.
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5.6.3. Notification of Migrated Lease
In the case of lease renewal, the client may not be submitting
requests for a filesystem that has been migrated to another server.
This can occur because of the implicit lease renewal mechanism. The
client renews a lease containing state of multiple filesystems when
submitting a request to any one filesystem at the server.
In order for the client to schedule renewal of leases that may have
been relocated to the new server, the client must find out about
lease relocation before those leases expire. To accomplish this, all
operations which implicitly renew leases for a client (such as OPEN,
CLOSE, READ, WRITE, RENEW, LOCK, and others), will return the error
NFS4ERR_LEASE_MOVED if responsibility for any of the leases to be
renewed has been transferred to a new server. Note that when the
transfer of responsibility leaves remaining state for that lease on
the source server, the lease is renewed just as it would have been in
the NFS4ERR_OK case, despite returning the error. The transfer of
responsibility happens when the server receives a
GETATTR(fs_locations) from the client for each filesystem for which a
lease has been moved to a new server. Normally it does this after
receiving an NFS4ERR_MOVED for an access to the filesystem but the
server is not required to verify that this happens in order to
terminate the return of NFS4ERR_LEASE_MOVED. By convention, the
compounds containing GETATTR(fs_locations) SHOULD include an appended
RENEW operation to permit the server to identify the client getting
the information.
Note that the NFS4ERR_LEASE_MOVED error is only required when
responsibility for at least one stateid has been transferred. In the
case of a null lease, where the only associated state is a clientid,
no NFS4ERR_LEASE_MOVED error need be generated.
Upon receiving the NFS4ERR_LEASE_MOVED error, a client that supports
filesystem migration MUST perform the necessary GETATTR operation for
each of the filesystems containing state that have been migrated and
so give the server evidence that it is aware of the migration of the
filesystem. Once the client has done this for all migrated
filesystems on which the client holds state, the server MUST resume
normal handling of stateful requests from that client.
One way in which clients can do this efficiently in the presence of
large numbers of filesystems is described below. This approach
divides the process into two phases, one devoted to finding the
migrated filesystems and the second devoted to doing the necessary
GETATTRs.
The client can find the migrated filesystems by building and issuing
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one or more COMPOUND requests, each consisting of a set of PUTFH/
GETFH pairs, each pair using an fh in one of the filesystems in
question. All such COMPOUND requests can be done in parallel. The
successful completion of such a request indicates that none of the
fs's interrogated have been migrated while termination with
NFS4ERR_MOVED indicates that the filesystem getting the error has
migrated while those interrogated before it in the same COMPOUND have
not. Those whose interrogation follows the error remain in an
uncertain state and can be interrogated by restarting the requests
from after the point at which NFS4ERR_MOVED was returned or by
issuing a new set of COMPOUND requests for the filesystems which
remain in an uncertain state.
Once the migrated filesystems have been found, all that is needed is
for client to give evidence to the server that it is aware of the
migrated status of filesystems found by this process, by
interrogating the fs_locations attribute for an fh each of the
migrated filesystems. The client can do this building and issuing
one or more COMPOUND requests, each of which consists of a set of
PUTFH operations, each followed by a GETATTR of the fs_locations
attribute. A RENEW follows to help tie the operations to the lease
returning NFS4ERR_LEASE_MOVED. Once the client has done this for all
migrated filesystems on which the client holds state, the server will
resume normal handling of stateful requests from that client.
In order to support legacy clients that do not handle the
NFS4ERR_LEASE_MOVED error correctly, the server SHOULD time out after
a wait of at least two lease periods, at which time it will resume
normal handling of stateful requests from all clients. If a client
attempts to access the migrated files, the server MUST reply
NFS4ERR_MOVED.
When the client receives an NFS4ERR_MOVED error, the client can
follow the normal process to obtain the new server information
(through the fs_locations attribute) and perform renewal of those
leases on the new server. If the server has not had state
transferred to it transparently, the client will receive either
NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from the new server,
as described above. The client can then recover state information as
it does in the event of server failure.
Aside from recovering from a migration, there are other reasons a
client may wish to retrieve fs_locations information from a server.
When a server becomes unresponsive, for example, a client may use
cached fs_locations data to discover an alternate server hosting the
same fs data. A client may periodically request fs_locations data
from a server in order to keep its cache of fs_locations data fresh.
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Since a GETATTR(fs_locations) operation would be used for refreshing
cached fs_locations data, a server could mistake such a request as
indicating recognition of an NFS4ERR_LEASE_MOVED condition.
Therefore a compound which is not intended to signal that a client
has recognized a migrated lease SHOULD be prefixed with a guard
operation which fails with NFS4ERR_MOVED if the file handle being
queried is no longer present on the server. The guard can be as
simple as a GETFH operation.
Though unlikely, it is possible that the target of such a compound
could be migrated in the time after the guard operation is executed
on the server but before the GETATTR(fs_locations) operation is
encountered. When a client issues a GETATTR(fs_locations) operation
as part of a compound not intended to signal recognition of a
migrated lease, it SHOULD be prepared to process fs_locations data in
the reply that shows the current location of the fs is gone.
5.6.4. Migration and the Lease_time Attribute
In order that the client may appropriately manage its leases in the
case of migration, the destination server must establish proper
values for the lease_time attribute.
When state is transferred transparently, that state should include
the correct value of the lease_time attribute. The lease_time
attribute on the destination server must never be less than that on
the source since this would result in premature expiration of leases
granted by the source server. Upon migration in which state is
transferred transparently, the client is under no obligation to re-
fetch the lease_time attribute and may continue to use the value
previously fetched (on the source server).
In the case in which lease merger occurs as part of state transfer,
the lease_time attribute of the destination lease remains in effect.
The client can simply renew that lease with its existing lease_time
attribute. State in the source lease is renewed at the time of
transfer so that it cannot expire, as long as the destination lease
is appropriately renewed.
If state has not been transferred transparently (i.e., the client
sees a real or simulated server reboot), the client should fetch the
value of lease_time on the new (i.e., destination) server, and use it
for subsequent locking requests. However the server must respect a
grace period at least as long as the lease_time on the source server,
in order to ensure that clients have ample time to reclaim their
locks before potentially conflicting non-reclaimed locks are granted.
The means by which the new server obtains the value of lease_time on
the old server is left to the server implementations. It is not
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specified by the NFS version 4.0 protocol.
6. Results of proposed changes
The purpose of this section is to examine the troubling results
reported in Section 3.1. We will look at the scenarios as they would
be handled within the proposal.
Because the choice of uniform vs. non-uniform nfs_client_id4 id
strings is a "SHOULD" in these cases, we will designate clients that
follow this recommendation by SHOULD-UF-CID.
We will also have to take account of the various merger-related
"SHOULD" clauses to better understand how they have addressed the
issues seen, we abbreviate these (collectively known as "SHOULD-
merges") as follows:
o SHOULD-SVR-AM refers to the server obeying the SHOULD which
RECOMMENDS that they merge leases with identical nfs_client_id4 id
strings and verifiers.
6.1. Results: Failure to free migrated state on client reboot
Let's look at the troublesome situation cited in Section 3.1.1. We
have already seen what happens when SHOULD-UF-CID does not hold. Now
let's look at the situation in which SHOULD-UF-CID holds, whether
SHOULD-SVR-AM is in effect or not.
o A client C establishes a clientid4 C1 with server ABC specifying
an nfs_client_id4 with "id" value "C" and verifier 0x111.
o The client begins to access files in filesystem F on server ABC,
resulting in generating stateids S1, S2, etc. under the lease for
clientid C1. It may also access files on other filesystems on the
same server.
o The filesystem is migrated from ABC to server XYZ. When
transparent state migration is in effect, stateids S1 and S2 and
lease {0x111, "C", C1} are now available for use by client C at
server XYZ. So far, so good.
o Client C reboots and attempts to access data on server XYZ,
whether in filesystem F or another. It does a SETCLIENID with an
nfs_client_id4 with "id" value "C" and verifier 0x112. The state
associated with lease {0x111, "C", C1} is deleted as part of
creating {0x112, "C", C2}. No problem.
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The correctness signature for this issue is
SHOULD-UF-CID
so if you have clients and servers that obey the SHOULD clauses, the
problem is gone regardless of the choice on the MAY.
6.2. Results: Server reboots resulting in confused lease situation
Now let's consider the scenario given in Section 3.1.2. We have
already seen what happens when SHOULD-UF-CID does not hold . Now
let's look at the situation in which SHOULD-UF-CID holds and SHOULD-
SVR-AM holds as well.
o Client C talks to server ABC using an nfs_client_id4 id like
"C-ABC" and verifier v1. As a result a lease with clientid4 c.i
established: {v1, "C-ABC", c.i}.
o fs_a1 migrates from server ABC to server XYZ along with its state.
Now server XYZ also has a lease: {v1, "C-ABC", c.i}
o Server ABC reboots.
o Client C talks to server ABC using an nfs_client_id4 id like
"C-ABC" and verifier v1. As a result a lease with clientid4 c.j
established: {v1, "C-ABC", c.j}.
o fs_a2 migrates from server ABC to server XYZ. As part of
migration the incoming lease is seen to denote same Nfs_client_id4
and so is merged with {v1, "C-ABC, c.i}.
o Now server XYZ has only one lease that matches {v1, "C_ABC", *},
so the problem is solved
Now let's consider the same scenario in the situation in which
SHOULD-UF-CID holds and SHOULD-SVR-AM holds as well.
o Client C talks to server ABC using an nfs_client_id4 id like "C"
and verifier v1. As a result a lease with clientid4 c.i is
established: {v1, "C", c.i}.
o fs_a1 migrates from server ABC to server XYZ along with its state.
Now XYZ also has a lease: {v1, "C", c.i}
o Server ABC reboots.
o Client C talks to server ABC using an nfs_client_id4 id like "C"
and verifier v1. As a result a lease with clientid4 c.j is
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established: {v1, "C", c.j}.
o fs_a2 migrates from server ABC to server XYZ. As part of
migration the incoming lease is seen to denote the same
nfs_client_id4 and so is merged with {v1, "C", c.i}.
o Now server XYZ has only one lease that matches {v1, "C", *}, so
the problem is solved
The correctness signature for this issue is
SHOULD-SVR-AM
so if you have clients and servers that obey the SHOULD clauses, the
problem is gone regardless of the choice on the MAY.
6.3. Results: Client complexity issues
Consider the following situation:
o There are a set of clients C1 through Cn accessing servers S1
through Sm. Each server manages some significant number of
filesystems with the filesystem count L being significantly
greater than m.
o Each client Cx will access a subset of the servers and so will
have up to m clientid's, which we will call Cxy for server Sy.
o Now assume that for load-balancing or other operational reasons,
numbers of filesystems are migrated among the servers. As a
result, depending on how this handled, the number of clientids may
explode. See below.
Now look what will happen under various scenarios:
o We have previously (in Section 3.1.3) looked at this in case of
client following the non-uniform client-string model. In that
case, each client-server pair could have up to m clientid's and
each client will have up to m**2 clientids. If we add the
possibility of server reboot, the only bound on a client's
clientid count is L.
o If we look at this in the SHOULD-UF-CID case in which the SHOULD-
SVR_AM condition holds, the situation is no different. Although
the server has the client identity information that could enable
same-client-same-server leases to be combined, it does not do so.
We still have up to L clientid's per client.
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o On the other hand, if we look at the SHOULD-UF-CID case in which
SHOULD-SVR-AM holds, the problem is gone. There can be no more
than m clientids per client, and n clientid's per server.
The correctness signature for this issue is
(SHOULD-UF-CID & SHOULD-SVR-AM)
so if you have clients and servers that obey the SHOULD clauses, the
problem is gone regardless of the choice on the MAY.
6.4. Result summary
We have seen that (SHOULD-SVR-AM & SHOULD-UF-CID) are sufficient to
solve the problems people have experienced.
7. Security Considerations
The current definitive definition of the NFSv4.0 protocol [RFC3530],
and the current pending draft of RFC3530bis [cur-v4.0-bis] both
agree. The section entitled "Security Considerations" encourages
that clients protect the integrity of the SECINFO operation, any
GETATTR operation for the fs_locations attribute, and the operations
SETCLIENTID/SETCLIENTID_CONFIRM. A migration recovery event can use
any or all of these operations. We do not recommend any change here.
8. IANA Considerations
This document does not require actions by IANA.
9. Acknowledgements
The editor and authors of this document gratefully acknowledge the
contributions of Trond Myklebust of NetApp and Robert Thurlow of
Oracle. We also thank Tom Haynes of NetApp and Spencer Shepler of
Microsoft for their guidance and suggestions.
Special thanks go to members of the Oracle Solaris NFS team,
especially Rick Mesta and James Wahlig, for their work implementing
an NFSv4.0 migration prototype and identifying many of the issues
documented here.
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, April 2003.
10.2. Informative References
[RFC5661] Shepler, S., Eisler, M., and D. Noveck, "Network File
System (NFS) Version 4 Minor Version 1 Protocol",
RFC 5661, January 2010.
[cur-v4.0-bis]
Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", 2011, <http://www.ietf.org/id/
draft-ietf-nfsv4-rfc3530bis-16.txt>.
Work in progress.
Authors' Addresses
David Noveck (editor)
EMC Corporation
228 South Street
Hopkinton, MA 01748
US
Phone: +1 508 249 5748
Email: david.noveck@emc.com
Piyush Shivam
Oracle Corporation
5300 Riata Park Ct.
Austin, TX 78727
US
Phone: +1 512 401 1019
Email: piyush.shivam@oracle.com
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Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
US
Phone: +1 248 614 5091
Email: chuck.lever@oracle.com
Bill Baker
Oracle Corporation
5300 Riata Park Ct.
Austin, TX 78727
US
Phone: +1 512 401 1081
Email: bill.baker@oracle.com
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