Internet DRAFT - draft-robinson-curstateid
draft-robinson-curstateid
NFSv4 D. Robinson
Internet-Draft Sun
Expires: January 14, 2006 July 13, 2005
NFSv4 current stateid
draft-robinson-curstateid-00
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Copyright (C) The Internet Society (2005).
Abstract
The semantics of the NFSv4 server are changed to include tracking the
value of current stateid in addition to the existing current
filehandle. This will allow the stateid generated by one operation
to be used by a subsequent operation without incurring an addition
round trip operation.
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1. Introduction
The NFSv4 COMPOUND operation introduced the concept of a current
filehandle to allow an operation that retrieve or creates a
filehandle to pass its value to a subsequent operation. In addition
it also created operations to allow the saving and restoring of the
current file handle. This has the benefit of both simplifying the
individual operations as well as removing unnecessary round trips
just to communicate the previous result to the next operation.
One deficiency in the NFSv4 COMPOUND model is that only the
filehandle may be passed from one operation to another. While a
complex scheme to pass any result as arguments to any other
operation's arguments is possible to be constructed, the most notable
state that cannot be passed is the stateid. During the initial
creation of the NFSv4 protocol, it was imagined that COMPOUND could
be used for optimizations such as opening, reading, and closing a
file all in a single RPC. Without the ability to pass the stateid
between operations this cannot be done without using a special
stateid that may potentially circumvent file record locking.
This proposal adds to the server's COMPOUND state a "current stateid"
and a "saved stateid" which are passed between operations. There are
no changes to the syntax of the protocol, only changes to the
semantics of a few operations in a manner that is mostly compatible
with the existing implementations.
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2. Current Stateid
A "current stateid" is the stateid that is associated with the
current filehandle. This proposal introduces no new operations that
independently manipulate the current stateid, it may only be
retrieved or modified using existing operations.
Any NFSv4 operation that modifies the current filehandle MUST also
modify the current stateid. Those operations that also return a
stateid MUST set the current stateid to that value. Those operations
that do not return a stateid MUST set the current stateid to the
special all-zeros stateid. As an example, PUTFH will change the
current server state from {ocfh, osid} to {cfh, 0} while LOCK will
change the current state from {cfh, osid} to {cfh, nsid}. The SAVEFH
and RESTOREFH operations will save and restore both the filehandle
and the stateid as a set.
Any NFSv4 operation that takes as an argument a stateid that is not
the special all-zeros stateid MUST set the current stateid to that
value before evaluating the operation. If the argument is the
special all-zeros stateid, the operation is evaluated using the
current stateid.
Examples of current stateid usage.
PUTFH fh1 - -> {fh1, 0}
READ sid1,0,1024 {fh1, sid1} -> {fh1, sid1}
PUTFH fh1 - -> {fh1, 0}
OPEN R,sid1,"foo" {fh1, sid1} -> {fh2, sid2}
READ 0,0,1024 {fh2, sid2} -> {fh2, sid2}
CLOSE 0 {fh2, sid2} -> {fh2, sid3}
PUTFH fh1 - -> {fh1, 0}
LOCK W,0,1024,sid1 {fh1, sid1} -> {fh1, sid2}
READ 0,0,1024 {fh1, sid2} -> {fh1, sid2}
LOCKU W,0,1024,0 {fh1, sid2} -> {fh1, sid3}
Figure 1
In Figure 1 the first example is the common case of a simple READ
operation with a supplied stateid showing that the PUTFH initializes
the current stateid to zero. The subsequent READ with stateid sid1
replaces the current stateid before evaluating the operation.
The second example performs an OPEN with the client provided stateid
sid1 and as a result generates stateid sid2. The next operation
specifies the READ with the special all-zero stateid but the current
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stateid set by the previous operation is actually used when the
operation is evaluated, allowing correct interaction with any
existing, potentially conflicting, locks.
The third example is similar to the second in how it passes the
stateid sid2 generated by the LOCK operation to the next READ
operation. This allows the client to explicitly surround a single
I/O operation with a lock and its appropriate stateid to guarantee
correctness with other client locks.
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3. Discussion
The current stateid is only used if the client has specified as an
argument to an operation a special stateid that is all-zeros. Most
current NFSv4 clients do not use the special stateids and as a
result, will see no semantic difference when the current stateid is
introduced. For those clients that do use the special stateid, most
will be doing so in situations that the current stateid is designed
to address, eliminating an extra round trip just to supply the
stateid. Using the current stateid will almost always be
semantically correct.
There may be a rare case where the client's use of a special stateid
is used with the intent of bypassing the actual state that client has
established, but this is not expected to be significant.
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4. IANA Considerations
This document has no actions for IANA.
5. Normative References
[1] Shepler, et al., S., "NFS Version 4 Protocol", RFC 3530,
April 2003.
Author's Address
David Robinson
Sun Microsystems, Inc.
5300 Riata Park Court
UAUS08-102
Austin, TX 78727
US
Phone: +1 512 401 1757
Email: David.Robinson@sun.com
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