Internet DRAFT - draft-keiser-afs3-directory-object
draft-keiser-afs3-directory-object
Network Working Group T. Keiser
Internet-Draft Sine Nomine
Intended status: Informational April 23, 2012
Expires: October 25, 2012
AFS-3 Directory Object Type Definition
draft-keiser-afs3-directory-object-00
Abstract
Directory lookups in the AFS-3 distributed file system are supported
by defining a canonical encoding for a directory object, and
transmitting all--or part--of that object from a file server to its
clients so that clients may resolve paths into AFS file IDs (FIDs).
This memo describes the AFS-3 directory object wire encoding.
Internet Draft Comments
Comments regarding this draft are solicited. Please include the
AFS-3 protocol standardization mailing list
(afs3-standardization@openafs.org) as a recipient of any comments.
Status of this Memo
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This Internet-Draft will expire on October 25, 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Directory Object Structure . . . . . . . . . . . . . . . . . . 5
5. Page Structure . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Record Index . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Page header . . . . . . . . . . . . . . . . . . . . . . . 6
5.2.1. Allocation Bitmap . . . . . . . . . . . . . . . . . . 7
6. Page N=0 structure . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Directory Header . . . . . . . . . . . . . . . . . . . . . 9
7. Page N>0 structure . . . . . . . . . . . . . . . . . . . . . . 10
8. Directory Entry . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Directory Entry Record . . . . . . . . . . . . . . . . . . 11
8.2. Directory Entry Extension Record . . . . . . . . . . . . . 12
9. Name Hash Function . . . . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. AFS Assign Numbers Registrar Considerations . . . . . . . . . 13
12. Security Considerations . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
13.1. Normative References . . . . . . . . . . . . . . . . . . . 14
13.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Example Directory Object . . . . . . . . . . . . . . 14
A.1. 18-character name string . . . . . . . . . . . . . . . . . 15
Appendix B. C Implementation of Directory Hash Function . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
AFS-3 [AFS1] [AFS2] is a distributed file system that has its origins
in the VICE project [CMU-ITC-84-020] [VICE1] at the Carnegie Mellon
University Information Technology Center [CMU-ITC-83-025], a joint
venture between CMU and IBM. VICE later became AFS when CMU moved
development to a new commercial venture called Transarc Corporation,
which later became IBM Pittsburgh Labs. AFS-3 is a suite of un-
standardized network protocols based on a remote procedure call (RPC)
suite known as Rx [AFS3-RX]. While de jure standards for AFS-3 fail
to exist, the various AFS-3 implementations have agreed upon certain
de facto standards, largely helped by the existence of an open source
fork called OpenAFS that has served the role of reference
implementation. In addition to using OpenAFS as a reference, IBM
wrote and donated developer documentation that contains somewhat
outdated specifications for the Rx protocol and all AFS-3 remote
procedure calls, as well as a detailed description of the AFS-3
system architecture.
Unlike most classical network file systems, AFS-3 explicitly eschews
remote procedure calls to facilitate file server-assisted directory
lookup operations. This was a conscious decision meant to limit
server load by placing lookup operations on the clients. In the
common cases, where there is significant locality of reference to
directory entries, this results in a substantial reduction in server
load (especially given the AFS-3 cache coherence model). It should
be noted that 23 years of empirical evidence have borne out this
decision as useful for many general-purpose workloads, while
disadvantageous for certain very specific workloads (e.g., large
directory objects with extremely non-uniform directory entry
reference distributions--where the server overhead of a lookup rpc
would would inconsequential compared to the directory file transfer
overhead of the existing model).
Due to the distributed nature of AFS-3 directory objects, a canonical
directory wire-format is an intrinsic part of the AFS-3 protocol.
This memo documents the directory object wire format; a future
document will document the lookup and modification algorithms, which
by the decentralized nature of AFS-3 directories, must be implemented
in the to-be-specified manner.
1.1. Abbreviations
AFS - Historically, AFS stood for the Andrew File System; AFS no
longer stands for anything
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RPC - Remote Procedure Call
Rx - The Remote Procedure Call mechanism utilized by AFS-3
XDR - eXternal Data Representation
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 RFC 2119 [RFC2119].
3. Constants
AFS_PAGESIZE = 2048
the size of each page in an AFS-3 directory object (in octets)
MAXPAGES = 128
the maximum number of pages in a legacy directory object
BIGMAXPAGES = 1023
the maximum number of pages in a new (circa 1988) directory
object
NHASHENT = 128
number of hash buckets in the entry name hash table
RECSIZE = 32
number of octets in a record
LRECSIZE = 5
base-2 logarithm of RECSIZE
EPP = 64
number of records per page
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LEPP = 6
base-2 logarithm of EPP
DHE = 12
number of records taken up in page 0 by the directory header
4. Directory Object Structure
An AFS-3 directory object consists of between 1 and BIGMAXPAGES
(1023) "pages" of length AFS_PAGESIZE (2048 octet).
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| page #0000 |
~ (2048 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| page #1022 |
~ (2048 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Structure
5. Page Structure
All pages in a directory object are AFS_PAGESIZE (2048 octets) in
length. All pages are subdivided into EPP (64) records, each RECSIZE
(32 octets) in length.
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(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| record #00 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| record #63 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Page Structure
5.1. Record Index
Each record within a directory object is referenced by an index
number. This number represents the offset from the start of the
file, in units of records, i.e., in multiples of RECSIZE. A record
index of 0 would thus point to record 0 in page 0, and a record index
of 67 would point to record 3 in page 1. Computing the file offset
from an index is simply a matter of left logical shifting the index
value by LRECSIZE (5) bits. Conversely, computing the index from a
file offset merely involves a right logical shift by LRECSIZE (5)
bits.
5.2. Page header
Each 2048-octet page within an AFS-3 directory object contains a 32-
octet (RECSIZE) header at offset 0 in the following form:
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(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pgcount | tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved | |
+-+-+-+-+-+-+-+-+ +
| allocation bitmap |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ +
| reserved |
~ (19 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Page Header
pgcount: 16 bits (unsigned integer)
This field only holds meaning for page 0. For the modern (post-
1988) directory format, this field holds the count of valid
directory pages (in network byte order) within the directory
object. When this value is zero, it denotes a legacy directory
object. Legacy directory objects are considered a deprecated and
historical vestige, and thus their format is not described in
this memo.
tag: 16 bits (unsigned integer)
This field MUST contain the magic value 1234 in network byte
order.
allocation bitmap: 8 octets (bit string)
The allocation bitmap is a bit field which contains one bit per
record slot within the page. The bit field is thus 8 (EPP/8)
octets in length. A bit value of zero indicates the entry is
free; a value of 1 indicates the entry has been allocated. The
allocation bitmap will be discussed further in Section 5.2.1.
5.2.1. Allocation Bitmap
The allocation bitmap contains one bit per record. The least
significant bit of the first octet within the bitmap references the
page header object (which is stored at offset 0). The second least
significant bit of the first octet within the bitmap references the
record index 1 (offset 32 octets into the page), and so on and so
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forth...until the most significant bit of the eighth octet of the
allocation bitmap references the 64th--and final--record of the page
(record 63 at page offset 2016).
The following invariants hold:
page_entry_offset = page_entry_index << LRECSIZE
alloc_bitmap_index = page_entry_index >> 3
alloc_bitmap_bit_num = page_entry_index & 0x7
alloc_bitmap_bit = 1 << alloc_bitmap_bit_num
The variable page_entry_index, which is an unsigned integer between 0
and 63, can be derived from the record index (Section 5.1) by bit-
wise ANDing it with EPP-1.
The equations above are written in C pseudocode--the variables are
all assumed to be unsigned integers, and the operators are assumed to
be identical to the ANSI C standard.
6. Page N=0 structure
Page 0 is special due to the directory header. It is structured as
follows:
1. a page header (see Section 5.2)
2. a directory header (see Section 6.1)
3. 51 data records (EPP-DHE-1=51)
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(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| page header |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| directory header |
~ (384 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data record #13 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data record #63 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Page N=0 Structure
6.1. Directory Header
The directory header structure is a 384-octet structure that is
stored at an offset of 32 octets from the beginning of the directory
object (i.e., directly following the page 0 page header--see
Section 5.2). The directory header layout is as follows:
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| page 0 map | page 1 map | page 2 map | page 3 map |
~ ~
| page 124 map | page 125 map | page 126 map | page 127 map |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hash chain 0 | hash chain 1 |
~ ~
| hash chain 126 | hash chain 127 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Header
page map:
Each of the MAXPAGES page map fields corresponds to one of the
MAXPAGES pages in a legacy directory object. Each field contains
the count of available entries in this directory page. A value
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of EPP indicates that this page has not yet been allocated (i.e.,
the page header has not been initialized).
hash chain:
Each of the NHASHENT hash chain fields contains a record index
for the first directory entry name that hashes to this bucket
(see Section 5.1, and see Section 9 for a description of the hash
function)
7. Page N>0 structure
Each page N>0 is structured as follows:
1. a page header (see Section 5.2)
2. EPP-1 (63) data records
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data record #01 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data record #63 |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Page N>0 Structure
8. Directory Entry
Since entry names are of variable length, directory entries are
structured as follows:
o a directory entry record (see Section 8.1)
o zero or more directory entry extension record(s) (see Section 8.2)
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This sequence of records MUST be contiguous, and MUST NOT cross a
directory page boundary.
8.1. Directory Entry Record
A directory entry record contains the dirent entry metadata (i.e.,
the vnode number and uniquifier, the name hash table next pointer,
flag bits, and the first twenty octets of the entry name string. Its
layout is as follows:
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | reserved | next |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| vnode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| uniquifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name |
~ (20 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Entry Record
flags: 8 bits
This is an 8-bit wide bit vector. Currently, only one bit is
defined: FFIRST (0x1). This flag bit is asserted in every record
of this type. There are vestiges in the OpenAFS implementation
suggesting that the original intention was to have a second flag
bit FNEXT (0x2)--being asserted in the first octet of extension
records--which would allow parsers to distinguish between entry
records and extension records. However, this idea was never
implemented (in fact, the concept of a flags field in an
extension record does not exist), and thus this flag bit has no
meaning. It should be noted that some tools have attempted to
use this bit to distinguish between entry records and entry
extension records (under the assumption that 0x1 is not a valid
ASCII value for a file name string). This technique is obviously
error-prone, and thus SHOULD NOT be relied upon.
next: 16 bits (unsigned integer)
This field contains a record index (see Section 5.1) pointing to
the next directory entry record on this name hash chain. A value
of zero indicates this is the last record on this chain. This
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entry MUST point to a record index which contains a data record
of the directory entry type (see Section 8).
vnode: 32 bits (unsigned integer)
This field contains the target vnode number for this directory
entry. Together with the uniquifier field, this forms the
equivalent of an inode number in the directory entry of a typical
on-disk Unix file system.
uniquifier: 32 bits (unsigned integer)
This field contains the target uniquifier for this directory
entry. Together with the vnode field, this forms the equivalent
of an inode number in the directory entry of a typical on-disk
Unix file system.
name: 20 octets (null-terminated string)
The first 20 octets of the directory entry's name string are
contained in this field. If the name string (including null
terminator) exceeds 20 octets, then this string will continue in
a sequence of one or more directory entry extension records (see
Section 8.2) that MUST directly proceed this record.
8.2. Directory Entry Extension Record
When a file name string exceeds the 20 octets set aside in an entry
record, one or more extension records MUST be allocated contiguously
following the base entry record in order to contain the rest of the
name string. The layout of an extension record is as follows:
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Directory Entry Extension Record
name: 32 octets (string)
This field contains part of an extended directory entry name
string. If this is the last directory entry extension record,
then this field MUST contain a null terminator character (octet
value zero) within its 32 octets.
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9. Name Hash Function
The hash function is a loop over each of the octets within the name
string. The hash is computed using integer arithmetic on an unsigned
32-bit integer. The hash MUST be initialized to zero before
commencing iteration over the characters in the name string. For
each character, the hash value is multiplied by the constant 173, and
then the value of the current character is added to the hash. When
the null terminator is encountered, the loop is terminated before the
hash is multiplied by 173.
For reasons unknown to the author, the resultant unsigned hash value
is then compared against the value 2^31. If the hash value is less
than 2^31 (i.e., what would be the sign bit--if the hash value were
signed-- is not asserted), then the resultant hash value will be the
value computed in the above loop bitwise ANDed with the constant
NHASHENT-1 (127). However, if the hash value is greater than or
equal to 2^31, then the resultant hash value will first be bitwise
anded with the constant NHASHENT-1 (127), and then the value will be
subtracted from the constant NHASHENT (128) to yield the final hash
value.
10. IANA Considerations
This memo includes no request to IANA.
11. AFS Assign Numbers Registrar Considerations
This memo includes no request to the AFS Assigned Numbers Registrar.
12. Security Considerations
Directory metadata can contain sensistive information. This memo
merely specifies the wire format encoding. Any implementation which
may be utilized to store and retrieve directories containing entries
whose name strings might reveal sensitive information should take
precautions to ensure that they are never transmitted in the clear,
and should take steps to ensure that those entries are not cached on
machines lacking appropriate physical and network security.
13. References
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13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13.2. Informative References
[AFS1] Howard, J., "An Overview of the Andrew File System"",
Proc. 1988 USENIX Winter Tech. Conf. pp. 23-26,
February 1988.
[AFS2] Howard, J., Kazar, M., Menees, S., Nichols, D.,
Satyanarayanan, M., Sidebotham, R., and M. West, "Scale
and Performance in a Distributed File System", ACM Trans.
Comp. Sys. Vol. 6, No. 1, pp. 51-81, February 1988.
[AFS3-RX] Zayas, E., "AFS-3 Programmer's Reference: Specification
for the Rx Remote Procedure Call Facility", Transarc Corp.
Tech. Rep. FS-00-D164, August 1991.
[CMU-ITC-83-025]
Morris, J., Van Houweling, D., and K. Slack, "The
Information Technology Center", CMU ITC Tech. Rep. CMU-
ITC-83-025, 1983.
[CMU-ITC-84-020]
West, M., "VICE File System Services", CMU ITC Tech.
Rep. CMU-ITC-84-020, August 1984.
[VICE1] Satyanarayanan, M., Howard, J., Nichols, D., Sidebotham,
R., Spector, A., and M. West, "The ITC Distributed File
System: Principles and Design", Proc. 10th ACM Symp.
Operating Sys. Princ. Vol. 19, No. 5, December 1985.
Appendix A. Example Directory Object
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A.1. 18-character name string
(MSB) (LSB)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pgcount=1 | tag=1234 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved |1| 0xfff |1 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| 0... |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ +
| reserved |
~ (19 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| map[0]=49 | |
+-+-+-+-+-+-+-+-+ +
| map[1...127]=0x40 0x40 0x40 ... |
~ (127 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hash[0...127]=0x0000 0x0000 ... |
~ (256 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags=0x1 | reserved | next=0x0000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| vnode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| uniquifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name="iamexactly018chars" |
~ (18 octets) ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | nul | ? |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| allocated extent record full of garbage |
~ (32 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unallocated garbage |
~ (1568 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
18-Character Name String
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Appendix B. C Implementation of Directory Hash Function
#define NHASHENT 128
unsigned int
dir_name_hash(const char * name)
{
unsigned int hash = 0;
while(*name++) {
hash *= 173;
hash += *name;
}
if (hash & 0x80000000) {
return NHASHENT - (hash & (NHASHENT-1));
} else {
return hash & (NHASHENT-1);
}
}
Figure 1
Author's Address
Thomas Keiser
Sine Nomine Associates
43596 Blacksmith Square
Ashburn, VA 20147
USA
Phone: +1 703 723 6673
Email: tkeiser@sinenomine.net
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