MFC r275385 (by bapt):

[FreeBSD/stable/10.git] / contrib / subversion / subversion / libsvn_fs_fs / structure

This file describes the design, layouts, and file formats of a
libsvn_fs_fs repository.

Design
------

In FSFS, each committed revision is represented as an immutable file
containing the new node-revisions, contents, and changed-path
information for the revision, plus a second, changeable file
containing the revision properties.

In contrast to the BDB back end, the contents of recent revision of
files are stored as deltas against earlier revisions, instead of the
other way around.  This is less efficient for common-case checkouts,
but brings greater simplicity and robustness, as well as the
flexibility to make commits work without write access to existing
revisions.  Skip-deltas and delta combination mitigate the checkout
cost.

In-progress transactions are represented with a prototype rev file
containing only the new text representations of files (appended to as
changed file contents come in), along with a separate file for each
node-revision, directory representation, or property representation
which has been changed or added in the transaction.  During the final
stage of the commit, these separate files are marshalled onto the end
of the prototype rev file to form the immutable revision file.

Layout of the FS directory
--------------------------

The layout of the FS directory (the "db" subdirectory of the
repository) is:

  revs/               Subdirectory containing revs
    <shard>/          Shard directory, if sharding is in use (see below)
      <revnum>        File containing rev <revnum>
    <shard>.pack/     Pack directory, if the repo has been packed (see below)
      pack            Pack file, if the repository has been packed (see below)
      manifest        Pack manifest file, if a pack file exists (see below)
  revprops/           Subdirectory containing rev-props
    <shard>/          Shard directory, if sharding is in use (see below)
      <revnum>        File containing rev-props for <revnum>
    <shard>.pack/     Pack directory, if the repo has been packed (see below)
      <rev>.<count>   Pack file, if the repository has been packed (see below)
      manifest        Pack manifest file, if a pack file exists (see below)
    revprops.db       SQLite database of the packed revprops (format 5 only)
  transactions/       Subdirectory containing transactions
    <txnid>.txn/      Directory containing transaction <txnid>
  txn-protorevs/      Subdirectory containing transaction proto-revision files
    <txnid>.rev       Proto-revision file for transaction <txnid>
    <txnid>.rev-lock  Write lock for proto-rev file
  txn-current         File containing the next transaction key
  locks/              Subdirectory containing locks
    <partial-digest>/ Subdirectory named for first 3 letters of an MD5 digest
      <digest>        File containing locks/children for path with <digest>
  node-origins/       Lazy cache of origin noderevs for nodes
    <partial-nodeid>  File containing noderev ID of origins of nodes
  current             File specifying current revision and next node/copy id
  fs-type             File identifying this filesystem as an FSFS filesystem
  write-lock          Empty file, locked to serialise writers
  pack-lock           Empty file, locked to serialise 'svnadmin pack' (f. 7+)
  txn-current-lock    Empty file, locked to serialise 'txn-current'
  uuid                File containing the repository IDs
  format              File containing the format number of this filesystem
  fsfs.conf           Configuration file
  min-unpacked-rev    File containing the oldest revision not in a pack file
  min-unpacked-revprop Same for revision properties (format 5 only)
  rep-cache.db        SQLite database mapping rep checksums to locations

Files in the revprops directory are in the hash dump format used by
svn_hash_write.

The format of the "current" file is:

 * Format 3 and above: a single line of the form
   "<youngest-revision>\n" giving the youngest revision for the
   repository.

 * Format 2 and below: a single line of the form "<youngest-revision>
   <next-node-id> <next-copy-id>\n" giving the youngest revision, the
   next unique node-ID, and the next unique copy-ID for the
   repository.

The "write-lock" file is an empty file which is locked before the
final stage of a commit and unlocked after the new "current" file has
been moved into place to indicate that a new revision is present.  It
is also locked during a revprop propchange while the revprop file is
read in, mutated, and written out again.  Furthermore, it will be used
to serialize the repository structure changes during 'svnadmin pack'
(see also next section).  Note that readers are never blocked by any
operation - writers must ensure that the filesystem is always in a
consistent state.

The "pack-lock" file is an empty file which is locked before an 'svnadmin
pack' operation commences.  Thus, only one process may attempt to modify
the repository structure at a time while other processes may still read
and write (commit) to the repository during most of the pack procedure.
It is only available with format 7 and newer repositories.  Older formats
use the global write-lock instead which disables commits completely
for the duration of the pack process.

The "txn-current" file is a file with a single line of text that
contains only a base-36 number.  The current value will be used in the
next transaction name, along with the revision number the transaction
is based on.  This sequence number ensures that transaction names are
not reused, even if the transaction is aborted and a new transaction
based on the same revision is begun.  The only operation that FSFS
performs on this file is "get and increment"; the "txn-current-lock"
file is locked during this operation.

"fsfs.conf" is a configuration file in the standard Subversion/Python
config format.  It is automatically generated when you create a new
repository; read the generated file for details on what it controls.

When representation sharing is enabled, the filesystem tracks
representation checksum and location mappings using a SQLite database in
"rep-cache.db".  The database has a single table, which stores the sha1
hash text as the primary key, mapped to the representation revision, offset,
size and expanded size.  This file is only consulted during writes and never
during reads.  Consequently, it is not required, and may be removed at an
abritrary time, with the subsequent loss of rep-sharing capabilities for
revisions written thereafter.

Filesystem formats
------------------

The "format" file defines what features are permitted within the
filesystem, and indicates changes that are not backward-compatible.
It serves the same purpose as the repository file of the same name.

The filesystem format file was introduced in Subversion 1.2, and so
will not be present if the repository was created with an older
version of Subversion.  An absent format file should be interpreted as
indicating a format 1 filesystem.

The format file is a single line of the form "<format number>\n",
followed by any number of lines specifying 'format options' -
additional information about the filesystem's format.  Each format
option line is of the form "<option>\n" or "<option> <parameters>\n".

Clients should raise an error if they encounter an option not
permitted by the format number in use.

The formats are:

  Format 1, understood by Subversion 1.1+
  Format 2, understood by Subversion 1.4+
  Format 3, understood by Subversion 1.5+
  Format 4, understood by Subversion 1.6+
  Format 5, understood by Subversion 1.7-dev, never released
  Format 6, understood by Subversion 1.8
  Format 7, understood by Subversion 1.9

The differences between the formats are:

Delta representation in revision files
  Format 1: svndiff0 only
  Formats 2+: svndiff0 or svndiff1

Format options
  Formats 1-2: none permitted
  Format 3+:   "layout" option
  Format 7+:   "addressing" option

Transaction name reuse
  Formats 1-2: transaction names may be reused
  Format 3+:   transaction names generated using txn-current file

Location of proto-rev file and its lock
  Formats 1-2: transactions/<txnid>/rev and
    transactions/<txnid>/rev-lock.
  Format 3+:   txn-protorevs/<txnid>.rev and
    txn-protorevs/<txnid>.rev-lock.

Node-ID and copy-ID generation
  Formats 1-2: Node-IDs and copy-IDs are guaranteed to form a
    monotonically increasing base36 sequence using the "current"
    file.
  Format 3+:   Node-IDs and copy-IDs use the new revision number to
    ensure uniqueness and the "current" file just contains the
    youngest revision.

Mergeinfo metadata:
  Format 1-2: minfo-here and minfo-count node-revision fields are not
    stored.  svn_fs_get_mergeinfo returns an error.
  Format 3+:  minfo-here and minfo-count node-revision fields are
    maintained.  svn_fs_get_mergeinfo works.

Revision changed paths list:
  Format 1-3: Does not contain the node's kind.
  Format 4+:  Contains the node's kind.
  Format 7+:  Contains the mergeinfo-mod flag.

Shard packing:
  Format 4:   Applied to revision data only.
  Format 5:   Revprops would be packed independently of revision data.
  Format 6+:  Applied equally to revision data and revprop data
    (i.e. same min packed revision)

Addressing:
  Format 1-6: Physical addressing; uses fixed positions within a rev file
  Format 7+:  Logical addressing; uses item index that will be translated
    on-the-fly to the actual rev / pack file location

Repository IDs:
  Format 1+:  The first line of db/uuid contains the repository UUID
  Format 7+:  The second line contains the instance ID (in UUID formatting)

# Incomplete list.  See SVN_FS_FS__MIN_*_FORMAT


Filesystem format options
-------------------------

Currently, the only recognised format options are "layout" and "addressing".
The first specifies the paths that will be used to store the revision
files and revision property files.  The second specifies that logical to
physical address translation is required.

The "layout" option is followed by the name of the filesystem layout
and any required parameters.  The default layout, if no "layout"
keyword is specified, is the 'linear' layout.

The known layouts, and the parameters they require, are as follows:

"linear"
  Revision files and rev-prop files are named after the revision they
  represent, and are placed directly in the revs/ and revprops/
  directories.  r1234 will be represented by the revision file
  revs/1234 and the rev-prop file revprops/1234.

"sharded <max-files-per-directory>"
  Revision files and rev-prop files are named after the revision they
  represent, and are placed in a subdirectory of the revs/ and
  revprops/ directories named according to the 'shard' they belong to.

  Shards are numbered from zero and contain between one and the
  maximum number of files per directory specified in the layout's
  parameters.

  For the "sharded 1000" layout, r1234 will be represented by the
  revision file revs/1/1234 and rev-prop file revprops/1/1234.  The
  revs/0/ directory will contain revisions 0-999, revs/1/ will contain
  1000-1999, and so on.

The "addressing" option is followed by the name of the addressing mode
and any required parameters.  The default addressing, if no "addressing"
keyword is specified, is the 'physical' addressing.

The supported modes, and the parameters they require, are as follows:

"physical"
  All existing and future revision files will use the traditional
  physical addressing scheme.  All references are given as rev/offset
  pairs with "offset" being the byte offset relative to the beginning of
  the revision in the respective rev or pack file.

"logical"
  All existing and future revision files will use logical
  addressing. It is illegal to use logical addressing on non-sharded
  repositories.


Addressing modes
----------------

Two addressing modes are supported in format 7: physical and logical
addressing.  Both use the same address format but apply a different
interpretation to it.  Older formats only support physical addressing.

All items are addressed using <rev> <item_index> pairs.  In physical
addressing mode, item_index is the (ASCII decimal) number of bytes from
the start of the revision file to the start of the respective item.  For
non-packed files that is also the absolute file offset.  Revision pack
files simply concatenate multiple rev files, i.e. the absolute file offset
is determined as

  absolute offset = rev offset taken from manifest + item_index
  
This simple addressing scheme makes it hard to change the location of
any item since that may break references from later revisions.
  
Logical addressing uses an index file to translate the rev / item_index
pairs into absolute file offsets.  There is one such index for every rev /
pack file using logical addressing and both are created in sync.  That
makes it possible to reorder items during pack file creation, particularly
to mix items from different revisions.

Some item_index values are pre-defined and apply to every revision:

  0 ... not used / invalid
  1 ... changed path list
  2 ... root node revision

A reverse index (phys-to-log) is being created as well that allows for
translating arbitrary file locations into item descriptions (type, rev,
item_index, on-disk length).  Known item types

  0 ... unused / empty section
  1 ... file representation
  2 ... directory representation
  3 ... file property representation
  4 ... directory property representation
  5 ... node revision
  6 ... changed paths list

The various representation types all share the same morphology.  The
distinction is only made to allow for more effective reordering heuristics.
Zero-length items are allowed.


Packing revisions
-----------------

A filesystem can optionally be "packed" to conserve space on disk.  The
packing process concatenates all the revision files in each full shard to
create a pack file.  The original shard is removed, and reads are
redirected to the pack file.

With physical addressing, a manifest file is created for each shard which
records the indexes of the corresponding revision files in the pack file.
The manifest file consists of a list of offsets, one for each revision in
the pack file.  The offsets are stored as ASCII decimal, and separated by
a newline character.

Revision pack files using logical addressing don't use manifest files but
appends index data to the revision contents.  The revisions inside a pack
file will also get interleaved to reduce I/O for typical access patterns.
There is no structural difference between packed and non-packed revision
files in that mode.


Packing revision properties (format 5: SQLite)
---------------------------

This was supported by 1.7-dev builds but never included in a blessed release.

See r1143829 of this file:
http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_fs_fs/structure?view=markup&pathrev=1143829


Packing revision properties (format 6+)
---------------------------

Similarly to the revision data, packing will concatenate multiple
revprops into a single file.  Since they are mutable data, we put an
upper limit to the size of these files:  We will concatenate the data
up to the limit and then use a new file for the following revisions.

The limit can be set and changed at will in the configuration file. 
It is 64kB by default.  Because a pack file must contain at least one
complete property list, files containing just one revision may exceed
that limit.

Furthermore, pack files can be compressed which saves about 75% of
disk space.  A configuration file flag enables the compression; it is
off by default and may be switched on and off at will.  The pack size
limit is always applied to the uncompressed data.  For this reason,
the default is 256kB while compression has been enabled.

Files are named after their start revision as "<rev>.<counter>" where
counter will be increased whenever we rewrite a pack file due to a
revprop change.  The manifest file contains the list of pack file
names, one line for each revision.

Many tools track repository global data in revision properties at 
revision 0.  To minimize I/O overhead for those applications,  we
will never pack that revision, i.e. its data is always being kept
in revprops/0/0.

Pack file format

  Top level: <packed container>

  We always apply data compression to the pack file - using the
  SVN_DELTA_COMPRESSION_LEVEL_NONE level if compression is disabled.
  (Note that compression at SVN_DELTA_COMPRESSION_LEVEL_NONE is not
  a no-op stream transformation although most of the data will remain
  human readable.)

  container := header '\n' (revprops)+
  header    := start_rev '\n' rev_count '\n' (size '\n')+

  All numbers in the header are given as ASCII decimals.  rev_count
  is the number of revisions packed into this container.  There must
  be exactly as many "size" and serialized "revprops".  The "size"
  values in the list are the length in bytes of the serialized
  revprops of the respective revision.

Writing to packed revprops

  The old pack file is being read and the new revprops serialized.
  If they fit into the same pack file, a temp file with the new
  content gets written and moved into place just like an non-packed
  revprop file would. No name change or manifest update required.

  If they don't fit into the same pack file,  i.e. exceed the pack
  size limit,  the pack will be split into 2 or 3 new packs just
  before and / or after the modified revision.

  In the current implementation, they will never be merged again.
  To minimize fragmentation, the initial packing process will only
  use about 90% of the limit, i.e. leave some room for growth.

  When a pack file gets split, its counter is being increased
  creating a new file and leaving the old content in place and
  available for concurrent readers.  Only after the new manifest
  file got moved into place, will the old pack files be deleted. 

  Write access to revprops is being serialized by the global
  filesystem write lock.  We only need to build a few retries into
  the reader code to gracefully handle manifest changes and pack
  file deletions.


Node-revision IDs
-----------------

A node-rev ID consists of the following three fields:

    node_revision_id ::= node_id '.' copy_id '.' txn_id

At this level, the form of the ID is the same as for BDB - see the
section called "ID's" in <../libsvn_fs_base/notes/structure>.

In order to support efficient lookup of node-revisions by their IDs
and to simplify the allocation of fresh node-IDs during a transaction,
we treat the fields of a node-rev ID in new and interesting ways.

Within a new transaction:

  New node-revision IDs assigned within a transaction have a txn-id
  field of the form "t<txnid>".

  When a new node-id or copy-id is assigned in a transaction, the ID
  used is a "_" followed by a base36 number unique to the transaction.

Within a revision:

  Within a revision file, node-revs have a txn-id field of the form
  "r<rev>/<item_index>", to support easy lookup.  See addressing modes
  for details.

  During the final phase of a commit, node-revision IDs are rewritten
  to have repository-wide unique node-ID and copy-ID fields, and to have
  "r<rev>/<item_index>" txn-id fields.

  In Format 3 and above, this uniqueness is done by changing a temporary
  id of "_<base36>" to "<base36>-<rev>".  Note that this means that the
  originating revision of a line of history or a copy can be determined
  by looking at the node ID.

  In Format 2 and below, the "current" file contains global base36
  node-ID and copy-ID counters; during the commit, the counter value is
  added to the transaction-specific base36 ID, and the value in
  "current" is adjusted.

  (It is legal for Format 3 repositories to contain Format 2-style IDs;
  this just prevents I/O-less node-origin-rev lookup for those nodes.)

The temporary assignment of node-ID and copy-ID fields has
implications for svn_fs_compare_ids and svn_fs_check_related.  The ID
_1.0.t1 is not related to the ID _1.0.t2 even though they have the
same node-ID, because temporary node-IDs are restricted in scope to
the transactions they belong to.

There is a lazily created cache mapping from node-IDs to the full
node-revision ID where they are created.  This is in the node-origins
directory; the file name is the node-ID without its last character (or
"0" for single-character node IDs) and the contents is a serialized
hash mapping from node-ID to node-revision ID.  This cache is only
used for node-IDs of the pre-Format 3 style.

Copy-IDs and copy roots
-----------------------

Copy-IDs are assigned in the same manner as they are in the BDB
implementation:

  * A node-rev resulting from a creation operation (with no copy
    history) receives the copy-ID of its parent directory.

  * A node-rev resulting from a copy operation receives a fresh
    copy-ID, as one would expect.

  * A node-rev resulting from a modification operation receives a
    copy-ID depending on whether its predecessor derives from a
    copy operation or whether it derives from a creation operation
    with no intervening copies:

      - If the predecessor does not derive from a copy, the new
        node-rev receives the copy-ID of its parent directory.  If the
        node-rev is being modified through its created-path, this will
        be the same copy-ID as the predecessor node-rev has; however,
        if the node-rev is being modified through a copied ancestor
        directory (i.e. we are performing a "lazy copy"), this will be
        a different copy-ID.

      - If the predecessor derives from a copy and the node-rev is
        being modified through its created-path, the new node-rev
        receives the copy-ID of the predecessor.

      - If the predecessor derives from a copy and the node-rev is not
        being modified through its created path, the new node-rev
        receives a fresh copy-ID.  This is called a "soft copy"
        operation, as distinct from a "true copy" operation which was
        actually requested through the svn_fs interface.  Soft copies
        exist to ensure that the same <node-ID,copy-ID> pair is not
        used twice within a transaction.

Unlike the BDB implementation, we do not have a "copies" table.
Instead, each node-revision record contains a "copyroot" field
identifying the node-rev resulting from the true copy operation most
proximal to the node-rev.  If the node-rev does not itself derive from
a copy operation, then the copyroot field identifies the copy of an
ancestor directory; if no ancestor directories derive from a copy
operation, then the copyroot field identifies the root directory of
rev 0.

Revision file format
--------------------

A revision file contains a concatenation of various kinds of data:

  * Text and property representations
  * Node-revisions
  * The changed-path data
  * Index data (logical addressing only)
  * Revision / pack file footer (logical addressing only)

A representation begins with a line containing either "PLAIN\n" or
"DELTA\n" or "DELTA <rev> <item_index> <length>\n", where <rev>,
<item_index>, and <length> give the location of the delta base of the
representation and the amount of data it contains (not counting the header
or trailer).  If no base location is given for a delta, the base is the
empty stream.  After the initial line comes raw svndiff data, followed
by a cosmetic trailer "ENDREP\n".

If the representation is for the text contents of a directory node,
the expanded contents are in hash dump format mapping entry names to
"<type> <id>" pairs, where <type> is "file" or "dir" and <id> gives
the ID of the child node-rev.

If a representation is for a property list, the expanded contents are
in the form of a dumped hash map mapping property names to property
values.

The marshalling syntax for node-revs is a series of fields terminated
by a blank line.  Fields have the syntax "<name>: <value>\n", where
<name> is a symbolic field name (each symbolic name is used only once
in a given node-rev) and <value> is the value data.  Unrecognized
fields are ignored, for extensibility.  The following fields are
defined:

  id        The ID of the node-rev
  type      "file" or "dir"
  pred      The ID of the predecessor node-rev
  count     Count of node-revs since the base of the node
  text      "<rev> <item_index> <length> <size> <digest>" for text rep
  props     "<rev> <item_index> <length> <size> <digest>" for props rep
            <rev> and <item_index> give location of rep
            <length> gives length of rep, sans header and trailer
            <size> gives size of expanded rep (*)
            <digest> gives hex MD5 digest of expanded rep
            ### in formats >=4, also present:
            <sha1-digest> gives hex SHA1 digest of expanded rep
            <uniquifier> see representation_t->uniquifier in fs.h
  cpath     FS pathname node was created at
  copyfrom  "<rev> <path>" of copyfrom data
  copyroot  "<rev> <created-path>" of the root of this copy
  minfo-cnt The number of nodes under (and including) this node
             which have svn:mergeinfo.
  minfo-here Exists if this node itself has svn:mergeinfo.

(*) Earlier versions of this document would state that <size> may be 0
    if the actual value matches <length>.  This is only true for property
    and directory representations and should be avoided in general.  File
    representations may not be handled correctly by SVN before 1.7.20,
    1.8.12 and 1.9.0, if they have 0 <size> fields for non-empty contents.
    Releases 1.8.0 through 1.8.11 may have falsely created instances of
    that (see issue #4554).  Finally, 0 <size> fields are only ever legal
    for DELTA representations if the reconstructed full-text is actually
    empty.

The predecessor of a node-rev crosses both soft and true copies;
together with the count field, it allows efficient determination of
the base for skip-deltas.  The first node-rev of a node contains no
"pred" field.  A node-revision with no properties may omit the "props"
field.  A node-revision with no contents (a zero-length file or an
empty directory) may omit the "text" field.  In a node-revision
resulting from a true copy operation, the "copyfrom" field gives the
copyfrom data.  The "copyroot" field identifies the root node-revision
of the copy; it may be omitted if the node-rev is its own copy root
(as is the case for node-revs with copy history, and for the root node
of revision 0).  Copy roots are identified by revision and
created-path, not by node-rev ID, because a copy root may be a
node-rev which exists later on within the same revision file, meaning
its location is not yet known.

The changed-path data is represented as a series of changed-path
items, each consisting of two lines.  The first line has the format
"<id> <action> <text-mod> <prop-mod> <mergeinfo-mod> <path>\n",
where <id> is the node-rev ID of the new node-rev, <action> is "add",
"delete", "replace", or "modify", <text-mod>, <prop-mod>, and
<mergeinfo-mod>  are "true" or "false" indicating whether the text,
properties and/or mergeinfo changed, and <path> is the changed pathname.
For deletes, <id> is the node-rev ID of the deleted node-rev, and
<text-mod> and <prop-mod> are always "false".  The second line has the
format "<rev> <path>\n" containing the node-rev's copyfrom information
if it has any; if it does not, the second line is blank.

Starting with FS format 4, <action> may contain the kind ("file" or
"dir") of the node, after a hyphen; for example, an added directory
may be represented as "add-dir".

Prior to FS format 7, <mergeinfo-mod> flag is not available.  It may
also be missing in revisions upgraded from pre-f7 formats.

In physical addressing mode, at the very end of a rev file is a pair of
lines containing "\n<root-offset> <cp-offset>\n", where <root-offset> is
the offset of the root directory node revision and <cp-offset> is the
offset of the changed-path data.

In logical addressing mode, the revision footer has the form

  <l2p offset> <l2p checksum> <p2l offset> <p2l checksum><terminal byte>

The terminal byte contains the length (as plain 8 bit value) of the footer
excluding that length byte.  The first offset is the start of the log-to-
phys index, followed by the digest of the MD5 checksum over its content.
The other pair gives the same of for the phys-to-log index.

All numbers in the rev file format are unsigned and are represented as
ASCII decimal.

Transaction layout
------------------

A transaction directory has the following layout:

  props                      Transaction props
  props-final                Final transaction props (optional)
  next-ids                   Next temporary node-ID and copy-ID
  changes                    Changed-path information so far
  node.<nid>.<cid>           New node-rev data for node
  node.<nid>.<cid>.props     Props for new node-rev, if changed
  node.<nid>.<cid>.children  Directory contents for node-rev
  <sha1>                     Text representation of that sha1

In FS formats 1 and 2, it also contains:

  rev                        Prototype rev file with new text reps
  rev-lock                   Lockfile for writing to the above

(In newer formats, these files are in the txn-protorevs/ directory.)

In format 7+ logical addressing mode, it contains two additional index
files (see structure-indexes for a detailed description) and one more
counter file:

  itemidx                    Next item_index value as decimal integer
  index.l2p                  Log-to-phys proto-index
  index.p2l                  Phys-to-log proto-index

The prototype rev file is used to store the text representations as
they are received from the client.  To ensure that only one client is
writing to the file at a given time, the "rev-lock" file is locked for
the duration of each write.

The three kinds of props files are all in hash dump format.  The "props"
file will always be present.  The "node.<nid>.<cid>.props" file will
only be present if the node-rev properties have been changed.  The
"props-final" only exists while converting the transaction into a revision.


The <sha1> files have been introduced in FS format 6. Their content
is that of text rep references: "<rev> <item_offset> <length> <size> <digest>"
They will be written for text reps in the current transaction and be
used to eliminate duplicate reps within that transaction.

The "next-ids" file contains a single line "<next-temp-node-id>
<next-temp-copy-id>\n" giving the next temporary node-ID and copy-ID
assignments (without the leading underscores).  The next node-ID is
also used as a uniquifier for representations which may share the same
underlying rep.

The "children" file for a node-revision begins with a copy of the hash
dump representation of the directory entries from the old node-rev (or
a dump of the empty hash for new directories), and then an incremental
hash dump entry for each change made to the directory.

The "changes" file contains changed-path entries in the same form as
the changed-path entries in a rev file, except that <id> and <action>
may both be "reset" (in which case <text-mod> and <prop-mod> are both
always "false") to indicate that all changes to a path should be
considered undone.  Reset entries are only used during the final merge
phase of a transaction.  Actions in the "changes" file always contain
a node kind, even if the FS format is older than format 4.

The node-rev files have the same format as node-revs in a revision
file, except that the "text" and "props" fields are augmented as
follows:

  * The "props" field may have the value "-1" if properties have
    been changed and are contained in a "props" file within the
    node-rev subdirectory.

  * For directory node-revs, the "text" field may have the value
    "-1" if entries have been changed and are contained in a
    "contents" file in the node-rev subdirectory.

  * For the directory node-rev representing the root of the
    transaction, the "is-fresh-txn-root" field indicates that it has
    not been made mutable yet (see Issue #2608).

  * For file node-revs, the "text" field may have the value "-1
    <offset> <length> <size> <digest>" if the text representation is
    within the prototype rev file.

  * The "copyroot" field may have the value "-1 <created-path>" if the
    copy root of the node-rev is part of the transaction in process.

Locks layout
------------

Locks in FSFS are stored in serialized hash format in files whose
names are MD5 digests of the FS path which the lock is associated
with.  For the purposes of keeping directory inode usage down, these
digest files live in subdirectories of the main lock directory whose
names are the first 3 characters of the digest filename.

Also stored in the digest file for a given FS path are pointers to
other digest files which contain information associated with other FS
paths that are beneath our path (an immediate child thereof, or a
grandchild, or a great-grandchild, ...).

To answer the question, "Does path FOO have a lock associated with
it?", one need only generate the MD5 digest of FOO's
absolute-in-the-FS path (say, 3b1b011fed614a263986b5c4869604e8), look
for a file located like so:

   /path/to/repos/locks/3b1/3b1b011fed614a263986b5c4869604e8

And then see if that file contains lock information.

To inquire about locks on children of the path FOO, you would
reference the same path as above, but look for a list of children in
that file (instead of lock information).  Children are listed as MD5
digests, too, so you would simply iterate over those digests and
consult the files they reference for lock information.


Index Data
----------

Format 7 introduces logical addressing that requires item indexes
to be translated / mapped to physical rev / pack file offsets.
These indexes are appended to the respective rev / pack file.

Details of the binary format used by these index files can be
found in structure-indexes.