Commit 2aef5bba authored by David Greaves's avatar David Greaves Committed by Linus Torvalds

[PATCH] Docs - delta object

Added delta documentation
Signed-off-by: David Greaves's avatarDavid Greaves <[email protected]>
Signed-off-by: default avatarLinus Torvalds <[email protected]>
parent 7096a645
......@@ -9,7 +9,7 @@ git-fsck-cache - Verifies the connectivity and validity of the objects in the da
SYNOPSIS
--------
'git-fsck-cache' [--tags] [--root] [[--unreachable] [--cache] <object>\*]
'git-fsck-cache' [--tags] [--root] [--delta-depth] [[--unreachable] [--cache] <object>\*]
DESCRIPTION
-----------
......@@ -34,6 +34,9 @@ OPTIONS
Consider any object recorded in the cache also as a head node for
an unreachability trace.
--delta-depth::
Report back the length of the longest delta chain found.
It tests SHA1 and general object sanity, and it does full tracking of
the resulting reachability and everything else. It prints out any
corruption it finds (missing or bad objects), and if you use the
......
git-mkdelta(1)
==============
May 2005
NAME
----
git-mkdelta - Creates a delta object
SYNOPSIS
--------
'git-mkdelta' [-v] [-d N | --max-depth=N ] <reference_object> <target_object> [ <next_object> ... ]
DESCRIPTION
-----------
Creates a delta object to replace <reference_object> by using an
ordered list of potential objects to deltafy against earlier objects
in the list.
A cap on the depth of delta references can be provided as well,
otherwise the default is to not have any limit. A limit of 0 will
also undeltafy a given object.
OPTIONS
-------
-v::
Verbose
-d|--max-depth::
limit the number of delta references in a chain
If 0 then all objects are undeltafied.
Author
------
Git is written by Linus Torvalds <[email protected]> and the git-list <[email protected]>.
Documentation
--------------
Documentation by David Greaves, Junio C Hamano and the git-list <[email protected]>.
GIT
---
Part of the link:git.html[git] suite
......@@ -51,6 +51,9 @@ link:git-init-db.html[git-init-db]::
link:git-merge-base.html[git-merge-base]::
Finds as good a common ancestor as possible for a merge
link:git-mkdelta.html[git-mkdelta]::
Creates a delta object
link:git-mktag.html[git-mktag]::
Creates a tag object
......
......@@ -32,8 +32,8 @@ build up a hierarchy of objects.
All objects have a statically determined "type" aka "tag", which is
determined at object creation time, and which identifies the format of
the object (i.e. how it is used, and how it can refer to other
objects). There are currently four different object types: "blob",
"tree", "commit" and "tag".
objects). There are currently five different object types: "blob",
"tree", "commit", "tag" and "delta"
A "blob" object cannot refer to any other object, and is, like the tag
implies, a pure storage object containing some user data. It is used to
......@@ -62,13 +62,17 @@ A "tag" object symbolically identifies and can be used to sign other
objects. It contains the identifier and type of another object, a
symbolic name (of course!) and, optionally, a signature.
Regardless of object type, all objects are share the following
characteristics: they are all in deflated with zlib, and have a header
that not only specifies their tag, but also size information about the
data in the object. It's worth noting that the SHA1 hash that is used
to name the object is the hash of the original data (historical note:
in the dawn of the age of git this was the sha1 of the _compressed_
object)
A "delta" object is used internally by the object database to minimise
disk usage. Instead of storing the entire contents of a revision, git
can behave in a similar manner to RCS et al and simply store a delta.
Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
that not only specifies their tag, but also provides size information
about the data in the object. It's worth noting that the SHA1 hash
that is used to name the object is the hash of the original data or
the delta. (Historical note: in the dawn of the age of git the hash
was the sha1 of the _compressed_ object)
As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
......@@ -215,6 +219,30 @@ verification) has to come from outside.
A tag is created with link:git-mktag.html[git-mktag] and
it's data can be accessed by link:git-cat-file.html[git-cat-file]
Delta Object
~~~~~~~~~~~~
The "delta" object is used internally by the object database to
minimise storage usage by using xdeltas (byte level diffs). Deltas can
form chains of arbitrary length as RCS does (although this is
configureable at creation time). Most operations won't see or even be
aware of delta objects as they are automatically 'applied' and appear
as 'real' git objects In other words, if you write your own routines
to look at the contents of the object database then you need to know
about this - otherwise you don't. Actually, that's not quite true -
one important area where deltas are likely to prove very valuable is
in reducing bandwidth loads - so the more sophisticated network tools
for git repositories will be aware of them too.
Finally, git repositories can (and must) be deltafied in the
background - the work to calculate the differences does not take place
automatically at commit time.
A delta can be created (or undeltafied) with
link:git-mkdelta.html[git-mkdelta] it's raw data cannot be accessed at
present.
The "index" aka "Current Directory Cache"
-----------------------------------------
The index is a simple binary file, which contains an efficient
......
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