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doc/repo: Describe object types

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Colin Walters 2013-08-23 17:21:31 -04:00
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<chapter id="repository-intro"> <chapter id="repository-intro">
<title>Core object types and data model</title> <title>Core object types and data model</title>
<para> <para>
OSTree is deeply inspired by git. It is worth taking some time OSTree is deeply inspired by git; like git, OSTree is a
userspace versioning filesystem. It is worth taking some time
to familiarize yourself with <ulink to familiarize yourself with <ulink
url="http://git-scm.com/book/en/Git-Internals">Git url="http://git-scm.com/book/en/Git-Internals">Git
Internals</ulink>, as this section will assume some knowledge of how Internals</ulink>, as this section will assume some knowledge of
git works. how git works.
</para> </para>
<para> <para>
Like git, OSTree is a userspace versioning filesystem. Its Its object types are similar to git; it has commit objects and
object types are similar to git; it has commit objects and
content objects. Git has "tree" objects, whereas OSTree splits content objects. Git has "tree" objects, whereas OSTree splits
them into "dirtree" and "dirmeta" objects. But unlike git, them into "dirtree" and "dirmeta" objects. But unlike git,
OSTree's checksums are SHA256. Its content objects include uid, OSTree's checksums are SHA256. And most crucially, its content
gid, and extended attributes (but still no timestamps). objects include uid, gid, and extended attributes (but still no
timestamps).
</para> </para>
<para> <simplesect id="commits">
Also like git, each commit in OSTree can have a parent. It is <title>Commit objects</title>
designed to store a history of your binary builds, just like git <para>
stores a history of source control. However, OSTree also makes A commit object contains metadata such as a timestamp, a log
it easy to delete data, under the assumption that you can message, and most importantly, a reference to a
regenerate it from source code. dirtree/dirmeta pair of checksums which describe the root
</para> directory of the filesystem.
</para>
<para>
Also like git, each commit in OSTree can have a parent. It is
designed to store a history of your binary builds, just like git
stores a history of source control. However, OSTree also makes
it easy to delete data, under the assumption that you can
regenerate it from source code.
</para>
</simplesect>
<simplesect id="dirtree">
<title>Dirtree objects</title>
<para>
A dirtree contains a sorted array of (filename, checksum)
pairs for content objects, and a second sorted array of
(filename, dirtree checksum, dirmeta checksum), which are
subdirectories.
</para>
</simplesect>
<simplesect id="dirmeta">
<title>Dirmeta objects</title>
<para>
In git, tree objects contain the metadata such as permissions
for their children. But OSTree splits this into a separate
object to avoid duplicating extended attribute listings.
</para>
</simplesect>
<simplesect id="content">
<title>Content objects</title>
<para>
Unlike the first three object types which are metadata,
designed to be <literal>mmap()ed</literal>, the content object
has a separate internal header and payload sections. The
header contains uid, gid, mode, and symbolic link target (for
symlinks), as well as extended attributes. After the header,
for regular files, the content follows.
</para>
</simplesect>
</chapter> </chapter>
<chapter id="repository-types"> <chapter id="repository-types">