git-annex/doc/internals.mdwn

In the world of git, we're not scared about internal implementation
details, and sometimes we like to dive in and tweak things by hand. Here's
some documentation to that end.

## `.git/annex/objects/aa/bb/*/*`

This is where locally available file contents are actually stored.
Files added to the annex get a symlink checked into git that points
to the file content.

First there are two levels of directories used for hashing, to prevent
too many things ending up in any one directory.

Each subdirectory has the name of a key in one of the
[[key-value_backends|backends]]. The file inside also has the name of the key.
This two-level structure is used because it allows the write bit to be removed
from the subdirectories as well as from the files. That prevents accidentially
deleting or changing the file contents.

## `.git-annex/uuid.log`

Records the UUIDs of known repositories, and associates them with a
description of the repository. This allows git-annex to display something
more useful than a UUID when it refers to a repository that does not have
a configured git remote pointing at it.

The file format is simply one line per repository, with the uuid followed by a
space and then the description through to the end of the line. Example:

	e605dca6-446a-11e0-8b2a-002170d25c55 laptop
	26339d22-446b-11e0-9101-002170d25c55 usb disk

## `git-annex/remotes.log`

Holds persistent configuration settings for [[special_remotes]] such as
Amazon S3.

The file format is one line per remote, starting with the uuid of the
remote, followed by a space, and then a series of key=value pairs,
each separated by whitespace.

## `.git-annex/trust.log`

Records the [[trust]] information for repositories. Does not exist unless
[[trust]] values are configured.

The file format is one line per repository, with the uuid followed by a
space, and then either 1 (trusted), 0 (untrusted), or ? (semi-trusted).
Repositories not listed are semi-trusted.

Example:

	e605dca6-446a-11e0-8b2a-002170d25c55 1
	26339d22-446b-11e0-9101-002170d25c55 ?

## `.git-annex/aaa/bbb/*.log`

The remainder of the log files record [[location_tracking]] information
for file contents. Again these are placed in two levels of subdirectories
for hashing. The name of the key is the filename, and the content
consists of a timestamp, either 1 (present) or 0 (not present), and
the UUID of the repository that has or lacks the file content.

Example:

	1287290776.765152s 1 e605dca6-446a-11e0-8b2a-002170d25c55
	1287290767.478634s 0 26339d22-446b-11e0-9101-002170d25c55

These files are designed to be auto-merged using git's union merge driver.
The timestamps allow the most recent information to be identified.