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In both the .git/annex directory and the git-annex branch, two levels of
hash directories are used, to avoid issues with too many files in one
directory.
Two separate hash methods are used.
* hashdirmixed is only used for non-bare git repositories.
(We'd like to stop using this, but it'd be too annoying to change
all the git-annex symlinks!)
* hashdirlower is used for bare git repositories, the
git-annex branch, and on special remotes as well.
Note that `git annex find` and `git annex examinekey` can be used with
the `--format` option to find the hash directories. The explanation
below is only for completeness.
## new hash format
This uses two directories, each with a three-letter name, such as "f87/4d5"
The directory names come from the md5sum of the [[key|key_format]].
For example:
echo -n "SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" | md5sum
## old hash format
This uses two directories, each with a two-letter name, such as "pX/1J"
It takes the md5sum of the key, but rather than a string, represents it as 4
32bit words. Only the first word is used. It is converted into a string by the
same mechanism that would be used to encode a normal md5sum value into a
string, but where that would normally encode the bits using the 16 characters
0-9a-f, this instead uses the 32 characters "0123456789zqjxkmvwgpfZQJXKMVWGPF".
The first 2 letters of the resulting string are the first directory, and the
second 2 are the second directory.
## chunk keys
The same hash directory is used for a chunk key as would be used for the
key that it's a chunk of.
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