gitformat-loose(5) Manual Page

Loose objects are how Git stores individual objects, where every object is written as a separate file.

Over the lifetime of a repository, objects are usually written as loose objects initially. Eventually, these loose objects will be compacted into packfiles via repository maintenance to improve disk space usage and speed up the lookup of these objects.

Each loose object contains a prefix, followed immediately by the data of the object. The prefix contains <type> <size>\0. <type> is one of blob, tree, commit, or tag and size is the size of the data (without the prefix) as a decimal integer expressed in ASCII.

The entire contents, prefix and data concatenated, is then compressed with zlib and the compressed data is stored in the file. The object ID of the object is the SHA-1 or SHA-256 (as appropriate) hash of the uncompressed data.

The file for the loose object is stored under the objects directory, with the first two hex characters of the object ID being the directory and the remaining characters being the file name. This is done to shard the data and avoid too many files being in one directory, since some file systems perform poorly with many items in a directory.

As an example, the empty tree contains the data (when uncompressed) tree 0\0 and, in a SHA-256 repository, would have the object ID 6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321 and would be stored under $GIT_DIR/objects/6e/f19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321.

Similarly, a blob containing the contents abc would have the uncompressed data of blob 3\0abc.

Part of the git(1) suite