gitformat-loose(5) ================== NAME ---- gitformat-loose - Git loose object format SYNOPSIS -------- [verse] $GIT_DIR/objects/[0-9a-f][0-9a-f]/* $GIT_DIR/objects/loose-object-idx $GIT_DIR/objects/loose-map/map-*.map DESCRIPTION ----------- Loose objects are how Git stores individual objects, where every object is written as a separate file. Over the lifetime of a repository, objects are usually written as loose objects initially. Eventually, these loose objects will be compacted into packfiles via repository maintenance to improve disk space usage and speed up the lookup of these objects. == Loose objects Each loose object contains a prefix, followed immediately by the data of the object. The prefix contains ` \0`. `` is one of `blob`, `tree`, `commit`, or `tag` and `size` is the size of the data (without the prefix) as a decimal integer expressed in ASCII. The entire contents, prefix and data concatenated, is then compressed with zlib and the compressed data is stored in the file. The object ID of the object is the SHA-1 or SHA-256 (as appropriate) hash of the uncompressed data. The file for the loose object is stored under the `objects` directory, with the first two hex characters of the object ID being the directory and the remaining characters being the file name. This is done to shard the data and avoid too many files being in one directory, since some file systems perform poorly with many items in a directory. As an example, the empty tree contains the data (when uncompressed) `tree 0\0` and, in a SHA-256 repository, would have the object ID `6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321` and would be stored under `$GIT_DIR/objects/6e/f19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321`. Similarly, a blob containing the contents `abc` would have the uncompressed data of `blob 3\0abc`. == Loose object mapping When the `compatObjectFormat` option is used, Git needs to store a mapping between the repository's main algorithm and the compatibility algorithm. There are two formats for this: the legacy mapping and the modern mapping. === Legacy mapping The compatibility mapping is stored in a file called `$GIT_DIR/objects/loose-object-idx`. The format of this file looks like this: # loose-object-idx (main-name SP compat-name LF)* `main-name` refers to hexadecimal object ID of the object in the main repository format and `compat-name` refers to the same thing, but for the compatibility format. This format is read if it exists but is not written. Note that carriage returns are not permitted in this file, regardless of the host system or configuration. === Modern mapping The modern mapping consists of a set of files under `$GIT_DIR/objects/loose` ending in `.map`. The portion of the filename before the extension is that of the hash checksum in hex format. `git pack-objects` will repack existing entries into one file, removing any unnecessary objects, such as obsolete shallow entries or loose objects that have been packed. ==== Mapping file format - A header appears at the beginning and consists of the following: * A 4-byte mapping signature: `LMAP` * 4-byte version number: 1 * 4-byte length of the header section. * 4-byte number of objects declared in this map file. * 4-byte number of object formats declared in this map file. * For each object format: ** 4-byte format identifier (e.g., `sha1` for SHA-1) ** 4-byte length in bytes of shortened object names. This is the shortest possible length needed to make names in the shortened object name table unambiguous. ** 8-byte integer, recording where tables relating to this format are stored in this index file, as an offset from the beginning. * 8-byte offset to the trailer from the beginning of this file. * Zero or more additional key/value pairs (4-byte key, 4-byte value), which may optionally declare one or more chunks. No chunks are currently defined. Readers must ignore unrecognized keys. - Zero or more NUL bytes. These are used to improve the alignment of the 4-byte quantities below. - Tables for the first object format: * A sorted table of shortened object names. These are prefixes of the names of all objects in this file, packed together without offset values to reduce the cache footprint of the binary search for a specific object name. * A sorted table of full object names. * A table of 4-byte metadata values. * Zero or more chunks. A chunk starts with a four-byte chunk identifier and a four-byte parameter (which, if unneeded, is all zeros) and an eight-byte size (not including the identifier, parameter, or size), plus the chunk data. - Zero or more NUL bytes. - Tables for subsequent object formats: * A sorted table of shortened object names. These are prefixes of the names of all objects in this file, packed together without offset values to reduce the cache footprint of the binary search for a specific object name. * A table of full object names in the order specified by the first object format. * A table of 4-byte values mapping object name order to the order of the first object format. For an object in the table of sorted shortened object names, the value at the corresponding index in this table is the index in the previous table for that same object. * Zero or more NUL bytes. - The trailer consists of the following: * Hash checksum of all of the above. The lower six bits of each metadata table contain a type field indicating the reason that this object is stored: 0:: Reserved. 1:: This object is stored as a loose object in the repository. 2:: This object is a shallow entry. The mapping refers to a shallow value returned by a remote server. 3:: This object is a submodule entry. The mapping refers to the commit stored representing a submodule. Other data may be stored in this field in the future. Bits that are not used must be zero. All 4-byte numbers are in network order and must be 4-byte aligned in the file, so the NUL padding may be required in some cases. Note that the hash at the end of the file is in whatever the repository's main algorithm is. In the usual case when there are multiple algorithms, the main algorithm will be SHA-256 and the compatibility algorithm will be SHA-1. GIT --- Part of the linkgit:git[1] suite