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Diffstat (limited to 'Documentation/gitformat-loose.adoc')
| -rw-r--r-- | Documentation/gitformat-loose.adoc | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/Documentation/gitformat-loose.adoc b/Documentation/gitformat-loose.adoc index 947993663e..b0b569761b 100644 --- a/Documentation/gitformat-loose.adoc +++ b/Documentation/gitformat-loose.adoc @@ -10,6 +10,7 @@ SYNOPSIS -------- [verse] $GIT_DIR/objects/[0-9a-f][0-9a-f]/* +$GIT_DIR/objects/object-map/map-*.map DESCRIPTION ----------- @@ -48,6 +49,83 @@ stored under 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 for +loose objects as well as some auxiliary information. + +The mapping consists of a set of files under `$GIT_DIR/objects/object-map` +ending in `.map`. The portion of the filename before the extension is that of +the main hash checksum (that is, the one specified in +`extensions.objectformat`) in hex format. + +`git gc` will repack existing entries into one file, removing any unnecessary +objects, such as obsolete shallow entries or loose objects that have been +packed. + +The file format is as follows. All values are in network byte order and all +4-byte and 8-byte values must be 4-byte aligned in the file, so the NUL padding +may be required in some cases. Git always uses the smallest number of NUL +bytes (including zero) that is required for the padding in order to make +writing files deterministic. + +- 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 (including reserved entries but + excluding any NUL padding). + * 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 (that is, prefixes of + the full 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. + * The remainder of the header section is reserved for future use. + Readers must ignore unrecognized data here. +- 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 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 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 using the main hash. + +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. + GIT --- Part of the linkgit:git[1] suite |