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author | Derrick Stolee <stolee@gmail.com> | 2018-07-12 15:39:19 -0400 |
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committer | Junio C Hamano <gitster@pobox.com> | 2018-07-12 13:55:02 -0700 |
commit | e0d1bcf82590aea8ee007ab087772f1c82f6890b (patch) | |
tree | bd6f79b1b75a63a95c9a433fa32f7f513846cd37 /Documentation | |
parent | multi-pack-index: add design document (diff) | |
download | tgif-e0d1bcf82590aea8ee007ab087772f1c82f6890b.tar.xz |
multi-pack-index: add format details
The multi-pack-index feature generalizes the existing pack-index
feature by indexing objects across multiple pack-files.
Describe the basic file format, using a 12-byte header followed by
a lookup table for a list of "chunks" which will be described later.
The file ends with a footer containing a checksum using the hash
algorithm.
The header allows later versions to create breaking changes by
advancing the version number. We can also change the hash algorithm
using a different version value.
We will add the individual chunk format information as we introduce
the code that writes that information.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/technical/pack-format.txt | 49 |
1 files changed, 49 insertions, 0 deletions
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt index 70a99fd142..e060e693f4 100644 --- a/Documentation/technical/pack-format.txt +++ b/Documentation/technical/pack-format.txt @@ -252,3 +252,52 @@ Pack file entry: <+ corresponding packfile. 20-byte SHA-1-checksum of all of the above. + +== multi-pack-index (MIDX) files have the following format: + +The multi-pack-index files refer to multiple pack-files and loose objects. + +In order to allow extensions that add extra data to the MIDX, we organize +the body into "chunks" and provide a lookup table at the beginning of the +body. The header includes certain length values, such as the number of packs, +the number of base MIDX files, hash lengths and types. + +All 4-byte numbers are in network order. + +HEADER: + + 4-byte signature: + The signature is: {'M', 'I', 'D', 'X'} + + 1-byte version number: + Git only writes or recognizes version 1. + + 1-byte Object Id Version + Git only writes or recognizes version 1 (SHA1). + + 1-byte number of "chunks" + + 1-byte number of base multi-pack-index files: + This value is currently always zero. + + 4-byte number of pack files + +CHUNK LOOKUP: + + (C + 1) * 12 bytes providing the chunk offsets: + First 4 bytes describe chunk id. Value 0 is a terminating label. + Other 8 bytes provide offset in current file for chunk to start. + (Chunks are provided in file-order, so you can infer the length + using the next chunk position if necessary.) + + The remaining data in the body is described one chunk at a time, and + these chunks may be given in any order. Chunks are required unless + otherwise specified. + +CHUNK DATA: + + (This section intentionally left incomplete.) + +TRAILER: + + 20-byte SHA1-checksum of the above contents. |