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

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 <>
 Documentation/technical/pack-format.txt | 49 +++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/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.
+       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
+       (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.
+       (This section intentionally left incomplete.)
+       20-byte SHA1-checksum of the above contents.

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