>+ * From v1.5.5, the pack.indexversion config option will default to 2,
>+ which is slightly more efficient, and makes repacking more immune to
>+ data corruptions. Git older than version 1.5.2 may revert to version 1
>+ of the pack index with a manual "git index-pack" to be able to directly
>+ access corresponding pack files.
You might want to mention that it's slightly more TIME efficient,
but takes 16% more space (28 bytes per object rather than 24).
If it helps, I documented the v2 index file format (a lot stolen
from commit c553ca25bd60dc9fd50b8bc7bd329601b81cee66 message).
(Public domain, copyright abandoned, if it breaks you get to keep both
pieces, yadda yadda.)
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index e5b31c8..a80baa4 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -1,9 +1,9 @@
GIT pack format
===============
-= pack-*.pack file has the following format:
+= pack-*.pack files have the following format:
- - The header appears at the beginning and consists of the following:
+ - A header appears at the beginning and consists of the following:
4-byte signature:
The signature is: {'P', 'A', 'C', 'K'}
@@ -34,18 +34,14 @@ GIT pack format
- The trailer records 20-byte SHA1 checksum of all of the above.
-= pack-*.idx file has the following format:
+= Original (version 1) pack-*.idx files have the following format:
- The header consists of 256 4-byte network byte order
integers. N-th entry of this table records the number of
objects in the corresponding pack, the first byte of whose
- object name are smaller than N. This is called the
+ object name is less than or equal to N. This is called the
'first-level fan-out' table.
- Observation: we would need to extend this to an array of
- 8-byte integers to go beyond 4G objects per pack, but it is
- not strictly necessary.
-
- The header is followed by sorted 24-byte entries, one entry
per object in the pack. Each entry is:
@@ -55,10 +51,6 @@ GIT pack format
20-byte object name.
- Observation: we would definitely need to extend this to
- 8-byte integer plus 20-byte object name to handle a packfile
- that is larger than 4GB.
-
- The file is concluded with a trailer:
A copy of the 20-byte SHA1 checksum at the end of
@@ -68,31 +60,30 @@ GIT pack format
Pack Idx file:
- idx
- +--------------------------------+
- | fanout[0] = 2 |-.
- +--------------------------------+ |
+ -- +--------------------------------+
+fanout | fanout[0] = 2 (for example) |-.
+table +--------------------------------+ |
| fanout[1] | |
+--------------------------------+ |
| fanout[2] | |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
- | fanout[255] | |
- +--------------------------------+ |
-main | offset | |
-index | object name 00XXXXXXXXXXXXXXXX | |
-table +--------------------------------+ |
- | offset | |
- | object name 00XXXXXXXXXXXXXXXX | |
- +--------------------------------+ |
- .-| offset |<+
- | | object name 01XXXXXXXXXXXXXXXX |
- | +--------------------------------+
- | | offset |
- | | object name 01XXXXXXXXXXXXXXXX |
- | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- | | offset |
- | | object name FFXXXXXXXXXXXXXXXX |
- | +--------------------------------+
+ | fanout[255] = total objects |---.
+ -- +--------------------------------+ | |
+main | offset | | |
+index | object name 00XXXXXXXXXXXXXXXX | | |
+table +--------------------------------+ | |
+ | offset | | |
+ | object name 00XXXXXXXXXXXXXXXX | | |
+ +--------------------------------+<+ |
+ .-| offset | |
+ | | object name 01XXXXXXXXXXXXXXXX | |
+ | +--------------------------------+ |
+ | | offset | |
+ | | object name 01XXXXXXXXXXXXXXXX | |
+ | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
+ | | offset | |
+ | | object name FFXXXXXXXXXXXXXXXX | |
+ --| +--------------------------------+<--+
trailer | | packfile checksum |
| +--------------------------------+
| | idxfile checksum |
@@ -116,3 +107,40 @@ Pack file entry: <+
20-byte base object name SHA1 (the size above is the
size of the delta data that follows).
delta data, deflated.
+
+
+= Version 2 pack-*.idx files support packs larger than 4 GiB, and
+ have some other reorganizations. They have the format:
+
+ - A 4-byte magic number '\377tOc' which is an unreasonable
+ fanout[0] value.
+
+ - A 4-byte version number (= 2)
+
+ - A 256-entry fan-out table just like v1.
+
+ - A table of sorted 20-byte SHA1 object names. These are
+ packed together without offset values to reduce the cache
+ footprint of the binary search for a specific object name.
+
+ - A table of 4-byte CRC32 values of the packed object data.
+ This is new in v2 so compressed data can be copied directly
+ from pack to pack during repacking withough undetected
+ data corruption.
+
+ - A table of 4-byte offset values (in network byte order).
+ These are usually 31-bit pack file offsets, but large
+ offsets are encoded as an index into the next table with
+ the msbit set.
+
+ - A table of 8-byte offset entries (empty for pack files less
+ than 2 GiB). Pack files are organized with heavily used
+ objects toward the front, so most object references should
+ not need to refer to this table.
+
+ - The same trailer as a v1 pack file:
+
+ A copy of the 20-byte SHA1 checksum at the end of
+ corresponding packfile.
+
+ 20-byte SHA1-checksum of all of the above.
-
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