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| -rw-r--r-- | Documentation/gitformat-pack.txt | 76 |
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diff --git a/Documentation/gitformat-pack.txt b/Documentation/gitformat-pack.txt index 9fcb29a..d6ae229 100644 --- a/Documentation/gitformat-pack.txt +++ b/Documentation/gitformat-pack.txt @@ -396,6 +396,15 @@ CHUNK DATA: is padded at the end with between 0 and 3 NUL bytes to make the chunk size a multiple of 4 bytes. + Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'}) + Stores a table of two 4-byte unsigned integers in network order. + Each table entry corresponds to a single pack (in the order that + they appear above in the `PNAM` chunk). The values for each table + entry are as follows: + - The first bit position (in pseudo-pack order, see below) to + contain an object from that pack. + - The number of bits whose objects are selected from that pack. + OID Fanout (ID: {'O', 'I', 'D', 'F'}) The ith entry, F[i], stores the number of OIDs with first byte at most i. Thus F[255] stores the total @@ -509,6 +518,73 @@ packs arranged in MIDX order (with the preferred pack coming first). The MIDX's reverse index is stored in the optional 'RIDX' chunk within the MIDX itself. +=== `BTMP` chunk + +The Bitmapped Packfiles (`BTMP`) chunk encodes additional information +about the objects in the multi-pack index's reachability bitmap. Recall +that objects from the MIDX are arranged in "pseudo-pack" order (see +above) for reachability bitmaps. + +From the example above, suppose we have packs "a", "b", and "c", with +10, 15, and 20 objects, respectively. In pseudo-pack order, those would +be arranged as follows: + + |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19| + +When working with single-pack bitmaps (or, equivalently, multi-pack +reachability bitmaps with a preferred pack), linkgit:git-pack-objects[1] +performs ``verbatim'' reuse, attempting to reuse chunks of the bitmapped +or preferred packfile instead of adding objects to the packing list. + +When a chunk of bytes is reused from an existing pack, any objects +contained therein do not need to be added to the packing list, saving +memory and CPU time. But a chunk from an existing packfile can only be +reused when the following conditions are met: + + - The chunk contains only objects which were requested by the caller + (i.e. does not contain any objects which the caller didn't ask for + explicitly or implicitly). + + - All objects stored in non-thin packs as offset- or reference-deltas + also include their base object in the resulting pack. + +The `BTMP` chunk encodes the necessary information in order to implement +multi-pack reuse over a set of packfiles as described above. +Specifically, the `BTMP` chunk encodes three pieces of information (all +32-bit unsigned integers in network byte-order) for each packfile `p` +that is stored in the MIDX, as follows: + +`bitmap_pos`:: The first bit position (in pseudo-pack order) in the + multi-pack index's reachability bitmap occupied by an object from `p`. + +`bitmap_nr`:: The number of bit positions (including the one at + `bitmap_pos`) that encode objects from that pack `p`. + +For example, the `BTMP` chunk corresponding to the above example (with +packs ``a'', ``b'', and ``c'') would look like: + +[cols="1,2,2"] +|=== +| |`bitmap_pos` |`bitmap_nr` + +|packfile ``a'' +|`0` +|`10` + +|packfile ``b'' +|`10` +|`15` + +|packfile ``c'' +|`25` +|`20` +|=== + +With this information in place, we can treat each packfile as +individually reusable in the same fashion as verbatim pack reuse is +performed on individual packs prior to the implementation of the `BTMP` +chunk. + == cruft packs The cruft packs feature offer an alternative to Git's traditional mechanism of |