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diff --git a/Documentation/gitformat-pack.txt b/Documentation/gitformat-pack.txt new file mode 100644 index 0000000..9fcb29a --- /dev/null +++ b/Documentation/gitformat-pack.txt @@ -0,0 +1,605 @@ +gitformat-pack(5) +================= + +NAME +---- +gitformat-pack - Git pack format + + +SYNOPSIS +-------- +[verse] +$GIT_DIR/objects/pack/pack-*.{pack,idx} +$GIT_DIR/objects/pack/pack-*.rev +$GIT_DIR/objects/pack/pack-*.mtimes +$GIT_DIR/objects/pack/multi-pack-index + +DESCRIPTION +----------- + +The Git pack format is how Git stores most of its primary repository +data. Over the lifetime of a repository, loose objects (if any) and +smaller packs are consolidated into larger pack(s). See +linkgit:git-gc[1] and linkgit:git-pack-objects[1]. + +The pack format is also used over-the-wire, see +e.g. linkgit:gitprotocol-v2[5], as well as being a part of +other container formats in the case of linkgit:gitformat-bundle[5]. + +== Checksums and object IDs + +In a repository using the traditional SHA-1, pack checksums, index checksums, +and object IDs (object names) mentioned below are all computed using SHA-1. +Similarly, in SHA-256 repositories, these values are computed using SHA-256. + +== pack-*.pack files have the following format: + + - A header appears at the beginning and consists of the following: + + 4-byte signature: + The signature is: {'P', 'A', 'C', 'K'} + + 4-byte version number (network byte order): + Git currently accepts version number 2 or 3 but + generates version 2 only. + + 4-byte number of objects contained in the pack (network byte order) + + Observation: we cannot have more than 4G versions ;-) and + more than 4G objects in a pack. + + - The header is followed by a number of object entries, each of + which looks like this: + + (undeltified representation) + n-byte type and length (3-bit type, (n-1)*7+4-bit length) + compressed data + + (deltified representation) + n-byte type and length (3-bit type, (n-1)*7+4-bit length) + base object name if OBJ_REF_DELTA or a negative relative + offset from the delta object's position in the pack if this + is an OBJ_OFS_DELTA object + compressed delta data + + Observation: the length of each object is encoded in a variable + length format and is not constrained to 32-bit or anything. + + - The trailer records a pack checksum of all of the above. + +=== Object types + +Valid object types are: + +- OBJ_COMMIT (1) +- OBJ_TREE (2) +- OBJ_BLOB (3) +- OBJ_TAG (4) +- OBJ_OFS_DELTA (6) +- OBJ_REF_DELTA (7) + +Type 5 is reserved for future expansion. Type 0 is invalid. + +=== Size encoding + +This document uses the following "size encoding" of non-negative +integers: From each byte, the seven least significant bits are +used to form the resulting integer. As long as the most significant +bit is 1, this process continues; the byte with MSB 0 provides the +last seven bits. The seven-bit chunks are concatenated. Later +values are more significant. + +This size encoding should not be confused with the "offset encoding", +which is also used in this document. + +=== Deltified representation + +Conceptually there are only four object types: commit, tree, tag and +blob. However to save space, an object could be stored as a "delta" of +another "base" object. These representations are assigned new types +ofs-delta and ref-delta, which is only valid in a pack file. + +Both ofs-delta and ref-delta store the "delta" to be applied to +another object (called 'base object') to reconstruct the object. The +difference between them is, ref-delta directly encodes base object +name. If the base object is in the same pack, ofs-delta encodes +the offset of the base object in the pack instead. + +The base object could also be deltified if it's in the same pack. +Ref-delta can also refer to an object outside the pack (i.e. the +so-called "thin pack"). When stored on disk however, the pack should +be self contained to avoid cyclic dependency. + +The delta data starts with the size of the base object and the +size of the object to be reconstructed. These sizes are +encoded using the size encoding from above. The remainder of +the delta data is a sequence of instructions to reconstruct the object +from the base object. If the base object is deltified, it must be +converted to canonical form first. Each instruction appends more and +more data to the target object until it's complete. There are two +supported instructions so far: one for copying a byte range from the +source object and one for inserting new data embedded in the +instruction itself. + +Each instruction has variable length. Instruction type is determined +by the seventh bit of the first octet. The following diagrams follow +the convention in RFC 1951 (Deflate compressed data format). + +==== Instruction to copy from base object + + +----------+---------+---------+---------+---------+-------+-------+-------+ + | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 | + +----------+---------+---------+---------+---------+-------+-------+-------+ + +This is the instruction format to copy a byte range from the source +object. It encodes the offset to copy from and the number of bytes to +copy. Offset and size are in little-endian order. + +All offset and size bytes are optional. This is to reduce the +instruction size when encoding small offsets or sizes. The first seven +bits in the first octet determine which of the next seven octets is +present. If bit zero is set, offset1 is present. If bit one is set +offset2 is present and so on. + +Note that a more compact instruction does not change offset and size +encoding. For example, if only offset2 is omitted like below, offset3 +still contains bits 16-23. It does not become offset2 and contains +bits 8-15 even if it's right next to offset1. + + +----------+---------+---------+ + | 10000101 | offset1 | offset3 | + +----------+---------+---------+ + +In its most compact form, this instruction only takes up one byte +(0x80) with both offset and size omitted, which will have default +values zero. There is another exception: size zero is automatically +converted to 0x10000. + +==== Instruction to add new data + + +----------+============+ + | 0xxxxxxx | data | + +----------+============+ + +This is the instruction to construct the target object without the base +object. The following data is appended to the target object. The first +seven bits of the first octet determine the size of data in +bytes. The size must be non-zero. + +==== Reserved instruction + + +----------+============ + | 00000000 | + +----------+============ + +This is the instruction reserved for future expansion. + +== 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 is less than or equal to N. This is called the + 'first-level fan-out' table. + + - The header is followed by sorted 24-byte entries, one entry + per object in the pack. Each entry is: + + 4-byte network byte order integer, recording where the + object is stored in the packfile as the offset from the + beginning. + + one object name of the appropriate size. + + - The file is concluded with a trailer: + + A copy of the pack checksum at the end of the corresponding + packfile. + + Index checksum of all of the above. + +Pack Idx file: + + -- +--------------------------------+ +fanout | fanout[0] = 2 (for example) |-. +table +--------------------------------+ | + | fanout[1] | | + +--------------------------------+ | + | fanout[2] | | + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | + | 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 | + | +--------------------------------+ + .-------. + | +Pack file entry: <+ + + packed object header: + 1-byte size extension bit (MSB) + type (next 3 bit) + size0 (lower 4-bit) + n-byte sizeN (as long as MSB is set, each 7-bit) + size0..sizeN form 4+7+7+..+7 bit integer, size0 + is the least significant part, and sizeN is the + most significant part. + packed object data: + If it is not DELTA, then deflated bytes (the size above + is the size before compression). + If it is REF_DELTA, then + base object name (the size above is the + size of the delta data that follows). + delta data, deflated. + If it is OFS_DELTA, then + n-byte offset (see below) interpreted as a negative + offset from the type-byte of the header of the + ofs-delta entry (the size above is the size of + the delta data that follows). + delta data, deflated. + + offset encoding: + n bytes with MSB set in all but the last one. + The offset is then the number constructed by + concatenating the lower 7 bit of each byte, and + for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1)) + to the result. + + + +== 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 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 without 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 pack checksum at the end of the + corresponding packfile. + + Index checksum of all of the above. + +== pack-*.rev files have the format: + + - A 4-byte magic number '0x52494458' ('RIDX'). + + - A 4-byte version identifier (= 1). + + - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256). + + - A table of index positions (one per packed object, num_objects in + total, each a 4-byte unsigned integer in network order), sorted by + their corresponding offsets in the packfile. + + - A trailer, containing a: + + checksum of the corresponding packfile, and + + a checksum of all of the above. + +All 4-byte numbers are in network order. + +== pack-*.mtimes files have the format: + +All 4-byte numbers are in network byte order. + + - A 4-byte magic number '0x4d544d45' ('MTME'). + + - A 4-byte version identifier (= 1). + + - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256). + + - A table of 4-byte unsigned integers. The ith value is the + modification time (mtime) of the ith object in the corresponding + pack by lexicographic (index) order. The mtimes count standard + epoch seconds. + + - A trailer, containing a checksum of the corresponding packfile, + and a checksum of all of the above (each having length according + to the specified hash function). + +== 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 + We infer the length of object IDs (OIDs) from this value: + 1 => SHA-1 + 2 => SHA-256 + If the hash type does not match the repository's hash algorithm, + the multi-pack-index file should be ignored with a warning + presented to the user. + + 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 CHUNK LOOKUP matches the table of contents from + the chunk-based file format, see linkgit:gitformat-chunk[5]. + + 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: + + Packfile Names (ID: {'P', 'N', 'A', 'M'}) + Store the names of packfiles as a sequence of NUL-terminated + strings. There is no extra padding between the filenames, + and they are listed in lexicographic order. The chunk itself + is padded at the end with between 0 and 3 NUL bytes to make the + chunk size a multiple of 4 bytes. + + 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 + number of objects. + + OID Lookup (ID: {'O', 'I', 'D', 'L'}) + The OIDs for all objects in the MIDX are stored in lexicographic + order in this chunk. + + Object Offsets (ID: {'O', 'O', 'F', 'F'}) + Stores two 4-byte values for every object. + 1: The pack-int-id for the pack storing this object. + 2: The offset within the pack. + If all offsets are less than 2^32, then the large offset chunk + will not exist and offsets are stored as in IDX v1. + If there is at least one offset value larger than 2^32-1, then + the large offset chunk must exist, and offsets larger than + 2^31-1 must be stored in it instead. If the large offset chunk + exists and the 31st bit is on, then removing that bit reveals + the row in the large offsets containing the 8-byte offset of + this object. + + [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'}) + 8-byte offsets into large packfiles. + + [Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'}) + A list of MIDX positions (one per object in the MIDX, num_objects in + total, each a 4-byte unsigned integer in network byte order), sorted + according to their relative bitmap/pseudo-pack positions. + +TRAILER: + + Index checksum of the above contents. + +== multi-pack-index reverse indexes + +Similar to the pack-based reverse index, the multi-pack index can also +be used to generate a reverse index. + +Instead of mapping between offset, pack-, and index position, this +reverse index maps between an object's position within the MIDX, and +that object's position within a pseudo-pack that the MIDX describes +(i.e., the ith entry of the multi-pack reverse index holds the MIDX +position of ith object in pseudo-pack order). + +To clarify the difference between these orderings, consider a multi-pack +reachability bitmap (which does not yet exist, but is what we are +building towards here). Each bit needs to correspond to an object in the +MIDX, and so we need an efficient mapping from bit position to MIDX +position. + +One solution is to let bits occupy the same position in the oid-sorted +index stored by the MIDX. But because oids are effectively random, their +resulting reachability bitmaps would have no locality, and thus compress +poorly. (This is the reason that single-pack bitmaps use the pack +ordering, and not the .idx ordering, for the same purpose.) + +So we'd like to define an ordering for the whole MIDX based around +pack ordering, which has far better locality (and thus compresses more +efficiently). We can think of a pseudo-pack created by the concatenation +of all of the packs in the MIDX. E.g., if we had a MIDX with three packs +(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an +ordering of the objects like: + + |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19| + +where the ordering of the packs is defined by the MIDX's pack list, +and then the ordering of objects within each pack is the same as the +order in the actual packfile. + +Given the list of packs and their counts of objects, you can +naïvely reconstruct that pseudo-pack ordering (e.g., the object at +position 27 must be (c,1) because packs "a" and "b" consumed 25 of the +slots). But there's a catch. Objects may be duplicated between packs, in +which case the MIDX only stores one pointer to the object (and thus we'd +want only one slot in the bitmap). + +Callers could handle duplicates themselves by reading objects in order +of their bit-position, but that's linear in the number of objects, and +much too expensive for ordinary bitmap lookups. Building a reverse index +solves this, since it is the logical inverse of the index, and that +index has already removed duplicates. But, building a reverse index on +the fly can be expensive. Since we already have an on-disk format for +pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack, +too. + +Objects from the MIDX are ordered as follows to string together the +pseudo-pack. Let `pack(o)` return the pack from which `o` was selected +by the MIDX, and define an ordering of packs based on their numeric ID +(as stored by the MIDX). Let `offset(o)` return the object offset of `o` +within `pack(o)`. Then, compare `o1` and `o2` as follows: + + - If one of `pack(o1)` and `pack(o2)` is preferred and the other + is not, then the preferred one sorts first. ++ +(This is a detail that allows the MIDX bitmap to determine which +pack should be used by the pack-reuse mechanism, since it can ask +the MIDX for the pack containing the object at bit position 0). + + - If `pack(o1) ≠ pack(o2)`, then sort the two objects in descending + order based on the pack ID. + + - Otherwise, `pack(o1) = pack(o2)`, and the objects are sorted in + pack-order (i.e., `o1` sorts ahead of `o2` exactly when `offset(o1) + < offset(o2)`). + +In short, a MIDX's pseudo-pack is the de-duplicated concatenation of +objects in packs stored by the MIDX, laid out in pack order, and the +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. + +== cruft packs + +The cruft packs feature offer an alternative to Git's traditional mechanism of +removing unreachable objects. This document provides an overview of Git's +pruning mechanism, and how a cruft pack can be used instead to accomplish the +same. + +=== Background + +To remove unreachable objects from your repository, Git offers `git repack -Ad` +(see linkgit:git-repack[1]). Quoting from the documentation: + +---- +[...] unreachable objects in a previous pack become loose, unpacked objects, +instead of being left in the old pack. [...] loose unreachable objects will be +pruned according to normal expiry rules with the next 'git gc' invocation. +---- + +Unreachable objects aren't removed immediately, since doing so could race with +an incoming push which may reference an object which is about to be deleted. +Instead, those unreachable objects are stored as loose objects and stay that way +until they are older than the expiration window, at which point they are removed +by linkgit:git-prune[1]. + +Git must store these unreachable objects loose in order to keep track of their +per-object mtimes. If these unreachable objects were written into one big pack, +then either freshening that pack (because an object contained within it was +re-written) or creating a new pack of unreachable objects would cause the pack's +mtime to get updated, and the objects within it would never leave the expiration +window. Instead, objects are stored loose in order to keep track of the +individual object mtimes and avoid a situation where all cruft objects are +freshened at once. + +This can lead to undesirable situations when a repository contains many +unreachable objects which have not yet left the grace period. Having large +directories in the shards of `.git/objects` can lead to decreased performance in +the repository. But given enough unreachable objects, this can lead to inode +starvation and degrade the performance of the whole system. Since we +can never pack those objects, these repositories often take up a large amount of +disk space, since we can only zlib compress them, but not store them in delta +chains. + +=== Cruft packs + +A cruft pack eliminates the need for storing unreachable objects in a loose +state by including the per-object mtimes in a separate file alongside a single +pack containing all loose objects. + +A cruft pack is written by `git repack --cruft` when generating a new pack. +linkgit:git-pack-objects[1]'s `--cruft` option. Note that `git repack --cruft` +is a classic all-into-one repack, meaning that everything in the resulting pack is +reachable, and everything else is unreachable. Once written, the `--cruft` +option instructs `git repack` to generate another pack containing only objects +not packed in the previous step (which equates to packing all unreachable +objects together). This progresses as follows: + + 1. Enumerate every object, marking any object which is (a) not contained in a + kept-pack, and (b) whose mtime is within the grace period as a traversal + tip. + + 2. Perform a reachability traversal based on the tips gathered in the previous + step, adding every object along the way to the pack. + + 3. Write the pack out, along with a `.mtimes` file that records the per-object + timestamps. + +This mode is invoked internally by linkgit:git-repack[1] when instructed to +write a cruft pack. Crucially, the set of in-core kept packs is exactly the set +of packs which will not be deleted by the repack; in other words, they contain +all of the repository's reachable objects. + +When a repository already has a cruft pack, `git repack --cruft` typically only +adds objects to it. An exception to this is when `git repack` is given the +`--cruft-expiration` option, which allows the generated cruft pack to omit +expired objects instead of waiting for linkgit:git-gc[1] to expire those objects +later on. + +It is linkgit:git-gc[1] that is typically responsible for removing expired +unreachable objects. + +=== Alternatives + +Notable alternatives to this design include: + + - The location of the per-object mtime data. + +On the location of mtime data, a new auxiliary file tied to the pack was chosen +to avoid complicating the `.idx` format. If the `.idx` format were ever to gain +support for optional chunks of data, it may make sense to consolidate the +`.mtimes` format into the `.idx` itself. + +GIT +--- +Part of the linkgit:git[1] suite |