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diff --git a/Documentation/Makefile b/Documentation/Makefile index 15d9d04f31..ecd0b340b1 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -93,6 +93,7 @@ TECH_DOCS += technical/protocol-capabilities TECH_DOCS += technical/protocol-common TECH_DOCS += technical/protocol-v2 TECH_DOCS += technical/racy-git +TECH_DOCS += technical/reftable TECH_DOCS += technical/send-pack-pipeline TECH_DOCS += technical/shallow TECH_DOCS += technical/signature-format diff --git a/Documentation/technical/reftable.txt b/Documentation/technical/reftable.txt new file mode 100644 index 0000000000..d652f42cbb --- /dev/null +++ b/Documentation/technical/reftable.txt @@ -0,0 +1,1069 @@ +reftable +-------- + +Overview +~~~~~~~~ + +Problem statement +^^^^^^^^^^^^^^^^^ + +Some repositories contain a lot of references (e.g. android at 866k, +rails at 31k). The existing packed-refs format takes up a lot of space +(e.g. 62M), and does not scale with additional references. Lookup of a +single reference requires linearly scanning the file. + +Atomic pushes modifying multiple references require copying the entire +packed-refs file, which can be a considerable amount of data moved +(e.g. 62M in, 62M out) for even small transactions (2 refs modified). + +Repositories with many loose references occupy a large number of disk +blocks from the local file system, as each reference is its own file +storing 41 bytes (and another file for the corresponding reflog). This +negatively affects the number of inodes available when a large number of +repositories are stored on the same filesystem. Readers can be penalized +due to the larger number of syscalls required to traverse and read the +`$GIT_DIR/refs` directory. + + +Objectives +^^^^^^^^^^ + +* Near constant time lookup for any single reference, even when the +repository is cold and not in process or kernel cache. +* Near constant time verification if a SHA-1 is referred to by at least +one reference (for allow-tip-sha1-in-want). +* Efficient enumeration of an entire namespace, such as `refs/tags/`. +* Support atomic push with `O(size_of_update)` operations. +* Combine reflog storage with ref storage for small transactions. +* Separate reflog storage for base refs and historical logs. + +Description +^^^^^^^^^^^ + +A reftable file is a portable binary file format customized for +reference storage. References are sorted, enabling linear scans, binary +search lookup, and range scans. + +Storage in the file is organized into variable sized blocks. Prefix +compression is used within a single block to reduce disk space. Block +size and alignment is tunable by the writer. + +Performance +^^^^^^^^^^^ + +Space used, packed-refs vs. reftable: + +[cols=",>,>,>,>,>",options="header",] +|=============================================================== +|repository |packed-refs |reftable |% original |avg ref |avg obj +|android |62.2 M |36.1 M |58.0% |33 bytes |5 bytes +|rails |1.8 M |1.1 M |57.7% |29 bytes |4 bytes +|git |78.7 K |48.1 K |61.0% |50 bytes |4 bytes +|git (heads) |332 b |269 b |81.0% |33 bytes |0 bytes +|=============================================================== + +Scan (read 866k refs), by reference name lookup (single ref from 866k +refs), and by SHA-1 lookup (refs with that SHA-1, from 866k refs): + +[cols=",>,>,>,>",options="header",] +|========================================================= +|format |cache |scan |by name |by SHA-1 +|packed-refs |cold |402 ms |409,660.1 usec |412,535.8 usec +|packed-refs |hot | |6,844.6 usec |20,110.1 usec +|reftable |cold |112 ms |33.9 usec |323.2 usec +|reftable |hot | |20.2 usec |320.8 usec +|========================================================= + +Space used for 149,932 log entries for 43,061 refs, reflog vs. reftable: + +[cols=",>,>",options="header",] +|================================ +|format |size |avg entry +|$GIT_DIR/logs |173 M |1209 bytes +|reftable |5 M |37 bytes +|================================ + +Details +~~~~~~~ + +Peeling +^^^^^^^ + +References stored in a reftable are peeled, a record for an annotated +(or signed) tag records both the tag object, and the object it refers +to. This is analogous to storage in the packed-refs format. + +Reference name encoding +^^^^^^^^^^^^^^^^^^^^^^^ + +Reference names are an uninterpreted sequence of bytes that must pass +linkgit:git-check-ref-format[1] as a valid reference name. + +Key unicity +^^^^^^^^^^^ + +Each entry must have a unique key; repeated keys are disallowed. + +Network byte order +^^^^^^^^^^^^^^^^^^ + +All multi-byte, fixed width fields are in network byte order. + +Varint encoding +^^^^^^^^^^^^^^^ + +Varint encoding is identical to the ofs-delta encoding method used +within pack files. + +Decoder works such as: + +.... +val = buf[ptr] & 0x7f +while (buf[ptr] & 0x80) { + ptr++ + val = ((val + 1) << 7) | (buf[ptr] & 0x7f) +} +.... + +Ordering +^^^^^^^^ + +Blocks are lexicographically ordered by their first reference. + +Directory/file conflicts +^^^^^^^^^^^^^^^^^^^^^^^^ + +The reftable format accepts both `refs/heads/foo` and +`refs/heads/foo/bar` as distinct references. + +This property is useful for retaining log records in reftable, but may +confuse versions of Git using `$GIT_DIR/refs` directory tree to maintain +references. Users of reftable may choose to continue to reject `foo` and +`foo/bar` type conflicts to prevent problems for peers. + +File format +~~~~~~~~~~~ + +Structure +^^^^^^^^^ + +A reftable file has the following high-level structure: + +.... +first_block { + header + first_ref_block +} +ref_block* +ref_index* +obj_block* +obj_index* +log_block* +log_index* +footer +.... + +A log-only file omits the `ref_block`, `ref_index`, `obj_block` and +`obj_index` sections, containing only the file header and log block: + +.... +first_block { + header +} +log_block* +log_index* +footer +.... + +in a log-only file the first log block immediately follows the file +header, without padding to block alignment. + +Block size +^^^^^^^^^^ + +The file's block size is arbitrarily determined by the writer, and does +not have to be a power of 2. The block size must be larger than the +longest reference name or log entry used in the repository, as +references cannot span blocks. + +Powers of two that are friendly to the virtual memory system or +filesystem (such as 4k or 8k) are recommended. Larger sizes (64k) can +yield better compression, with a possible increased cost incurred by +readers during access. + +The largest block size is `16777215` bytes (15.99 MiB). + +Block alignment +^^^^^^^^^^^^^^^ + +Writers may choose to align blocks at multiples of the block size by +including `padding` filled with NUL bytes at the end of a block to round +out to the chosen alignment. When alignment is used, writers must +specify the alignment with the file header's `block_size` field. + +Block alignment is not required by the file format. Unaligned files must +set `block_size = 0` in the file header, and omit `padding`. Unaligned +files with more than one ref block must include the link:#Ref-index[ref +index] to support fast lookup. Readers must be able to read both aligned +and non-aligned files. + +Very small files (e.g. a single ref block) may omit `padding` and the ref +index to reduce total file size. + +Header +^^^^^^ + +A 24-byte header appears at the beginning of the file: + +.... +'REFT' +uint8( version_number = 1 ) +uint24( block_size ) +uint64( min_update_index ) +uint64( max_update_index ) +.... + +Aligned files must specify `block_size` to configure readers with the +expected block alignment. Unaligned files must set `block_size = 0`. + +The `min_update_index` and `max_update_index` describe bounds for the +`update_index` field of all log records in this file. When reftables are +used in a stack for link:#Update-transactions[transactions], these +fields can order the files such that the prior file's +`max_update_index + 1` is the next file's `min_update_index`. + +First ref block +^^^^^^^^^^^^^^^ + +The first ref block shares the same block as the file header, and is 24 +bytes smaller than all other blocks in the file. The first block +immediately begins after the file header, at position 24. + +If the first block is a log block (a log-only file), its block header +begins immediately at position 24. + +Ref block format +^^^^^^^^^^^^^^^^ + +A ref block is written as: + +.... +'r' +uint24( block_len ) +ref_record+ +uint24( restart_offset )+ +uint16( restart_count ) + +padding? +.... + +Blocks begin with `block_type = 'r'` and a 3-byte `block_len` which +encodes the number of bytes in the block up to, but not including the +optional `padding`. This is always less than or equal to the file's +block size. In the first ref block, `block_len` includes 24 bytes for +the file header. + +The 2-byte `restart_count` stores the number of entries in the +`restart_offset` list, which must not be empty. Readers can use +`restart_count` to binary search between restarts before starting a +linear scan. + +Exactly `restart_count` 3-byte `restart_offset` values precedes the +`restart_count`. Offsets are relative to the start of the block and +refer to the first byte of any `ref_record` whose name has not been +prefix compressed. Entries in the `restart_offset` list must be sorted, +ascending. Readers can start linear scans from any of these records. + +A variable number of `ref_record` fill the middle of the block, +describing reference names and values. The format is described below. + +As the first ref block shares the first file block with the file header, +all `restart_offset` in the first block are relative to the start of the +file (position 0), and include the file header. This forces the first +`restart_offset` to be `28`. + +ref record +++++++++++ + +A `ref_record` describes a single reference, storing both the name and +its value(s). Records are formatted as: + +.... +varint( prefix_length ) +varint( (suffix_length << 3) | value_type ) +suffix +varint( update_index_delta ) +value? +.... + +The `prefix_length` field specifies how many leading bytes of the prior +reference record's name should be copied to obtain this reference's +name. This must be 0 for the first reference in any block, and also must +be 0 for any `ref_record` whose offset is listed in the `restart_offset` +table at the end of the block. + +Recovering a reference name from any `ref_record` is a simple concat: + +.... +this_name = prior_name[0..prefix_length] + suffix +.... + +The `suffix_length` value provides the number of bytes available in +`suffix` to copy from `suffix` to complete the reference name. + +The `update_index` that last modified the reference can be obtained by +adding `update_index_delta` to the `min_update_index` from the file +header: `min_update_index + update_index_delta`. + +The `value` follows. Its format is determined by `value_type`, one of +the following: + +* `0x0`: deletion; no value data (see transactions, below) +* `0x1`: one 20-byte object name; value of the ref +* `0x2`: two 20-byte object names; value of the ref, peeled target +* `0x3`: symbolic reference: `varint( target_len ) target` + +Symbolic references use `0x3`, followed by the complete name of the +reference target. No compression is applied to the target name. + +Types `0x4..0x7` are reserved for future use. + +Ref index +^^^^^^^^^ + +The ref index stores the name of the last reference from every ref block +in the file, enabling reduced disk seeks for lookups. Any reference can +be found by searching the index, identifying the containing block, and +searching within that block. + +The index may be organized into a multi-level index, where the 1st level +index block points to additional ref index blocks (2nd level), which may +in turn point to either additional index blocks (e.g. 3rd level) or ref +blocks (leaf level). Disk reads required to access a ref go up with +higher index levels. Multi-level indexes may be required to ensure no +single index block exceeds the file format's max block size of +`16777215` bytes (15.99 MiB). To achieve constant O(1) disk seeks for +lookups the index must be a single level, which is permitted to exceed +the file's configured block size, but not the format's max block size of +15.99 MiB. + +If present, the ref index block(s) appears after the last ref block. + +If there are at least 4 ref blocks, a ref index block should be written +to improve lookup times. Cold reads using the index require 2 disk reads +(read index, read block), and binary searching < 4 blocks also requires +<= 2 reads. Omitting the index block from smaller files saves space. + +If the file is unaligned and contains more than one ref block, the ref +index must be written. + +Index block format: + +.... +'i' +uint24( block_len ) +index_record+ +uint24( restart_offset )+ +uint16( restart_count ) + +padding? +.... + +The index blocks begin with `block_type = 'i'` and a 3-byte `block_len` +which encodes the number of bytes in the block, up to but not including +the optional `padding`. + +The `restart_offset` and `restart_count` fields are identical in format, +meaning and usage as in ref blocks. + +To reduce the number of reads required for random access in very large +files the index block may be larger than other blocks. However, readers +must hold the entire index in memory to benefit from this, so it's a +time-space tradeoff in both file size and reader memory. + +Increasing the file's block size decreases the index size. Alternatively +a multi-level index may be used, keeping index blocks within the file's +block size, but increasing the number of blocks that need to be +accessed. + +index record +++++++++++++ + +An index record describes the last entry in another block. Index records +are written as: + +.... +varint( prefix_length ) +varint( (suffix_length << 3) | 0 ) +suffix +varint( block_position ) +.... + +Index records use prefix compression exactly like `ref_record`. + +Index records store `block_position` after the suffix, specifying the +absolute position in bytes (from the start of the file) of the block +that ends with this reference. Readers can seek to `block_position` to +begin reading the block header. + +Readers must examine the block header at `block_position` to determine +if the next block is another level index block, or the leaf-level ref +block. + +Reading the index ++++++++++++++++++ + +Readers loading the ref index must first read the footer (below) to +obtain `ref_index_position`. If not present, the position will be 0. The +`ref_index_position` is for the 1st level root of the ref index. + +Obj block format +^^^^^^^^^^^^^^^^ + +Object blocks are optional. Writers may choose to omit object blocks, +especially if readers will not use the SHA-1 to ref mapping. + +Object blocks use unique, abbreviated 2-20 byte SHA-1 keys, mapping to +ref blocks containing references pointing to that object directly, or as +the peeled value of an annotated tag. Like ref blocks, object blocks use +the file's standard block size. The abbrevation length is available in +the footer as `obj_id_len`. + +To save space in small files, object blocks may be omitted if the ref +index is not present, as brute force search will only need to read a few +ref blocks. When missing, readers should brute force a linear search of +all references to lookup by SHA-1. + +An object block is written as: + +.... +'o' +uint24( block_len ) +obj_record+ +uint24( restart_offset )+ +uint16( restart_count ) + +padding? +.... + +Fields are identical to ref block. Binary search using the restart table +works the same as in reference blocks. + +Because object names are abbreviated by writers to the shortest +unique abbreviation within the reftable, obj key lengths are variable +between 2 and 20 bytes. Readers must compare only for common prefix +match within an obj block or obj index. + +obj record +++++++++++ + +An `obj_record` describes a single object abbreviation, and the blocks +containing references using that unique abbreviation: + +.... +varint( prefix_length ) +varint( (suffix_length << 3) | cnt_3 ) +suffix +varint( cnt_large )? +varint( position_delta )* +.... + +Like in reference blocks, abbreviations are prefix compressed within an +obj block. On large reftables with many unique objects, higher block +sizes (64k), and higher restart interval (128), a `prefix_length` of 2 +or 3 and `suffix_length` of 3 may be common in obj records (unique +abbreviation of 5-6 raw bytes, 10-12 hex digits). + +Each record contains `position_count` number of positions for matching +ref blocks. For 1-7 positions the count is stored in `cnt_3`. When +`cnt_3 = 0` the actual count follows in a varint, `cnt_large`. + +The use of `cnt_3` bets most objects are pointed to by only a single +reference, some may be pointed to by a couple of references, and very +few (if any) are pointed to by more than 7 references. + +A special case exists when `cnt_3 = 0` and `cnt_large = 0`: there are no +`position_delta`, but at least one reference starts with this +abbreviation. A reader that needs exact reference names must scan all +references to find which specific references have the desired object. +Writers should use this format when the `position_delta` list would have +overflowed the file's block size due to a high number of references +pointing to the same object. + +The first `position_delta` is the position from the start of the file. +Additional `position_delta` entries are sorted ascending and relative to +the prior entry, e.g. a reader would perform: + +.... +pos = position_delta[0] +prior = pos +for (j = 1; j < position_count; j++) { + pos = prior + position_delta[j] + prior = pos +} +.... + +With a position in hand, a reader must linearly scan the ref block, +starting from the first `ref_record`, testing each reference's SHA-1s +(for `value_type = 0x1` or `0x2`) for full equality. Faster searching by +SHA-1 within a single ref block is not supported by the reftable format. +Smaller block sizes reduce the number of candidates this step must +consider. + +Obj index +^^^^^^^^^ + +The obj index stores the abbreviation from the last entry for every obj +block in the file, enabling reduced disk seeks for all lookups. It is +formatted exactly the same as the ref index, but refers to obj blocks. + +The obj index should be present if obj blocks are present, as obj blocks +should only be written in larger files. + +Readers loading the obj index must first read the footer (below) to +obtain `obj_index_position`. If not present, the position will be 0. + +Log block format +^^^^^^^^^^^^^^^^ + +Unlike ref and obj blocks, log blocks are always unaligned. + +Log blocks are variable in size, and do not match the `block_size` +specified in the file header or footer. Writers should choose an +appropriate buffer size to prepare a log block for deflation, such as +`2 * block_size`. + +A log block is written as: + +.... +'g' +uint24( block_len ) +zlib_deflate { + log_record+ + uint24( restart_offset )+ + uint16( restart_count ) +} +.... + +Log blocks look similar to ref blocks, except `block_type = 'g'`. + +The 4-byte block header is followed by the deflated block contents using +zlib deflate. The `block_len` in the header is the inflated size +(including 4-byte block header), and should be used by readers to +preallocate the inflation output buffer. A log block's `block_len` may +exceed the file's block size. + +Offsets within the log block (e.g. `restart_offset`) still include the +4-byte header. Readers may prefer prefixing the inflation output buffer +with the 4-byte header. + +Within the deflate container, a variable number of `log_record` describe +reference changes. The log record format is described below. See ref +block format (above) for a description of `restart_offset` and +`restart_count`. + +Because log blocks have no alignment or padding between blocks, readers +must keep track of the bytes consumed by the inflater to know where the +next log block begins. + +log record +++++++++++ + +Log record keys are structured as: + +.... +ref_name '\0' reverse_int64( update_index ) +.... + +where `update_index` is the unique transaction identifier. The +`update_index` field must be unique within the scope of a `ref_name`. +See the update transactions section below for further details. + +The `reverse_int64` function inverses the value so lexicographical +ordering the network byte order encoding sorts the more recent records +with higher `update_index` values first: + +.... +reverse_int64(int64 t) { + return 0xffffffffffffffff - t; +} +.... + +Log records have a similar starting structure to ref and index records, +utilizing the same prefix compression scheme applied to the log record +key described above. + +.... + varint( prefix_length ) + varint( (suffix_length << 3) | log_type ) + suffix + log_data { + old_id + new_id + varint( name_length ) name + varint( email_length ) email + varint( time_seconds ) + sint16( tz_offset ) + varint( message_length ) message + }? +.... + +Log record entries use `log_type` to indicate what follows: + +* `0x0`: deletion; no log data. +* `0x1`: standard git reflog data using `log_data` above. + +The `log_type = 0x0` is mostly useful for `git stash drop`, removing an +entry from the reflog of `refs/stash` in a transaction file (below), +without needing to rewrite larger files. Readers reading a stack of +reflogs must treat this as a deletion. + +For `log_type = 0x1`, the `log_data` section follows +linkgit:git-update-ref[1] logging and includes: + +* two 20-byte SHA-1s (old id, new id) +* varint string of committer's name +* varint string of committer's email +* varint time in seconds since epoch (Jan 1, 1970) +* 2-byte timezone offset in minutes (signed) +* varint string of message + +`tz_offset` is the absolute number of minutes from GMT the committer was +at the time of the update. For example `GMT-0800` is encoded in reftable +as `sint16(-480)` and `GMT+0230` is `sint16(150)`. + +The committer email does not contain `<` or `>`, it's the value normally +found between the `<>` in a git commit object header. + +The `message_length` may be 0, in which case there was no message +supplied for the update. + +Contrary to traditional reflog (which is a file), renames are encoded as +a combination of ref deletion and ref creation. A deletion is a log +record with a zero new_id, and a creation is a log record with a zero old_id. + +Reading the log ++++++++++++++++ + +Readers accessing the log must first read the footer (below) to +determine the `log_position`. The first block of the log begins at +`log_position` bytes since the start of the file. The `log_position` is +not block aligned. + +Importing logs +++++++++++++++ + +When importing from `$GIT_DIR/logs` writers should globally order all +log records roughly by timestamp while preserving file order, and assign +unique, increasing `update_index` values for each log line. Newer log +records get higher `update_index` values. + +Although an import may write only a single reftable file, the reftable +file must span many unique `update_index`, as each log line requires its +own `update_index` to preserve semantics. + +Log index +^^^^^^^^^ + +The log index stores the log key +(`refname \0 reverse_int64(update_index)`) for the last log record of +every log block in the file, supporting bounded-time lookup. + +A log index block must be written if 2 or more log blocks are written to +the file. If present, the log index appears after the last log block. +There is no padding used to align the log index to block alignment. + +Log index format is identical to ref index, except the keys are 9 bytes +longer to include `'\0'` and the 8-byte `reverse_int64(update_index)`. +Records use `block_position` to refer to the start of a log block. + +Reading the index ++++++++++++++++++ + +Readers loading the log index must first read the footer (below) to +obtain `log_index_position`. If not present, the position will be 0. + +Footer +^^^^^^ + +After the last block of the file, a file footer is written. It begins +like the file header, but is extended with additional data. + +A 68-byte footer appears at the end: + +.... + 'REFT' + uint8( version_number = 1 ) + uint24( block_size ) + uint64( min_update_index ) + uint64( max_update_index ) + + uint64( ref_index_position ) + uint64( (obj_position << 5) | obj_id_len ) + uint64( obj_index_position ) + + uint64( log_position ) + uint64( log_index_position ) + + uint32( CRC-32 of above ) +.... + +If a section is missing (e.g. ref index) the corresponding position +field (e.g. `ref_index_position`) will be 0. + +* `obj_position`: byte position for the first obj block. +* `obj_id_len`: number of bytes used to abbreviate object names in +obj blocks. +* `log_position`: byte position for the first log block. +* `ref_index_position`: byte position for the start of the ref index. +* `obj_index_position`: byte position for the start of the obj index. +* `log_index_position`: byte position for the start of the log index. + +Reading the footer +++++++++++++++++++ + +Readers must seek to `file_length - 68` to access the footer. A trusted +external source (such as `stat(2)`) is necessary to obtain +`file_length`. When reading the footer, readers must verify: + +* 4-byte magic is correct +* 1-byte version number is recognized +* 4-byte CRC-32 matches the other 64 bytes (including magic, and +version) + +Once verified, the other fields of the footer can be accessed. + +Binary search +^^^^^^^^^^^^^ + +Binary search within a block is supported by the `restart_offset` fields +at the end of the block. Readers can binary search through the restart +table to locate between which two restart points the sought reference or +key should appear. + +Each record identified by a `restart_offset` stores the complete key in +the `suffix` field of the record, making the compare operation during +binary search straightforward. + +Once a restart point lexicographically before the sought reference has +been identified, readers can linearly scan through the following record +entries to locate the sought record, terminating if the current record +sorts after (and therefore the sought key is not present). + +Restart point selection ++++++++++++++++++++++++ + +Writers determine the restart points at file creation. The process is +arbitrary, but every 16 or 64 records is recommended. Every 16 may be +more suitable for smaller block sizes (4k or 8k), every 64 for larger +block sizes (64k). + +More frequent restart points reduces prefix compression and increases +space consumed by the restart table, both of which increase file size. + +Less frequent restart points makes prefix compression more effective, +decreasing overall file size, with increased penalties for readers +walking through more records after the binary search step. + +A maximum of `65535` restart points per block is supported. + +Considerations +~~~~~~~~~~~~~~ + +Lightweight refs dominate +^^^^^^^^^^^^^^^^^^^^^^^^^ + +The reftable format assumes the vast majority of references are single +SHA-1 valued with common prefixes, such as Gerrit Code Review's +`refs/changes/` namespace, GitHub's `refs/pulls/` namespace, or many +lightweight tags in the `refs/tags/` namespace. + +Annotated tags storing the peeled object cost an additional 20 bytes per +reference. + +Low overhead +^^^^^^^^^^^^ + +A reftable with very few references (e.g. git.git with 5 heads) is 269 +bytes for reftable, vs. 332 bytes for packed-refs. This supports +reftable scaling down for transaction logs (below). + +Block size +^^^^^^^^^^ + +For a Gerrit Code Review type repository with many change refs, larger +block sizes (64 KiB) and less frequent restart points (every 64) yield +better compression due to more references within the block compressing +against the prior reference. + +Larger block sizes reduce the index size, as the reftable will require +fewer blocks to store the same number of references. + +Minimal disk seeks +^^^^^^^^^^^^^^^^^^ + +Assuming the index block has been loaded into memory, binary searching +for any single reference requires exactly 1 disk seek to load the +containing block. + +Scans and lookups dominate +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Scanning all references and lookup by name (or namespace such as +`refs/heads/`) are the most common activities performed on repositories. +SHA-1s are stored directly with references to optimize this use case. + +Logs are infrequently read +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Logs are infrequently accessed, but can be large. Deflating log blocks +saves disk space, with some increased penalty at read time. + +Logs are stored in an isolated section from refs, reducing the burden on +reference readers that want to ignore logs. Further, historical logs can +be isolated into log-only files. + +Logs are read backwards +^^^^^^^^^^^^^^^^^^^^^^^ + +Logs are frequently accessed backwards (most recent N records for master +to answer `master@{4}`), so log records are grouped by reference, and +sorted descending by update index. + +Repository format +~~~~~~~~~~~~~~~~~ + +Version 1 +^^^^^^^^^ + +A repository must set its `$GIT_DIR/config` to configure reftable: + +.... +[core] + repositoryformatversion = 1 +[extensions] + refStorage = reftable +.... + +Layout +^^^^^^ + +A collection of reftable files are stored in the `$GIT_DIR/reftable/` +directory: + +.... +00000001-00000001.log +00000002-00000002.ref +00000003-00000003.ref +.... + +where reftable files are named by a unique name such as produced by the +function `${min_update_index}-${max_update_index}.ref`. + +Log-only files use the `.log` extension, while ref-only and mixed ref +and log files use `.ref`. extension. + +The stack ordering file is `$GIT_DIR/reftable/tables.list` and lists the +current files, one per line, in order, from oldest (base) to newest +(most recent): + +.... +$ cat .git/reftable/tables.list +00000001-00000001.log +00000002-00000002.ref +00000003-00000003.ref +.... + +Readers must read `$GIT_DIR/reftable/tables.list` to determine which +files are relevant right now, and search through the stack in reverse +order (last reftable is examined first). + +Reftable files not listed in `tables.list` may be new (and about to be +added to the stack by the active writer), or ancient and ready to be +pruned. + +Backward compatibility +^^^^^^^^^^^^^^^^^^^^^^ + +Older clients should continue to recognize the directory as a git +repository so they don't look for an enclosing repository in parent +directories. To this end, a reftable-enabled repository must contain the +following dummy files + +* `.git/HEAD`, a regular file containing `ref: refs/heads/.invalid`. +* `.git/refs/`, a directory +* `.git/refs/heads`, a regular file + +Readers +^^^^^^^ + +Readers can obtain a consistent snapshot of the reference space by +following: + +1. Open and read the `tables.list` file. +2. Open each of the reftable files that it mentions. +3. If any of the files is missing, goto 1. +4. Read from the now-open files as long as necessary. + +Update transactions +^^^^^^^^^^^^^^^^^^^ + +Although reftables are immutable, mutations are supported by writing a +new reftable and atomically appending it to the stack: + +1. Acquire `tables.list.lock`. +2. Read `tables.list` to determine current reftables. +3. Select `update_index` to be most recent file's +`max_update_index + 1`. +4. Prepare temp reftable `tmp_XXXXXX`, including log entries. +5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`. +6. Copy `tables.list` to `tables.list.lock`, appending file from (5). +7. Rename `tables.list.lock` to `tables.list`. + +During step 4 the new file's `min_update_index` and `max_update_index` +are both set to the `update_index` selected by step 3. All log records +for the transaction use the same `update_index` in their keys. This +enables later correlation of which references were updated by the same +transaction. + +Because a single `tables.list.lock` file is used to manage locking, the +repository is single-threaded for writers. Writers may have to busy-spin +(with backoff) around creating `tables.list.lock`, for up to an +acceptable wait period, aborting if the repository is too busy to +mutate. Application servers wrapped around repositories (e.g. Gerrit +Code Review) can layer their own lock/wait queue to improve fairness to +writers. + +Reference deletions +^^^^^^^^^^^^^^^^^^^ + +Deletion of any reference can be explicitly stored by setting the `type` +to `0x0` and omitting the `value` field of the `ref_record`. This serves +as a tombstone, overriding any assertions about the existence of the +reference from earlier files in the stack. + +Compaction +^^^^^^^^^^ + +A partial stack of reftables can be compacted by merging references +using a straightforward merge join across reftables, selecting the most +recent value for output, and omitting deleted references that do not +appear in remaining, lower reftables. + +A compacted reftable should set its `min_update_index` to the smallest +of the input files' `min_update_index`, and its `max_update_index` +likewise to the largest input `max_update_index`. + +For sake of illustration, assume the stack currently consists of +reftable files (from oldest to newest): A, B, C, and D. The compactor is +going to compact B and C, leaving A and D alone. + +1. Obtain lock `tables.list.lock` and read the `tables.list` file. +2. Obtain locks `B.lock` and `C.lock`. Ownership of these locks +prevents other processes from trying to compact these files. +3. Release `tables.list.lock`. +4. Compact `B` and `C` into a temp file +`${min_update_index}-${max_update_index}_XXXXXX`. +5. Reacquire lock `tables.list.lock`. +6. Verify that `B` and `C` are still in the stack, in that order. This +should always be the case, assuming that other processes are adhering to +the locking protocol. +7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to +`${min_update_index}-${max_update_index}.ref`. +8. Write the new stack to `tables.list.lock`, replacing `B` and `C` +with the file from (4). +9. Rename `tables.list.lock` to `tables.list`. +10. Delete `B` and `C`, perhaps after a short sleep to avoid forcing +readers to backtrack. + +This strategy permits compactions to proceed independently of updates. + +Each reftable (compacted or not) is uniquely identified by its name, so +open reftables can be cached by their name. + +Alternatives considered +~~~~~~~~~~~~~~~~~~~~~~~ + +bzip packed-refs +^^^^^^^^^^^^^^^^ + +`bzip2` can significantly shrink a large packed-refs file (e.g. 62 MiB +compresses to 23 MiB, 37%). However the bzip format does not support +random access to a single reference. Readers must inflate and discard +while performing a linear scan. + +Breaking packed-refs into chunks (individually compressing each chunk) +would reduce the amount of data a reader must inflate, but still leaves +the problem of indexing chunks to support readers efficiently locating +the correct chunk. + +Given the compression achieved by reftable's encoding, it does not seem +necessary to add the complexity of bzip/gzip/zlib. + +Michael Haggerty's alternate format +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Michael Haggerty proposed +link:https://lore.kernel.org/git/CAMy9T_HCnyc1g8XWOOWhe7nN0aEFyyBskV2aOMb_fe%2BwGvEJ7A%40mail.gmail.com/[an +alternate] format to reftable on the Git mailing list. This format uses +smaller chunks, without the restart table, and avoids block alignment +with padding. Reflog entries immediately follow each ref, and are thus +interleaved between refs. + +Performance testing indicates reftable is faster for lookups (51% +faster, 11.2 usec vs. 5.4 usec), although reftable produces a slightly +larger file (+ ~3.2%, 28.3M vs 29.2M): + +[cols=">,>,>,>",options="header",] +|===================================== +|format |size |seek cold |seek hot +|mh-alt |28.3 M |23.4 usec |11.2 usec +|reftable |29.2 M |19.9 usec |5.4 usec +|===================================== + +JGit Ketch RefTree +^^^^^^^^^^^^^^^^^^ + +https://dev.eclipse.org/mhonarc/lists/jgit-dev/msg03073.html[JGit Ketch] +proposed +link:https://lore.kernel.org/git/CAJo%3DhJvnAPNAdDcAAwAvU9C4RVeQdoS3Ev9WTguHx4fD0V_nOg%40mail.gmail.com/[RefTree], +an encoding of references inside Git tree objects stored as part of the +repository's object database. + +The RefTree format adds additional load on the object database storage +layer (more loose objects, more objects in packs), and relies heavily on +the packer's delta compression to save space. Namespaces which are flat +(e.g. thousands of tags in refs/tags) initially create very large loose +objects, and so RefTree does not address the problem of copying many +references to modify a handful. + +Flat namespaces are not efficiently searchable in RefTree, as tree +objects in canonical formatting cannot be binary searched. This fails +the need to handle a large number of references in a single namespace, +such as GitHub's `refs/pulls`, or a project with many tags. + +LMDB +^^^^ + +David Turner proposed +https://lore.kernel.org/git/1455772670-21142-26-git-send-email-dturner@twopensource.com/[using +LMDB], as LMDB is lightweight (64k of runtime code) and GPL-compatible +license. + +A downside of LMDB is its reliance on a single C implementation. This +makes embedding inside JGit (a popular reimplementation of Git) +difficult, and hoisting onto virtual storage (for JGit DFS) virtually +impossible. + +A common format that can be supported by all major Git implementations +(git-core, JGit, libgit2) is strongly preferred. + +Future +~~~~~~ + +Longer hashes +^^^^^^^^^^^^^ + +Version will bump (e.g. 2) to indicate `value` uses a different object +name length other than 20. The length could be stored in an expanded file +heaer, or hardcoded as part of the version. |