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 a SHA-1 is referred to by at least one reference (for allow-tip-sha1-in-want).
  • Efficient lookup of an entire namespace, such as refs/tags/.
  • Support atomic push O(size_of_update) operations.
  • Combine reflog storage with ref storage.

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 blocks. Prefix compression is used within a single block to reduce disk space. Block size is tunable by the writer.

Performance

Space used, packed-refs vs. reftable:

repositorypacked-refsreftable% originalavg refavg obj
android62.2 M34.4 M55.2%33 bytes8 bytes
rails1.8 M1.1 M57.7%29 bytes6 bytes
git78.7 K44.0 K60.0%50 bytes6 bytes
git (heads)332 b239 b72.0%31 bytes0 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):

formatcachescanby nameby SHA-1
packed-refscold402 ms409,660.1 usec412,535.8 usec
packed-refshot6,844.6 usec20,110.1 usec
reftablecold112 ms33.9 usec323.2 usec
reftablehot20.2 usec320.8 usec

Space used for 149,932 log entries for 43,061 refs, reflog vs. reftable:

formatsizeavg log
$GIT_DIR/logs173 M1209 bytes
reftable4 M30 bytes

Details

Peeling

References in a reftable are always peeled.

Reference name encoding

Reference names should be encoded with UTF-8.

Network byte order

All multi-byte, fixed width fields are in network byte order.

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_blocks*
ref_index?
obj_blocks*
obj_index?
log_blocks*
log_index?
footer

Block size

The 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 deflated 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).

Header

An 8-byte header appears at the beginning of the file:

'\1REF'
uint8( version_number = 1 )
uint24( block_size )

First ref block

The first ref block shares the same block as the file header, and is 8 bytes smaller than all other blocks in the file. The first block immediately begins after the file header, at offset 8.

Ref block format

A ref block is written as:

'r'
uint24 ( block_len )
ref_record+
uint32( 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 almost always shorter than the file's block_size. In the first ref block, block_len includes 8 bytes for the file header.

The 4-byte block header is followed by a variable number of ref_record, describing reference names and values. The format is described below.

A variable number of 4-byte restart_offset values follow the records. 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. Offsets in the first block are relative to the start of the file (position 0), and include the file header. This requires the first restart in the first block to be at offset 8.

The 2-byte restart_count stores the number of entries in the restart_offset list, which must not be empty.

Readers can use the restart count to binary search between restarts before starting a linear scan. The restart_count field must be the last 2 bytes of the block as specified by block_len from the block header.

The end of the record may be filled with padding NUL bytes to fill out the block to the common block_size as specified in the file header. Padding may be necessary to ensure the following block starts at a block alignment, and does not spill into the tail of this block. Padding may be omitted if the block is the last block of the file, or there is no index block. This allows reftable to efficiently scale down to a small number of refs.

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
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 to copy from suffix to complete the reference name.

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 id; value of the ref
  • 0x2: two 20-byte object ids; value of the ref, peeled target
  • 0x3: symref and text: varint( text_len ) text
  • 0x4: index record (see below)
  • 0x5: log record (see below)

Symbolic references use 0x3 with a text string starting with "ref: ", followed by the complete name of the reference target. No compression is applied to the target name. Other types of contents that are also reference like, such as FETCH_HEAD and MERGE_HEAD, may also be stored using type 0x3.

Types 0x6..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 constant O(1) disk seeks for all lookups. Any reference can be found by binary searching the index, identifying the containing block, and searching within that block.

If present, the ref index block appears after the last ref block. The prior ref block should be padded to ensure the ref index starts on a block alignment.

An index block should only be written if there are at least 4 blocks in the file, as cold reads using the index requires 2 disk reads, and binary searching <= 4 blocks also requires <= 2 reads. Omitting the index block from smaller files saves space.

Index block format:

uint32( (0x80 << 24) | block_len )
index_record+
uint32( restart_offset )+
uint16( restart_count )
padding?

The index block header starts with the high bit set. This identifies the block as an index block, and not as a ref block, log block or file footer. The block_len field in an index block is 30-bits network byte order, and allowed to occupy space normally used by the block type in other blocks. This supports indexes significantly larger than the file's block_size.

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 the 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 block size in the writer decreases the index size.

When object blocks are present the ref index block is padded with padding to maintain alignment for the next block. No padding is necessary if log blocks or the file trailer follows the ref index.

index record

An index record describes the last entry in another block. Index records are written as:

varint( prefix_length )
varint( (suffix_length << 3) | 0x4 )
suffix
varint( block_offset )

Index records use prefix compression exactly like ref_record.

Index records store block_offset after the suffix, specifying the offset in bytes (from the start of the file) of the block that ends with this reference.

Reading the index

Readers loading the ref index must first read the footer (below) to obtain ref_index_offset. If not present, the offset will be 0.

Obj block format

Object blocks use unique, abbreviated 2-20 byte SHA-1s 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.

To save space in small files, object blocks may be omitted if the ref index is not present. 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+
uint32( 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 identifiers 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.

Object blocks should be block aligned, according to block_size from the file header. The padding field is filled with NULs to maintain alignment for the next block.

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_rest )?
varint( block_delta )+

Like in reference blocks, abbreviations are prefix compressed within an obj block. On large reftable files 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 block_count number of block identifiers for ref blocks. The block_count is determined by:

block_count = cnt_3
if (cnt_3 == 0x7) {
  block_count += cnt_rest
}

The cnt_rest field is only present when block_count >= 0x7 and could overflow the cnt_3 field available in the record start. This encoding scheme is used as the vast majority of abbreviations are only one reference (or at most a few), and unlikely to exceed 6 blocks.

The first block_delta is the absolute block identifier counting from the start of the file. The offset of that block can be obtained by block_delta[0] * block_size. Additional block_delta entries are relative to the prior entry, e.g. a reader would perform:

block_id = block_delta[0]
prior = block_id
for (j = 1; j < block_count; j++) {
  block_id = prior + block_delta[j]
  prior = block_id
}

With a block_id in hand, a reader must linearly scan the ref block at block_id * block_size offset in the file, 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 reduces 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 constant O(1) 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.

The obj index should be block aligned, according to block_size from the file header. This requires padding the last obj block to maintain alignment.

Readers loading the obj index must first read the footer (below) to obtain obj_index_offset. If not present, the offset will be 0.

Log block format

Unlike ref and obj blocks, log block sizes 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+
  int32( restart_offset )+
  int16( restart_count )
}

Log blocks look similar to ref blocks, except block_type = 'g'.

The 4-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. Offsets within the 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.

Unlike ref blocks, log blocks are written at any alignment, without padding. The first log block immediately follows the end of the prior block, which omits its trailing padding. In very small files the log block may appear in the first block.

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( time_usec )

where time_usec is the update time in microseconds since the epoch. The reverse_int64 function inverses the value so lexographical ordering the network byte order time sorts more recent records first:

reverse_int64(int64 t) {
  return 0xffffffffffffffff - t;
}

The time_usec field must be unique within the scope of a ref_name. Writers working from seconds precision source are recomended to add 999999 microseconds to the timestamp, and decrement microseconds from older entries within the same second to prevent duplicates. Truncating to seconds on read will restore the original values.

Log records have a similar starting structure to ref and index records, utilizing the same prefix compression scheme applied to the key described above.

    varint( prefix_length )
    varint( (suffix_length << 3) | 0x5 )
    suffix

    old_id
    new_id
    sint16( tz_offset )
    varint( name_length  )   name
    varint( email_length )   email
    varint( message_length ) message

The value data following the key suffix is complex:

  • two 20-byte SHA-1s (old id, new id)
  • 2-byte timezone offset (signed)
  • varint string of committer's name
  • varint string of committer's email
  • 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 int16(-480) and GMT+0230 is int16(150).

The message_length may be 0, in which case there was no message supplied for the update.

Reading the log

Readers accessing the log must first read the footer (below) to determine the log_offset. The first block of the log begins at log_offset bytes since the start of the file. The log_offset is not block aligned.

Log index

The log index stores the log key (refname \0 reverse_int64(time_sec)) 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 first 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 5 bytes longer to include '\0' and the 4-byte reverse_int64(time). Records use block_offset 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_offset. If not present, the offset 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 52-byte footer appears at the end:

    '\1REF'
    uint8( version_number = 1 )
    uint24( block_size )
    uint64( ref_index_offset )

    uint64( obj_offset )
    uint64( obj_index_offset )

    uint64( log_offset )
    uint64( log_index_offset )

    uint32( CRC-32 of prior )

If a section is missing (e.g. ref index) the corresponding offset field (e.g. ref_index_offset) will be 0.

  • obj_offset: byte offset for the first obj block.
  • log_offset: byte offset for the first log block.
  • ref_index_offset: byte offset for the start of the ref index.
  • obj_index_offset: byte offset for the start of the obj index.
  • log_index_offset: byte offset for the start of the log index.

Reading the footer

Readers must seek to file_length - 64 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 48 bytes (including magic, and version)

Once verified, the other fields of the footer can be accessed.

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 = val << 7
  val = val | (buf[ptr] & 0x7f)
}

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 penalities 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 only an additional 20 bytes per reference.

Low overhead

A reftable with very few references (e.g. git.git with 5 heads) is 251 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 reduces 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/) is the most common activity performed by repositories. SHA-1s are stored twice when obj blocks are present, avoiding disk seeks for the common cases of scan and lookup by name.

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.

Logs are read backwards

Logs are frequently accessed backwards (most recent N records for master), so log records are grouped by reference, and sorted descending by time.

Repository format

Version 1

A repository must set its $GIT_DIR/config to configure reftable:

[core]
    repositoryformatversion = 1
[extensions]
    reftable = 1

Layout

The $GIT_DIR/refs path is a file when reftable is configured, not a directory. This prevents loose references from being stored.

A collection of reftable files are stored in the $GIT_DIR/reftable/ directory:

1500398516_UF4paF
1500398517_bUVgy4

where reftable files are named by a unique name such as produced by the function:

mktemp "${seconds_since_epoch}_XXXXXX"

The stack ordering file is $GIT_DIR/refs and lists the current files, one per line, in order, from oldest (base) to newest (most recent):

$ cat .git/refs
1500398516_UF4paF
1500398517_bUVgy4

Readers must read $GIT_DIR/refs 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 refs may be new (and about to be added to the stack by the active writer), or ancient and ready to be pruned.

Update transactions

Although reftables are immutable, mutations are supported by writing a new reftable and atomically appending it to the stack:

  1. Read refs to determine current reftables.
  2. Prepare new reftable ${time}_XXXXXX, including log entries.
  3. Acquire refs.lock.
  4. If refs differs from (1), verify file from (2) does not conflict.
  5. Copy refs to refs.lock, appending file from (2).
  6. Rename refs.lock to refs.

Because a single refs.lock file is used to manage locking, the repository is single-threaded for writers. Writers may have to busy-spin (with some small backoff) around creating refs.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 in memory thread lock/wait queue to improve fairness.

By preparing the update in steps 1-2 without the lock held, concurrent updaters may be able to update unrelated references in a safe way, with a minimal critical section. This requires updaters to verify in 4 that the stack has not changed in a meaningful way, and to gracefully abort by deleting ${time}_XXXXXX if a conflict is detected.

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 entry shadows the reference in 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.

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 refs.lock and read the refs file.
  2. Obtain locks B.lock and C.lock. Ownership of these locks prevents other processes from trying to compact these files.
  3. Release refs.lock.
  4. Compact B and C into a new file ${time}_XXXXXX, where ${time} is the latest time prefix, likely from C.
  5. Reacquire lock refs.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. Write the new stack to refs.lock, replacing B and C with the file from (4).
  8. Rename refs.lock to refs.
  9. Delete B and C, perhaps after a short sleep to avoid forcing readers to backtrack.

This strategy permits compactions to proceed independently of updates.

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.

JGit Ketch RefTree

JGit Ketch proposed 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 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 reimplemenation 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 id length other than 20. The length could be stored in an expanded file header, or hardcoded as part of the version.