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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.</p></div> <div class="paragraph"><p>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).</p></div> <div class="paragraph"><p>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 <code>$GIT_DIR/refs</code> directory.</p></div> </div> <div class="sect3"> <h4 id="_objectives">Objectives</h4> <div class="ulist"><ul> <li> <p> Near constant time lookup for any single reference, even when the repository is cold and not in process or kernel cache. </p> </li> <li> <p> Near constant time verification if an object name is referred to by at least one reference (for allow-tip-sha1-in-want). </p> </li> <li> <p> Efficient enumeration of an entire namespace, such as <code>refs/tags/</code>. </p> </li> <li> <p> Support atomic push with <code>O(size_of_update)</code> operations. </p> </li> <li> <p> Combine reflog storage with ref storage for small transactions. </p> </li> <li> <p> Separate reflog storage for base refs and historical logs. </p> </li> </ul></div> </div> <div class="sect3"> <h4 id="_description">Description</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>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 are tunable by the writer.</p></div> </div> <div class="sect3"> <h4 id="_performance">Performance</h4> <div class="paragraph"><p>Space used, packed-refs vs. reftable:</p></div> <div class="tableblock"> <table rules="all" width="100%" frame="border" cellspacing="0" cellpadding="4"> <col width="16%" /> <col width="16%" /> <col width="16%" /> <col width="16%" /> <col width="16%" /> <col width="16%" /> <thead> <tr> <th align="left" valign="top">repository </th> <th align="right" valign="top">packed-refs </th> <th align="right" valign="top">reftable </th> <th align="right" valign="top">% original </th> <th align="right" valign="top">avg ref </th> <th align="right" valign="top">avg obj</th> </tr> </thead> <tbody> <tr> <td align="left" valign="top"><p class="table">android</p></td> <td align="right" valign="top"><p class="table">62.2 M</p></td> <td align="right" valign="top"><p class="table">36.1 M</p></td> <td align="right" valign="top"><p class="table">58.0%</p></td> <td align="right" valign="top"><p class="table">33 bytes</p></td> <td align="right" valign="top"><p class="table">5 bytes</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">rails</p></td> <td align="right" valign="top"><p class="table">1.8 M</p></td> <td align="right" valign="top"><p class="table">1.1 M</p></td> <td align="right" valign="top"><p class="table">57.7%</p></td> <td align="right" valign="top"><p class="table">29 bytes</p></td> <td align="right" valign="top"><p class="table">4 bytes</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">git</p></td> <td align="right" valign="top"><p class="table">78.7 K</p></td> <td align="right" valign="top"><p class="table">48.1 K</p></td> <td align="right" valign="top"><p class="table">61.0%</p></td> <td align="right" valign="top"><p class="table">50 bytes</p></td> <td align="right" valign="top"><p class="table">4 bytes</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">git (heads)</p></td> <td align="right" valign="top"><p class="table">332 b</p></td> <td align="right" valign="top"><p class="table">269 b</p></td> <td align="right" valign="top"><p class="table">81.0%</p></td> <td align="right" valign="top"><p class="table">33 bytes</p></td> <td align="right" valign="top"><p class="table">0 bytes</p></td> </tr> </tbody> </table> </div> <div class="paragraph"><p>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):</p></div> <div class="tableblock"> <table rules="all" width="100%" frame="border" cellspacing="0" cellpadding="4"> <col width="20%" /> <col width="20%" /> <col width="20%" /> <col width="20%" /> <col width="20%" /> <thead> <tr> <th align="left" valign="top">format </th> <th align="right" valign="top">cache </th> <th align="right" valign="top">scan </th> <th align="right" valign="top">by name </th> <th align="right" valign="top">by SHA-1</th> </tr> </thead> <tbody> <tr> <td align="left" valign="top"><p class="table">packed-refs</p></td> <td align="right" valign="top"><p class="table">cold</p></td> <td align="right" valign="top"><p class="table">402 ms</p></td> <td align="right" valign="top"><p class="table">409,660.1 usec</p></td> <td align="right" valign="top"><p class="table">412,535.8 usec</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">packed-refs</p></td> <td align="right" valign="top"><p class="table">hot</p></td> <td align="right" valign="top"><p class="table"></p></td> <td align="right" valign="top"><p class="table">6,844.6 usec</p></td> <td align="right" valign="top"><p class="table">20,110.1 usec</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">reftable</p></td> <td align="right" valign="top"><p class="table">cold</p></td> <td align="right" valign="top"><p class="table">112 ms</p></td> <td align="right" valign="top"><p class="table">33.9 usec</p></td> <td align="right" valign="top"><p class="table">323.2 usec</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">reftable</p></td> <td align="right" valign="top"><p class="table">hot</p></td> <td align="right" valign="top"><p class="table"></p></td> <td align="right" valign="top"><p class="table">20.2 usec</p></td> <td align="right" valign="top"><p class="table">320.8 usec</p></td> </tr> </tbody> </table> </div> <div class="paragraph"><p>Space used for 149,932 log entries for 43,061 refs, reflog vs. reftable:</p></div> <div class="tableblock"> <table rules="all" width="100%" frame="border" cellspacing="0" cellpadding="4"> <col width="33%" /> <col width="33%" /> <col width="33%" /> <thead> <tr> <th align="left" valign="top">format </th> <th align="right" valign="top">size </th> <th align="right" valign="top">avg entry</th> </tr> </thead> <tbody> <tr> <td align="left" valign="top"><p class="table">$GIT_DIR/logs</p></td> <td align="right" valign="top"><p class="table">173 M</p></td> <td align="right" valign="top"><p class="table">1209 bytes</p></td> </tr> <tr> <td align="left" valign="top"><p class="table">reftable</p></td> <td align="right" valign="top"><p class="table">5 M</p></td> <td align="right" valign="top"><p class="table">37 bytes</p></td> </tr> </tbody> </table> </div> </div> </div> <div class="sect2"> <h3 id="_details">Details</h3> <div class="sect3"> <h4 id="_peeling">Peeling</h4> <div class="paragraph"><p>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.</p></div> </div> <div class="sect3"> <h4 id="_reference_name_encoding">Reference name encoding</h4> <div class="paragraph"><p>Reference names are an uninterpreted sequence of bytes that must pass <a href="../git-check-ref-format.html">git-check-ref-format(1)</a> as a valid reference name.</p></div> </div> <div class="sect3"> <h4 id="_key_unicity">Key unicity</h4> <div class="paragraph"><p>Each entry must have a unique key; repeated keys are disallowed.</p></div> </div> <div class="sect3"> <h4 id="_network_byte_order">Network byte order</h4> <div class="paragraph"><p>All multi-byte, fixed width fields are in network byte order.</p></div> </div> <div class="sect3"> <h4 id="_varint_encoding">Varint encoding</h4> <div class="paragraph"><p>Varint encoding is identical to the ofs-delta encoding method used within pack files.</p></div> <div class="paragraph"><p>Decoder works as follows:</p></div> <div class="literalblock"> <div class="content"> <pre><code>val = buf[ptr] & 0x7f while (buf[ptr] & 0x80) { ptr++ val = ((val + 1) << 7) | (buf[ptr] & 0x7f) }</code></pre> </div></div> </div> <div class="sect3"> <h4 id="_ordering">Ordering</h4> <div class="paragraph"><p>Blocks are lexicographically ordered by their first reference.</p></div> </div> <div class="sect3"> <h4 id="_directory_file_conflicts">Directory/file conflicts</h4> <div class="paragraph"><p>The reftable format accepts both <code>refs/heads/foo</code> and <code>refs/heads/foo/bar</code> as distinct references.</p></div> <div class="paragraph"><p>This property is useful for retaining log records in reftable, but may confuse versions of Git using <code>$GIT_DIR/refs</code> directory tree to maintain references. Users of reftable may choose to continue to reject <code>foo</code> and <code>foo/bar</code> type conflicts to prevent problems for peers.</p></div> </div> </div> <div class="sect2"> <h3 id="_file_format">File format</h3> <div class="sect3"> <h4 id="_structure">Structure</h4> <div class="paragraph"><p>A reftable file has the following high-level structure:</p></div> <div class="literalblock"> <div class="content"> <pre><code>first_block { header first_ref_block } ref_block* ref_index* obj_block* obj_index* log_block* log_index* footer</code></pre> </div></div> <div class="paragraph"><p>A log-only file omits the <code>ref_block</code>, <code>ref_index</code>, <code>obj_block</code> and <code>obj_index</code> sections, containing only the file header and log block:</p></div> <div class="literalblock"> <div class="content"> <pre><code>first_block { header } log_block* log_index* footer</code></pre> </div></div> <div class="paragraph"><p>In a log-only file, the first log block immediately follows the file header, without padding to block alignment.</p></div> </div> <div class="sect3"> <h4 id="_block_size">Block size</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>The largest block size is <code>16777215</code> bytes (15.99 MiB).</p></div> </div> <div class="sect3"> <h4 id="_block_alignment">Block alignment</h4> <div class="paragraph"><p>Writers may choose to align blocks at multiples of the block size by including <code>padding</code> 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 <code>block_size</code> field.</p></div> <div class="paragraph"><p>Block alignment is not required by the file format. Unaligned files must set <code>block_size = 0</code> in the file header, and omit <code>padding</code>. Unaligned files with more than one ref block must include the <a href="#Ref-index">ref index</a> to support fast lookup. Readers must be able to read both aligned and non-aligned files.</p></div> <div class="paragraph"><p>Very small files (e.g. a single ref block) may omit <code>padding</code> and the ref index to reduce total file size.</p></div> </div> <div class="sect3"> <h4 id="_header_version_1">Header (version 1)</h4> <div class="paragraph"><p>A 24-byte header appears at the beginning of the file:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'REFT' uint8( version_number = 1 ) uint24( block_size ) uint64( min_update_index ) uint64( max_update_index )</code></pre> </div></div> <div class="paragraph"><p>Aligned files must specify <code>block_size</code> to configure readers with the expected block alignment. Unaligned files must set <code>block_size = 0</code>.</p></div> <div class="paragraph"><p>The <code>min_update_index</code> and <code>max_update_index</code> describe bounds for the <code>update_index</code> field of all log records in this file. When reftables are used in a stack for <a href="#Update-transactions">transactions</a>, these fields can order the files such that the prior file’s <code>max_update_index + 1</code> is the next file’s <code>min_update_index</code>.</p></div> </div> <div class="sect3"> <h4 id="_header_version_2">Header (version 2)</h4> <div class="paragraph"><p>A 28-byte header appears at the beginning of the file:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'REFT' uint8( version_number = 2 ) uint24( block_size ) uint64( min_update_index ) uint64( max_update_index ) uint32( hash_id )</code></pre> </div></div> <div class="paragraph"><p>The header is identical to <code>version_number=1</code>, with the 4-byte hash ID ("sha1" for SHA1 and "s256" for SHA-256) appended to the header.</p></div> <div class="paragraph"><p>For maximum backward compatibility, it is recommended to use version 1 when writing SHA1 reftables.</p></div> </div> <div class="sect3"> <h4 id="_first_ref_block">First ref block</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>If the first block is a log block (a log-only file), its block header begins immediately at position 24.</p></div> </div> <div class="sect3"> <h4 id="_ref_block_format">Ref block format</h4> <div class="paragraph"><p>A ref block is written as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'r' uint24( block_len ) ref_record+ uint24( restart_offset )+ uint16( restart_count ) padding?</code></pre> </div></div> <div class="paragraph"><p>Blocks begin with <code>block_type = 'r'</code> and a 3-byte <code>block_len</code> which encodes the number of bytes in the block up to, but not including the optional <code>padding</code>. This is always less than or equal to the file’s block size. In the first ref block, <code>block_len</code> includes 24 bytes for the file header.</p></div> <div class="paragraph"><p>The 2-byte <code>restart_count</code> stores the number of entries in the <code>restart_offset</code> list, which must not be empty. Readers can use <code>restart_count</code> to binary search between restarts before starting a linear scan.</p></div> <div class="paragraph"><p>Exactly <code>restart_count</code> 3-byte <code>restart_offset</code> values precede the <code>restart_count</code>. Offsets are relative to the start of the block and refer to the first byte of any <code>ref_record</code> whose name has not been prefix compressed. Entries in the <code>restart_offset</code> list must be sorted, ascending. Readers can start linear scans from any of these records.</p></div> <div class="paragraph"><p>A variable number of <code>ref_record</code> fill the middle of the block, describing reference names and values. The format is described below.</p></div> <div class="paragraph"><p>As the first ref block shares the first file block with the file header, all <code>restart_offset</code> in the first block are relative to the start of the file (position 0), and include the file header. This forces the first <code>restart_offset</code> to be <code>28</code>.</p></div> <div class="sect4"> <h5 id="_ref_record">ref record</h5> <div class="paragraph"><p>A <code>ref_record</code> describes a single reference, storing both the name and its value(s). Records are formatted as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>varint( prefix_length ) varint( (suffix_length << 3) | value_type ) suffix varint( update_index_delta ) value?</code></pre> </div></div> <div class="paragraph"><p>The <code>prefix_length</code> 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 <code>ref_record</code> whose offset is listed in the <code>restart_offset</code> table at the end of the block.</p></div> <div class="paragraph"><p>Recovering a reference name from any <code>ref_record</code> is a simple concat:</p></div> <div class="literalblock"> <div class="content"> <pre><code>this_name = prior_name[0..prefix_length] + suffix</code></pre> </div></div> <div class="paragraph"><p>The <code>suffix_length</code> value provides the number of bytes available in <code>suffix</code> to copy from <code>suffix</code> to complete the reference name.</p></div> <div class="paragraph"><p>The <code>update_index</code> that last modified the reference can be obtained by adding <code>update_index_delta</code> to the <code>min_update_index</code> from the file header: <code>min_update_index + update_index_delta</code>.</p></div> <div class="paragraph"><p>The <code>value</code> follows. Its format is determined by <code>value_type</code>, one of the following:</p></div> <div class="ulist"><ul> <li> <p> <code>0x0</code>: deletion; no value data (see transactions, below) </p> </li> <li> <p> <code>0x1</code>: one object name; value of the ref </p> </li> <li> <p> <code>0x2</code>: two object names; value of the ref, peeled target </p> </li> <li> <p> <code>0x3</code>: symbolic reference: <code>varint( target_len ) target</code> </p> </li> </ul></div> <div class="paragraph"><p>Symbolic references use <code>0x3</code>, followed by the complete name of the reference target. No compression is applied to the target name.</p></div> <div class="paragraph"><p>Types <code>0x4..0x7</code> are reserved for future use.</p></div> </div> </div> <div class="sect3"> <h4 id="_ref_index">Ref index</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>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 <code>16777215</code> 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.</p></div> <div class="paragraph"><p>If present, the ref index block(s) appears after the last ref block.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>If the file is unaligned and contains more than one ref block, the ref index must be written.</p></div> <div class="paragraph"><p>Index block format:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'i' uint24( block_len ) index_record+ uint24( restart_offset )+ uint16( restart_count ) padding?</code></pre> </div></div> <div class="paragraph"><p>The index blocks begin with <code>block_type = 'i'</code> and a 3-byte <code>block_len</code> which encodes the number of bytes in the block, up to but not including the optional <code>padding</code>.</p></div> <div class="paragraph"><p>The <code>restart_offset</code> and <code>restart_count</code> fields are identical in format, meaning and usage as in ref blocks.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>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.</p></div> <div class="sect4"> <h5 id="_index_record">index record</h5> <div class="paragraph"><p>An index record describes the last entry in another block. Index records are written as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>varint( prefix_length ) varint( (suffix_length << 3) | 0 ) suffix varint( block_position )</code></pre> </div></div> <div class="paragraph"><p>Index records use prefix compression exactly like <code>ref_record</code>.</p></div> <div class="paragraph"><p>Index records store <code>block_position</code> 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 <code>block_position</code> to begin reading the block header.</p></div> <div class="paragraph"><p>Readers must examine the block header at <code>block_position</code> to determine if the next block is another level index block, or the leaf-level ref block.</p></div> </div> <div class="sect4"> <h5 id="_reading_the_index">Reading the index</h5> <div class="paragraph"><p>Readers loading the ref index must first read the footer (below) to obtain <code>ref_index_position</code>. If not present, the position will be 0. The <code>ref_index_position</code> is for the 1st level root of the ref index.</p></div> </div> </div> <div class="sect3"> <h4 id="_obj_block_format">Obj block format</h4> <div class="paragraph"><p>Object blocks are optional. Writers may choose to omit object blocks, especially if readers will not use the object name to ref mapping.</p></div> <div class="paragraph"><p>Object blocks use unique, abbreviated 2-31 byte object name 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 abbreviation length is available in the footer as <code>obj_id_len</code>.</p></div> <div class="paragraph"><p>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 object name.</p></div> <div class="paragraph"><p>An object block is written as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'o' uint24( block_len ) obj_record+ uint24( restart_offset )+ uint16( restart_count ) padding?</code></pre> </div></div> <div class="paragraph"><p>Fields are identical to ref block. Binary search using the restart table works the same as in reference blocks.</p></div> <div class="paragraph"><p>Because object names are abbreviated by writers to the shortest unique abbreviation within the reftable, obj key lengths have a variable length. Their length must be at least 2 bytes. Readers must compare only for common prefix match within an obj block or obj index.</p></div> <div class="sect4"> <h5 id="_obj_record">obj record</h5> <div class="paragraph"><p>An <code>obj_record</code> describes a single object abbreviation, and the blocks containing references using that unique abbreviation:</p></div> <div class="literalblock"> <div class="content"> <pre><code>varint( prefix_length ) varint( (suffix_length << 3) | cnt_3 ) suffix varint( cnt_large )? varint( position_delta )*</code></pre> </div></div> <div class="paragraph"><p>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 <code>prefix_length</code> of 2 or 3 and <code>suffix_length</code> of 3 may be common in obj records (unique abbreviation of 5-6 raw bytes, 10-12 hex digits).</p></div> <div class="paragraph"><p>Each record contains <code>position_count</code> number of positions for matching ref blocks. For 1-7 positions the count is stored in <code>cnt_3</code>. When <code>cnt_3 = 0</code> the actual count follows in a varint, <code>cnt_large</code>.</p></div> <div class="paragraph"><p>The use of <code>cnt_3</code> 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.</p></div> <div class="paragraph"><p>A special case exists when <code>cnt_3 = 0</code> and <code>cnt_large = 0</code>: there are no <code>position_delta</code>, 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 <code>position_delta</code> list would have overflowed the file’s block size due to a high number of references pointing to the same object.</p></div> <div class="paragraph"><p>The first <code>position_delta</code> is the position from the start of the file. Additional <code>position_delta</code> entries are sorted ascending and relative to the prior entry, e.g. a reader would perform:</p></div> <div class="literalblock"> <div class="content"> <pre><code>pos = position_delta[0] prior = pos for (j = 1; j < position_count; j++) { pos = prior + position_delta[j] prior = pos }</code></pre> </div></div> <div class="paragraph"><p>With a position in hand, a reader must linearly scan the ref block, starting from the first <code>ref_record</code>, testing each reference’s object names (for <code>value_type = 0x1</code> or <code>0x2</code>) for full equality. Faster searching by object name within a single ref block is not supported by the reftable format. Smaller block sizes reduce the number of candidates this step must consider.</p></div> </div> </div> <div class="sect3"> <h4 id="_obj_index">Obj index</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>The obj index should be present if obj blocks are present, as obj blocks should only be written in larger files.</p></div> <div class="paragraph"><p>Readers loading the obj index must first read the footer (below) to obtain <code>obj_index_position</code>. If not present, the position will be 0.</p></div> </div> <div class="sect3"> <h4 id="_log_block_format">Log block format</h4> <div class="paragraph"><p>Unlike ref and obj blocks, log blocks are always unaligned.</p></div> <div class="paragraph"><p>Log blocks are variable in size, and do not match the <code>block_size</code> specified in the file header or footer. Writers should choose an appropriate buffer size to prepare a log block for deflation, such as <code>2 * block_size</code>.</p></div> <div class="paragraph"><p>A log block is written as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>'g' uint24( block_len ) zlib_deflate { log_record+ uint24( restart_offset )+ uint16( restart_count ) }</code></pre> </div></div> <div class="paragraph"><p>Log blocks look similar to ref blocks, except <code>block_type = 'g'</code>.</p></div> <div class="paragraph"><p>The 4-byte block header is followed by the deflated block contents using zlib deflate. The <code>block_len</code> 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 <code>block_len</code> may exceed the file’s block size.</p></div> <div class="paragraph"><p>Offsets within the log block (e.g. <code>restart_offset</code>) still include the 4-byte header. Readers may prefer prefixing the inflation output buffer with the 4-byte header.</p></div> <div class="paragraph"><p>Within the deflate container, a variable number of <code>log_record</code> describe reference changes. The log record format is described below. See ref block format (above) for a description of <code>restart_offset</code> and <code>restart_count</code>.</p></div> <div class="paragraph"><p>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.</p></div> <div class="sect4"> <h5 id="_log_record">log record</h5> <div class="paragraph"><p>Log record keys are structured as:</p></div> <div class="literalblock"> <div class="content"> <pre><code>ref_name '\0' reverse_int64( update_index )</code></pre> </div></div> <div class="paragraph"><p>where <code>update_index</code> is the unique transaction identifier. The <code>update_index</code> field must be unique within the scope of a <code>ref_name</code>. See the update transactions section below for further details.</p></div> <div class="paragraph"><p>The <code>reverse_int64</code> function inverses the value so lexicographical ordering the network byte order encoding sorts the more recent records with higher <code>update_index</code> values first:</p></div> <div class="literalblock"> <div class="content"> <pre><code>reverse_int64(int64 t) { return 0xffffffffffffffff - t; }</code></pre> </div></div> <div class="paragraph"><p>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.</p></div> <div class="literalblock"> <div class="content"> <pre><code> 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 }?</code></pre> </div></div> <div class="paragraph"><p>Log record entries use <code>log_type</code> to indicate what follows:</p></div> <div class="ulist"><ul> <li> <p> <code>0x0</code>: deletion; no log data. </p> </li> <li> <p> <code>0x1</code>: standard git reflog data using <code>log_data</code> above. </p> </li> </ul></div> <div class="paragraph"><p>The <code>log_type = 0x0</code> is mostly useful for <code>git stash drop</code>, removing an entry from the reflog of <code>refs/stash</code> in a transaction file (below), without needing to rewrite larger files. Readers reading a stack of reflogs must treat this as a deletion.</p></div> <div class="paragraph"><p>For <code>log_type = 0x1</code>, the <code>log_data</code> section follows <a href="../git-update-ref.html">git-update-ref(1)</a> logging and includes:</p></div> <div class="ulist"><ul> <li> <p> two object names (old id, new id) </p> </li> <li> <p> varint string of committer’s name </p> </li> <li> <p> varint string of committer’s email </p> </li> <li> <p> varint time in seconds since epoch (Jan 1, 1970) </p> </li> <li> <p> 2-byte timezone offset in minutes (signed) </p> </li> <li> <p> varint string of message </p> </li> </ul></div> <div class="paragraph"><p><code>tz_offset</code> is the absolute number of minutes from GMT the committer was at the time of the update. For example <code>GMT-0800</code> is encoded in reftable as <code>sint16(-480)</code> and <code>GMT+0230</code> is <code>sint16(150)</code>.</p></div> <div class="paragraph"><p>The committer email does not contain <code><</code> or <code>></code>, it’s the value normally found between the <code><></code> in a git commit object header.</p></div> <div class="paragraph"><p>The <code>message_length</code> may be 0, in which case there was no message supplied for the update.</p></div> <div class="paragraph"><p>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.</p></div> </div> <div class="sect4"> <h5 id="_reading_the_log">Reading the log</h5> <div class="paragraph"><p>Readers accessing the log must first read the footer (below) to determine the <code>log_position</code>. The first block of the log begins at <code>log_position</code> bytes since the start of the file. The <code>log_position</code> is not block aligned.</p></div> </div> <div class="sect4"> <h5 id="_importing_logs">Importing logs</h5> <div class="paragraph"><p>When importing from <code>$GIT_DIR/logs</code> writers should globally order all log records roughly by timestamp while preserving file order, and assign unique, increasing <code>update_index</code> values for each log line. Newer log records get higher <code>update_index</code> values.</p></div> <div class="paragraph"><p>Although an import may write only a single reftable file, the reftable file must span many unique <code>update_index</code>, as each log line requires its own <code>update_index</code> to preserve semantics.</p></div> </div> </div> <div class="sect3"> <h4 id="_log_index">Log index</h4> <div class="paragraph"><p>The log index stores the log key (<code>refname \0 reverse_int64(update_index)</code>) for the last log record of every log block in the file, supporting bounded-time lookup.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>Log index format is identical to ref index, except the keys are 9 bytes longer to include <code>'\0'</code> and the 8-byte <code>reverse_int64(update_index)</code>. Records use <code>block_position</code> to refer to the start of a log block.</p></div> <div class="sect4"> <h5 id="_reading_the_index_2">Reading the index</h5> <div class="paragraph"><p>Readers loading the log index must first read the footer (below) to obtain <code>log_index_position</code>. If not present, the position will be 0.</p></div> </div> </div> <div class="sect3"> <h4 id="_footer">Footer</h4> <div class="paragraph"><p>After the last block of the file, a file footer is written. It begins like the file header, but is extended with additional data.</p></div> <div class="literalblock"> <div class="content"> <pre><code> HEADER 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 )</code></pre> </div></div> <div class="paragraph"><p>If a section is missing (e.g. ref index) the corresponding position field (e.g. <code>ref_index_position</code>) will be 0.</p></div> <div class="ulist"><ul> <li> <p> <code>obj_position</code>: byte position for the first obj block. </p> </li> <li> <p> <code>obj_id_len</code>: number of bytes used to abbreviate object names in obj blocks. </p> </li> <li> <p> <code>log_position</code>: byte position for the first log block. </p> </li> <li> <p> <code>ref_index_position</code>: byte position for the start of the ref index. </p> </li> <li> <p> <code>obj_index_position</code>: byte position for the start of the obj index. </p> </li> <li> <p> <code>log_index_position</code>: byte position for the start of the log index. </p> </li> </ul></div> <div class="paragraph"><p>The size of the footer is 68 bytes for version 1, and 72 bytes for version 2.</p></div> <div class="sect4"> <h5 id="_reading_the_footer">Reading the footer</h5> <div class="paragraph"><p>Readers must first read the file start to determine the version number. Then they seek to <code>file_length - FOOTER_LENGTH</code> to access the footer. A trusted external source (such as <code>stat(2)</code>) is necessary to obtain <code>file_length</code>. When reading the footer, readers must verify:</p></div> <div class="ulist"><ul> <li> <p> 4-byte magic is correct </p> </li> <li> <p> 1-byte version number is recognized </p> </li> <li> <p> 4-byte CRC-32 matches the other 64 bytes (including magic, and version) </p> </li> </ul></div> <div class="paragraph"><p>Once verified, the other fields of the footer can be accessed.</p></div> </div> <div class="sect4"> <h5 id="_empty_tables">Empty tables</h5> <div class="paragraph"><p>A reftable may be empty. In this case, the file starts with a header and is immediately followed by a footer.</p></div> </div> </div> <div class="sect3"> <h4 id="_binary_search">Binary search</h4> <div class="paragraph"><p>Binary search within a block is supported by the <code>restart_offset</code> 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.</p></div> <div class="paragraph"><p>Each record identified by a <code>restart_offset</code> stores the complete key in the <code>suffix</code> field of the record, making the compare operation during binary search straightforward.</p></div> <div class="paragraph"><p>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).</p></div> <div class="sect4"> <h5 id="_restart_point_selection">Restart point selection</h5> <div class="paragraph"><p>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).</p></div> <div class="paragraph"><p>More frequent restart points reduces prefix compression and increases space consumed by the restart table, both of which increase file size.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>A maximum of <code>65535</code> restart points per block is supported.</p></div> </div> </div> </div> <div class="sect2"> <h3 id="_considerations">Considerations</h3> <div class="sect3"> <h4 id="_lightweight_refs_dominate">Lightweight refs dominate</h4> <div class="paragraph"><p>The reftable format assumes the vast majority of references are single object names valued with common prefixes, such as Gerrit Code Review’s <code>refs/changes/</code> namespace, GitHub’s <code>refs/pulls/</code> namespace, or many lightweight tags in the <code>refs/tags/</code> namespace.</p></div> <div class="paragraph"><p>Annotated tags storing the peeled object cost an additional object name per reference.</p></div> </div> <div class="sect3"> <h4 id="_low_overhead">Low overhead</h4> <div class="paragraph"><p>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).</p></div> </div> <div class="sect3"> <h4 id="_block_size_2">Block size</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>Larger block sizes reduce the index size, as the reftable will require fewer blocks to store the same number of references.</p></div> </div> <div class="sect3"> <h4 id="_minimal_disk_seeks">Minimal disk seeks</h4> <div class="paragraph"><p>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.</p></div> </div> <div class="sect3"> <h4 id="_scans_and_lookups_dominate">Scans and lookups dominate</h4> <div class="paragraph"><p>Scanning all references and lookup by name (or namespace such as <code>refs/heads/</code>) are the most common activities performed on repositories. Object names are stored directly with references to optimize this use case.</p></div> </div> <div class="sect3"> <h4 id="_logs_are_infrequently_read">Logs are infrequently read</h4> <div class="paragraph"><p>Logs are infrequently accessed, but can be large. Deflating log blocks saves disk space, with some increased penalty at read time.</p></div> <div class="paragraph"><p>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.</p></div> </div> <div class="sect3"> <h4 id="_logs_are_read_backwards">Logs are read backwards</h4> <div class="paragraph"><p>Logs are frequently accessed backwards (most recent N records for master to answer <code>master@{4}</code>), so log records are grouped by reference, and sorted descending by update index.</p></div> </div> </div> <div class="sect2"> <h3 id="_repository_format">Repository format</h3> <div class="sect3"> <h4 id="_version_1">Version 1</h4> <div class="paragraph"><p>A repository must set its <code>$GIT_DIR/config</code> to configure reftable:</p></div> <div class="literalblock"> <div class="content"> <pre><code>[core] repositoryformatversion = 1 [extensions] refStorage = reftable</code></pre> </div></div> </div> <div class="sect3"> <h4 id="_layout">Layout</h4> <div class="paragraph"><p>A collection of reftable files are stored in the <code>$GIT_DIR/reftable/</code> directory. Their names should have a random element, such that each filename is globally unique; this helps avoid spurious failures on Windows, where open files cannot be removed or overwritten. It suggested to use <code>${min_update_index}-${max_update_index}-${random}.ref</code> as a naming convention.</p></div> <div class="paragraph"><p>Log-only files use the <code>.log</code> extension, while ref-only and mixed ref and log files use <code>.ref</code>. extension.</p></div> <div class="paragraph"><p>The stack ordering file is <code>$GIT_DIR/reftable/tables.list</code> and lists the current files, one per line, in order, from oldest (base) to newest (most recent):</p></div> <div class="literalblock"> <div class="content"> <pre><code>$ cat .git/reftable/tables.list 00000001-00000001-RANDOM1.log 00000002-00000002-RANDOM2.ref 00000003-00000003-RANDOM3.ref</code></pre> </div></div> <div class="paragraph"><p>Readers must read <code>$GIT_DIR/reftable/tables.list</code> to determine which files are relevant right now, and search through the stack in reverse order (last reftable is examined first).</p></div> <div class="paragraph"><p>Reftable files not listed in <code>tables.list</code> may be new (and about to be added to the stack by the active writer), or ancient and ready to be pruned.</p></div> </div> <div class="sect3"> <h4 id="_backward_compatibility">Backward compatibility</h4> <div class="paragraph"><p>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</p></div> <div class="ulist"><ul> <li> <p> <code>.git/HEAD</code>, a regular file containing <code>ref: refs/heads/.invalid</code>. </p> </li> <li> <p> <code>.git/refs/</code>, a directory </p> </li> <li> <p> <code>.git/refs/heads</code>, a regular file </p> </li> </ul></div> </div> <div class="sect3"> <h4 id="_readers">Readers</h4> <div class="paragraph"><p>Readers can obtain a consistent snapshot of the reference space by following:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Open and read the <code>tables.list</code> file. </p> </li> <li> <p> Open each of the reftable files that it mentions. </p> </li> <li> <p> If any of the files is missing, goto 1. </p> </li> <li> <p> Read from the now-open files as long as necessary. </p> </li> </ol></div> </div> <div class="sect3"> <h4 id="_update_transactions">Update transactions</h4> <div class="paragraph"><p>Although reftables are immutable, mutations are supported by writing a new reftable and atomically appending it to the stack:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Acquire <code>tables.list.lock</code>. </p> </li> <li> <p> Read <code>tables.list</code> to determine current reftables. </p> </li> <li> <p> Select <code>update_index</code> to be most recent file’s <code>max_update_index + 1</code>. </p> </li> <li> <p> Prepare temp reftable <code>tmp_XXXXXX</code>, including log entries. </p> </li> <li> <p> Rename <code>tmp_XXXXXX</code> to <code>${update_index}-${update_index}-${random}.ref</code>. </p> </li> <li> <p> Copy <code>tables.list</code> to <code>tables.list.lock</code>, appending file from (5). </p> </li> <li> <p> Rename <code>tables.list.lock</code> to <code>tables.list</code>. </p> </li> </ol></div> <div class="paragraph"><p>During step 4 the new file’s <code>min_update_index</code> and <code>max_update_index</code> are both set to the <code>update_index</code> selected by step 3. All log records for the transaction use the same <code>update_index</code> in their keys. This enables later correlation of which references were updated by the same transaction.</p></div> <div class="paragraph"><p>Because a single <code>tables.list.lock</code> file is used to manage locking, the repository is single-threaded for writers. Writers may have to busy-spin (with backoff) around creating <code>tables.list.lock</code>, 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.</p></div> </div> <div class="sect3"> <h4 id="_reference_deletions">Reference deletions</h4> <div class="paragraph"><p>Deletion of any reference can be explicitly stored by setting the <code>type</code> to <code>0x0</code> and omitting the <code>value</code> field of the <code>ref_record</code>. This serves as a tombstone, overriding any assertions about the existence of the reference from earlier files in the stack.</p></div> </div> <div class="sect3"> <h4 id="_compaction">Compaction</h4> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>A compacted reftable should set its ‘min_update_index` to the smallest of the input files’ <code>min_update_index</code>, and its <code>max_update_index</code> likewise to the largest input <code>max_update_index</code>.</p></div> <div class="paragraph"><p>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.</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Obtain lock <code>tables.list.lock</code> and read the <code>tables.list</code> file. </p> </li> <li> <p> Obtain locks <code>B.lock</code> and <code>C.lock</code>. Ownership of these locks prevents other processes from trying to compact these files. </p> </li> <li> <p> Release <code>tables.list.lock</code>. </p> </li> <li> <p> Compact <code>B</code> and <code>C</code> into a temp file <code>${min_update_index}-${max_update_index}_XXXXXX</code>. </p> </li> <li> <p> Reacquire lock <code>tables.list.lock</code>. </p> </li> <li> <p> Verify that <code>B</code> and <code>C</code> are still in the stack, in that order. This should always be the case, assuming that other processes are adhering to the locking protocol. </p> </li> <li> <p> Rename <code>${min_update_index}-${max_update_index}_XXXXXX</code> to <code>${min_update_index}-${max_update_index}-${random}.ref</code>. </p> </li> <li> <p> Write the new stack to <code>tables.list.lock</code>, replacing <code>B</code> and <code>C</code> with the file from (4). </p> </li> <li> <p> Rename <code>tables.list.lock</code> to <code>tables.list</code>. </p> </li> <li> <p> Delete <code>B</code> and <code>C</code>, perhaps after a short sleep to avoid forcing readers to backtrack. </p> </li> </ol></div> <div class="paragraph"><p>This strategy permits compactions to proceed independently of updates.</p></div> <div class="paragraph"><p>Each reftable (compacted or not) is uniquely identified by its name, so open reftables can be cached by their name.</p></div> </div> <div class="sect3"> <h4 id="_windows">Windows</h4> <div class="paragraph"><p>On windows, and other systems that do not allow deleting or renaming to open files, compaction may succeed, but other readers may prevent obsolete tables from being deleted.</p></div> <div class="paragraph"><p>On these platforms, the following strategy can be followed: on closing a reftable stack, reload <code>tables.list</code>, and delete any tables no longer mentioned in <code>tables.list</code>.</p></div> <div class="paragraph"><p>Irregular program exit may still leave about unused files. In this case, a cleanup operation should proceed as follows:</p></div> <div class="ulist"><ul> <li> <p> take a lock <code>tables.list.lock</code> to prevent concurrent modifications </p> </li> <li> <p> refresh the reftable stack, by reading <code>tables.list</code> </p> </li> <li> <p> for each <code>*.ref</code> file, remove it if </p> <div class="ulist"><ul> <li> <p> it is not mentioned in <code>tables.list</code>, and </p> </li> <li> <p> its max update_index is not beyond the max update_index of the stack </p> </li> </ul></div> </li> </ul></div> </div> </div> <div class="sect2"> <h3 id="_alternatives_considered">Alternatives considered</h3> <div class="sect3"> <h4 id="_bzip_packed_refs">bzip packed-refs</h4> <div class="paragraph"><p><code>bzip2</code> 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.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>Given the compression achieved by reftable’s encoding, it does not seem necessary to add the complexity of bzip/gzip/zlib.</p></div> </div> <div class="sect3"> <h4 id="_michael_haggerty_8217_s_alternate_format">Michael Haggerty’s alternate format</h4> <div class="paragraph"><p>Michael Haggerty proposed <a href="https://lore.kernel.org/git/CAMy9T_HCnyc1g8XWOOWhe7nN0aEFyyBskV2aOMb_fe%2BwGvEJ7A%40mail.gmail.com/">an alternate</a> 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.</p></div> <div class="paragraph"><p>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):</p></div> <div class="tableblock"> <table rules="all" width="100%" frame="border" cellspacing="0" cellpadding="4"> <col width="25%" /> <col width="25%" /> <col width="25%" /> <col width="25%" /> <thead> <tr> <th align="right" valign="top">format </th> <th align="right" valign="top">size </th> <th align="right" valign="top">seek cold </th> <th align="right" valign="top">seek hot</th> </tr> </thead> <tbody> <tr> <td align="right" valign="top"><p class="table">mh-alt</p></td> <td align="right" valign="top"><p class="table">28.3 M</p></td> <td align="right" valign="top"><p class="table">23.4 usec</p></td> <td align="right" valign="top"><p class="table">11.2 usec</p></td> </tr> <tr> <td align="right" valign="top"><p class="table">reftable</p></td> <td align="right" valign="top"><p class="table">29.2 M</p></td> <td align="right" valign="top"><p class="table">19.9 usec</p></td> <td align="right" valign="top"><p class="table">5.4 usec</p></td> </tr> </tbody> </table> </div> </div> <div class="sect3"> <h4 id="_jgit_ketch_reftree">JGit Ketch RefTree</h4> <div class="paragraph"><p><a href="https://dev.eclipse.org/mhonarc/lists/jgit-dev/msg03073.html">JGit Ketch</a> proposed <a href="https://lore.kernel.org/git/CAJo%3DhJvnAPNAdDcAAwAvU9C4RVeQdoS3Ev9WTguHx4fD0V_nOg%40mail.gmail.com/">RefTree</a>, an encoding of references inside Git tree objects stored as part of the repository’s object database.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>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 <code>refs/pulls</code>, or a project with many tags.</p></div> </div> <div class="sect3"> <h4 id="_lmdb">LMDB</h4> <div class="paragraph"><p>David Turner proposed <a href="https://lore.kernel.org/git/1455772670-21142-26-git-send-email-dturner@twopensource.com/">using LMDB</a>, as LMDB is lightweight (64k of runtime code) and GPL-compatible license.</p></div> <div class="paragraph"><p>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.</p></div> <div class="paragraph"><p>A common format that can be supported by all major Git implementations (git-core, JGit, libgit2) is strongly preferred.</p></div> </div> </div> </div> </div> </div> <div id="footnotes"><hr /></div> <div id="footer"> <div id="footer-text"> Last updated 2024-05-31 00:41:06 UTC </div> </div> </body> </html>