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chromium / external / gitster / git-htmldocs / b96f40a0cb74a86e910bb0f415c4df5182ea5ccf / . / technical / reftable.html
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Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame^]1<!DOCTYPE html>
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435</head>
436<body class="article">
437<div id="header">
438</div>
439<div id="content">
440<div class="sect1">
441<h2 id="_reftable">reftable</h2>
442<div class="sectionbody">
443<div class="sect2">
444<h3 id="_overview">Overview</h3>
445<div class="sect3">
446<h4 id="_problem_statement">Problem statement</h4>
447<div class="paragraph">
448<p>Some repositories contain a lot of references (e.g. android at 866k,
449rails at 31k). The existing packed-refs format takes up a lot of space
450(e.g. 62M), and does not scale with additional references. Lookup of a
451single reference requires linearly scanning the file.</p>
452</div>
453<div class="paragraph">
454<p>Atomic pushes modifying multiple references require copying the entire
455packed-refs file, which can be a considerable amount of data moved
456(e.g. 62M in, 62M out) for even small transactions (2 refs modified).</p>
457</div>
458<div class="paragraph">
459<p>Repositories with many loose references occupy a large number of disk
460blocks from the local file system, as each reference is its own file
461storing 41 bytes (and another file for the corresponding reflog). This
462negatively affects the number of inodes available when a large number of
463repositories are stored on the same filesystem. Readers can be penalized
464due to the larger number of syscalls required to traverse and read the
465<code>$GIT_DIR/refs</code> directory.</p>
466</div>
467</div>
468<div class="sect3">
469<h4 id="_objectives">Objectives</h4>
470<div class="ulist">
471<ul>
472<li>
473<p>Near constant time lookup for any single reference, even when the
474repository is cold and not in process or kernel cache.</p>
475</li>
476<li>
477<p>Near constant time verification if an object name is referred to by at least
478one reference (for allow-tip-sha1-in-want).</p>
479</li>
480<li>
481<p>Efficient enumeration of an entire namespace, such as <code>refs/tags/</code>.</p>
482</li>
483<li>
484<p>Support atomic push with <code>O(size_of_update)</code> operations.</p>
485</li>
486<li>
487<p>Combine reflog storage with ref storage for small transactions.</p>
488</li>
489<li>
490<p>Separate reflog storage for base refs and historical logs.</p>
491</li>
492</ul>
493</div>
494</div>
495<div class="sect3">
496<h4 id="_description">Description</h4>
497<div class="paragraph">
498<p>A reftable file is a portable binary file format customized for
499reference storage. References are sorted, enabling linear scans, binary
500search lookup, and range scans.</p>
501</div>
502<div class="paragraph">
503<p>Storage in the file is organized into variable sized blocks. Prefix
504compression is used within a single block to reduce disk space. Block
505size and alignment are tunable by the writer.</p>
506</div>
507</div>
508<div class="sect3">
509<h4 id="_performance">Performance</h4>
510<div class="paragraph">
511<p>Space used, packed-refs vs. reftable:</p>
512</div>
513<table class="tableblock frame-all grid-all stretch">
514<colgroup>
515<col style="width: 16.6666%;"/>
516<col style="width: 16.6666%;"/>
517<col style="width: 16.6666%;"/>
518<col style="width: 16.6666%;"/>
519<col style="width: 16.6666%;"/>
520<col style="width: 16.667%;"/>
521</colgroup>
522<thead>
523<tr>
524<th class="tableblock halign-left valign-top">repository</th>
525<th class="tableblock halign-right valign-top">packed-refs</th>
526<th class="tableblock halign-right valign-top">reftable</th>
527<th class="tableblock halign-right valign-top">% original</th>
528<th class="tableblock halign-right valign-top">avg ref</th>
529<th class="tableblock halign-right valign-top">avg obj</th>
530</tr>
531</thead>
532<tbody>
533<tr>
534<td class="tableblock halign-left valign-top"><p class="tableblock">android</p></td>
535<td class="tableblock halign-right valign-top"><p class="tableblock">62.2 M</p></td>
536<td class="tableblock halign-right valign-top"><p class="tableblock">36.1 M</p></td>
537<td class="tableblock halign-right valign-top"><p class="tableblock">58.0%</p></td>
538<td class="tableblock halign-right valign-top"><p class="tableblock">33 bytes</p></td>
539<td class="tableblock halign-right valign-top"><p class="tableblock">5 bytes</p></td>
540</tr>
541<tr>
542<td class="tableblock halign-left valign-top"><p class="tableblock">rails</p></td>
543<td class="tableblock halign-right valign-top"><p class="tableblock">1.8 M</p></td>
544<td class="tableblock halign-right valign-top"><p class="tableblock">1.1 M</p></td>
545<td class="tableblock halign-right valign-top"><p class="tableblock">57.7%</p></td>
546<td class="tableblock halign-right valign-top"><p class="tableblock">29 bytes</p></td>
547<td class="tableblock halign-right valign-top"><p class="tableblock">4 bytes</p></td>
548</tr>
549<tr>
550<td class="tableblock halign-left valign-top"><p class="tableblock">git</p></td>
551<td class="tableblock halign-right valign-top"><p class="tableblock">78.7 K</p></td>
552<td class="tableblock halign-right valign-top"><p class="tableblock">48.1 K</p></td>
553<td class="tableblock halign-right valign-top"><p class="tableblock">61.0%</p></td>
554<td class="tableblock halign-right valign-top"><p class="tableblock">50 bytes</p></td>
555<td class="tableblock halign-right valign-top"><p class="tableblock">4 bytes</p></td>
556</tr>
557<tr>
558<td class="tableblock halign-left valign-top"><p class="tableblock">git (heads)</p></td>
559<td class="tableblock halign-right valign-top"><p class="tableblock">332 b</p></td>
560<td class="tableblock halign-right valign-top"><p class="tableblock">269 b</p></td>
561<td class="tableblock halign-right valign-top"><p class="tableblock">81.0%</p></td>
562<td class="tableblock halign-right valign-top"><p class="tableblock">33 bytes</p></td>
563<td class="tableblock halign-right valign-top"><p class="tableblock">0 bytes</p></td>
564</tr>
565</tbody>
566</table>
567<div class="paragraph">
568<p>Scan (read 866k refs), by reference name lookup (single ref from 866k
569refs), and by SHA-1 lookup (refs with that SHA-1, from 866k refs):</p>
570</div>
571<table class="tableblock frame-all grid-all stretch">
572<colgroup>
573<col style="width: 20%;"/>
574<col style="width: 20%;"/>
575<col style="width: 20%;"/>
576<col style="width: 20%;"/>
577<col style="width: 20%;"/>
578</colgroup>
579<thead>
580<tr>
581<th class="tableblock halign-left valign-top">format</th>
582<th class="tableblock halign-right valign-top">cache</th>
583<th class="tableblock halign-right valign-top">scan</th>
584<th class="tableblock halign-right valign-top">by name</th>
585<th class="tableblock halign-right valign-top">by SHA-1</th>
586</tr>
587</thead>
588<tbody>
589<tr>
590<td class="tableblock halign-left valign-top"><p class="tableblock">packed-refs</p></td>
591<td class="tableblock halign-right valign-top"><p class="tableblock">cold</p></td>
592<td class="tableblock halign-right valign-top"><p class="tableblock">402 ms</p></td>
593<td class="tableblock halign-right valign-top"><p class="tableblock">409,660.1 usec</p></td>
594<td class="tableblock halign-right valign-top"><p class="tableblock">412,535.8 usec</p></td>
595</tr>
596<tr>
597<td class="tableblock halign-left valign-top"><p class="tableblock">packed-refs</p></td>
598<td class="tableblock halign-right valign-top"><p class="tableblock">hot</p></td>
599<td class="tableblock halign-right valign-top"></td>
600<td class="tableblock halign-right valign-top"><p class="tableblock">6,844.6 usec</p></td>
601<td class="tableblock halign-right valign-top"><p class="tableblock">20,110.1 usec</p></td>
602</tr>
603<tr>
604<td class="tableblock halign-left valign-top"><p class="tableblock">reftable</p></td>
605<td class="tableblock halign-right valign-top"><p class="tableblock">cold</p></td>
606<td class="tableblock halign-right valign-top"><p class="tableblock">112 ms</p></td>
607<td class="tableblock halign-right valign-top"><p class="tableblock">33.9 usec</p></td>
608<td class="tableblock halign-right valign-top"><p class="tableblock">323.2 usec</p></td>
609</tr>
610<tr>
611<td class="tableblock halign-left valign-top"><p class="tableblock">reftable</p></td>
612<td class="tableblock halign-right valign-top"><p class="tableblock">hot</p></td>
613<td class="tableblock halign-right valign-top"></td>
614<td class="tableblock halign-right valign-top"><p class="tableblock">20.2 usec</p></td>
615<td class="tableblock halign-right valign-top"><p class="tableblock">320.8 usec</p></td>
616</tr>
617</tbody>
618</table>
619<div class="paragraph">
620<p>Space used for 149,932 log entries for 43,061 refs, reflog vs. reftable:</p>
621</div>
622<table class="tableblock frame-all grid-all stretch">
623<colgroup>
624<col style="width: 33.3333%;"/>
625<col style="width: 33.3333%;"/>
626<col style="width: 33.3334%;"/>
627</colgroup>
628<thead>
629<tr>
630<th class="tableblock halign-left valign-top">format</th>
631<th class="tableblock halign-right valign-top">size</th>
632<th class="tableblock halign-right valign-top">avg entry</th>
633</tr>
634</thead>
635<tbody>
636<tr>
637<td class="tableblock halign-left valign-top"><p class="tableblock">$GIT_DIR/logs</p></td>
638<td class="tableblock halign-right valign-top"><p class="tableblock">173 M</p></td>
639<td class="tableblock halign-right valign-top"><p class="tableblock">1209 bytes</p></td>
640</tr>
641<tr>
642<td class="tableblock halign-left valign-top"><p class="tableblock">reftable</p></td>
643<td class="tableblock halign-right valign-top"><p class="tableblock">5 M</p></td>
644<td class="tableblock halign-right valign-top"><p class="tableblock">37 bytes</p></td>
645</tr>
646</tbody>
647</table>
648</div>
649</div>
650<div class="sect2">
651<h3 id="_details">Details</h3>
652<div class="sect3">
653<h4 id="_peeling">Peeling</h4>
654<div class="paragraph">
655<p>References stored in a reftable are peeled, a record for an annotated
656(or signed) tag records both the tag object, and the object it refers
657to. This is analogous to storage in the packed-refs format.</p>
658</div>
659</div>
660<div class="sect3">
661<h4 id="_reference_name_encoding">Reference name encoding</h4>
662<div class="paragraph">
663<p>Reference names are an uninterpreted sequence of bytes that must pass
664<a href="../git-check-ref-format.html">git-check-ref-format(1)</a> as a valid reference name.</p>
665</div>
666</div>
667<div class="sect3">
668<h4 id="_key_unicity">Key unicity</h4>
669<div class="paragraph">
670<p>Each entry must have a unique key; repeated keys are disallowed.</p>
671</div>
672</div>
673<div class="sect3">
674<h4 id="_network_byte_order">Network byte order</h4>
675<div class="paragraph">
676<p>All multi-byte, fixed width fields are in network byte order.</p>
677</div>
678</div>
679<div class="sect3">
680<h4 id="_varint_encoding">Varint encoding</h4>
681<div class="paragraph">
682<p>Varint encoding is identical to the ofs-delta encoding method used
683within pack files.</p>
684</div>
685<div class="paragraph">
686<p>Decoder works as follows:</p>
687</div>
688<div class="literalblock">
689<div class="content">
690<pre>val = buf[ptr] &amp; 0x7f
691while (buf[ptr] &amp; 0x80) {
692 ptr++
693 val = ((val + 1) &lt;&lt; 7) | (buf[ptr] &amp; 0x7f)
694}</pre>
695</div>
696</div>
697</div>
698<div class="sect3">
699<h4 id="_ordering">Ordering</h4>
700<div class="paragraph">
701<p>Blocks are lexicographically ordered by their first reference.</p>
702</div>
703</div>
704<div class="sect3">
705<h4 id="_directoryfile_conflicts">Directory/file conflicts</h4>
706<div class="paragraph">
707<p>The reftable format accepts both <code>refs/heads/foo</code> and
708<code>refs/heads/foo/bar</code> as distinct references.</p>
709</div>
710<div class="paragraph">
711<p>This property is useful for retaining log records in reftable, but may
712confuse versions of Git using <code>$GIT_DIR/refs</code> directory tree to maintain
713references. Users of reftable may choose to continue to reject <code>foo</code> and
714<code>foo/bar</code> type conflicts to prevent problems for peers.</p>
715</div>
716</div>
717</div>
718<div class="sect2">
719<h3 id="_file_format">File format</h3>
720<div class="sect3">
721<h4 id="_structure">Structure</h4>
722<div class="paragraph">
723<p>A reftable file has the following high-level structure:</p>
724</div>
725<div class="literalblock">
726<div class="content">
727<pre>first_block {
728 header
729 first_ref_block
730}
731ref_block*
732ref_index*
733obj_block*
734obj_index*
735log_block*
736log_index*
737footer</pre>
738</div>
739</div>
740<div class="paragraph">
741<p>A log-only file omits the <code>ref_block</code>, <code>ref_index</code>, <code>obj_block</code> and
742<code>obj_index</code> sections, containing only the file header and log block:</p>
743</div>
744<div class="literalblock">
745<div class="content">
746<pre>first_block {
747 header
748}
749log_block*
750log_index*
751footer</pre>
752</div>
753</div>
754<div class="paragraph">
755<p>In a log-only file, the first log block immediately follows the file
756header, without padding to block alignment.</p>
757</div>
758</div>
759<div class="sect3">
760<h4 id="_block_size">Block size</h4>
761<div class="paragraph">
762<p>The file&#8217;s block size is arbitrarily determined by the writer, and does
763not have to be a power of 2. The block size must be larger than the
764longest reference name or log entry used in the repository, as
765references cannot span blocks.</p>
766</div>
767<div class="paragraph">
768<p>Powers of two that are friendly to the virtual memory system or
769filesystem (such as 4k or 8k) are recommended. Larger sizes (64k) can
770yield better compression, with a possible increased cost incurred by
771readers during access.</p>
772</div>
773<div class="paragraph">
774<p>The largest block size is <code>16777215</code> bytes (15.99 MiB).</p>
775</div>
776</div>
777<div class="sect3">
778<h4 id="_block_alignment">Block alignment</h4>
779<div class="paragraph">
780<p>Writers may choose to align blocks at multiples of the block size by
781including <code>padding</code> filled with NUL bytes at the end of a block to round
782out to the chosen alignment. When alignment is used, writers must
783specify the alignment with the file header&#8217;s <code>block_size</code> field.</p>
784</div>
785<div class="paragraph">
786<p>Block alignment is not required by the file format. Unaligned files must
787set <code>block_size = 0</code> in the file header, and omit <code>padding</code>. Unaligned
788files with more than one ref block must include the <a href="#Ref-index">ref
789index</a> to support fast lookup. Readers must be able to read both aligned
790and non-aligned files.</p>
791</div>
792<div class="paragraph">
793<p>Very small files (e.g. a single ref block) may omit <code>padding</code> and the ref
794index to reduce total file size.</p>
795</div>
796</div>
797<div class="sect3">
798<h4 id="_header_version_1">Header (version 1)</h4>
799<div class="paragraph">
800<p>A 24-byte header appears at the beginning of the file:</p>
801</div>
802<div class="literalblock">
803<div class="content">
804<pre>'REFT'
805uint8( version_number = 1 )
806uint24( block_size )
807uint64( min_update_index )
808uint64( max_update_index )</pre>
809</div>
810</div>
811<div class="paragraph">
812<p>Aligned files must specify <code>block_size</code> to configure readers with the
813expected block alignment. Unaligned files must set <code>block_size = 0</code>.</p>
814</div>
815<div class="paragraph">
816<p>The <code>min_update_index</code> and <code>max_update_index</code> describe bounds for the
817<code>update_index</code> field of all log records in this file. When reftables are
818used in a stack for <a href="#Update-transactions">transactions</a>, these
819fields can order the files such that the prior file&#8217;s
820<code>max_update_index + 1</code> is the next file&#8217;s <code>min_update_index</code>.</p>
821</div>
822</div>
823<div class="sect3">
824<h4 id="_header_version_2">Header (version 2)</h4>
825<div class="paragraph">
826<p>A 28-byte header appears at the beginning of the file:</p>
827</div>
828<div class="literalblock">
829<div class="content">
830<pre>'REFT'
831uint8( version_number = 2 )
832uint24( block_size )
833uint64( min_update_index )
834uint64( max_update_index )
835uint32( hash_id )</pre>
836</div>
837</div>
838<div class="paragraph">
839<p>The header is identical to <code>version_number=1</code>, with the 4-byte hash ID
840("sha1" for SHA1 and "s256" for SHA-256) appended to the header.</p>
841</div>
842<div class="paragraph">
843<p>For maximum backward compatibility, it is recommended to use version 1 when
844writing SHA1 reftables.</p>
845</div>
846</div>
847<div class="sect3">
848<h4 id="_first_ref_block">First ref block</h4>
849<div class="paragraph">
850<p>The first ref block shares the same block as the file header, and is 24
851bytes smaller than all other blocks in the file. The first block
852immediately begins after the file header, at position 24.</p>
853</div>
854<div class="paragraph">
855<p>If the first block is a log block (a log-only file), its block header
856begins immediately at position 24.</p>
857</div>
858</div>
859<div class="sect3">
860<h4 id="_ref_block_format">Ref block format</h4>
861<div class="paragraph">
862<p>A ref block is written as:</p>
863</div>
864<div class="literalblock">
865<div class="content">
866<pre>'r'
867uint24( block_len )
868ref_record+
869uint24( restart_offset )+
870uint16( restart_count )
871
872padding?</pre>
873</div>
874</div>
875<div class="paragraph">
876<p>Blocks begin with <code>block_type = 'r'</code> and a 3-byte <code>block_len</code> which
877encodes the number of bytes in the block up to, but not including the
878optional <code>padding</code>. This is always less than or equal to the file&#8217;s
879block size. In the first ref block, <code>block_len</code> includes 24 bytes for
880the file header.</p>
881</div>
882<div class="paragraph">
883<p>The 2-byte <code>restart_count</code> stores the number of entries in the
884<code>restart_offset</code> list, which must not be empty. Readers can use
885<code>restart_count</code> to binary search between restarts before starting a
886linear scan.</p>
887</div>
888<div class="paragraph">
889<p>Exactly <code>restart_count</code> 3-byte <code>restart_offset</code> values precede the
890<code>restart_count</code>. Offsets are relative to the start of the block and
891refer to the first byte of any <code>ref_record</code> whose name has not been
892prefix compressed. Entries in the <code>restart_offset</code> list must be sorted,
893ascending. Readers can start linear scans from any of these records.</p>
894</div>
895<div class="paragraph">
896<p>A variable number of <code>ref_record</code> fill the middle of the block,
897describing reference names and values. The format is described below.</p>
898</div>
899<div class="paragraph">
900<p>As the first ref block shares the first file block with the file header,
901all <code>restart_offset</code> in the first block are relative to the start of the
902file (position 0), and include the file header. This forces the first
903<code>restart_offset</code> to be <code>28</code>.</p>
904</div>
905<div class="sect4">
906<h5 id="_ref_record">ref record</h5>
907<div class="paragraph">
908<p>A <code>ref_record</code> describes a single reference, storing both the name and
909its value(s). Records are formatted as:</p>
910</div>
911<div class="literalblock">
912<div class="content">
913<pre>varint( prefix_length )
914varint( (suffix_length &lt;&lt; 3) | value_type )
915suffix
916varint( update_index_delta )
917value?</pre>
918</div>
919</div>
920<div class="paragraph">
921<p>The <code>prefix_length</code> field specifies how many leading bytes of the prior
922reference record&#8217;s name should be copied to obtain this reference&#8217;s
923name. This must be 0 for the first reference in any block, and also must
924be 0 for any <code>ref_record</code> whose offset is listed in the <code>restart_offset</code>
925table at the end of the block.</p>
926</div>
927<div class="paragraph">
928<p>Recovering a reference name from any <code>ref_record</code> is a simple concat:</p>
929</div>
930<div class="literalblock">
931<div class="content">
932<pre>this_name = prior_name[0..prefix_length] + suffix</pre>
933</div>
934</div>
935<div class="paragraph">
936<p>The <code>suffix_length</code> value provides the number of bytes available in
937<code>suffix</code> to copy from <code>suffix</code> to complete the reference name.</p>
938</div>
939<div class="paragraph">
940<p>The <code>update_index</code> that last modified the reference can be obtained by
941adding <code>update_index_delta</code> to the <code>min_update_index</code> from the file
942header: <code>min_update_index + update_index_delta</code>.</p>
943</div>
944<div class="paragraph">
945<p>The <code>value</code> follows. Its format is determined by <code>value_type</code>, one of
946the following:</p>
947</div>
948<div class="ulist">
949<ul>
950<li>
951<p><code>0x0</code>: deletion; no value data (see transactions, below)</p>
952</li>
953<li>
954<p><code>0x1</code>: one object name; value of the ref</p>
955</li>
956<li>
957<p><code>0x2</code>: two object names; value of the ref, peeled target</p>
958</li>
959<li>
960<p><code>0x3</code>: symbolic reference: <code>varint( target_len ) target</code></p>
961</li>
962</ul>
963</div>
964<div class="paragraph">
965<p>Symbolic references use <code>0x3</code>, followed by the complete name of the
966reference target. No compression is applied to the target name.</p>
967</div>
968<div class="paragraph">
969<p>Types <code>0x4..0x7</code> are reserved for future use.</p>
970</div>
971</div>
972</div>
973<div class="sect3">
974<h4 id="_ref_index">Ref index</h4>
975<div class="paragraph">
976<p>The ref index stores the name of the last reference from every ref block
977in the file, enabling reduced disk seeks for lookups. Any reference can
978be found by searching the index, identifying the containing block, and
979searching within that block.</p>
980</div>
981<div class="paragraph">
982<p>The index may be organized into a multi-level index, where the 1st level
983index block points to additional ref index blocks (2nd level), which may
984in turn point to either additional index blocks (e.g. 3rd level) or ref
985blocks (leaf level). Disk reads required to access a ref go up with
986higher index levels. Multi-level indexes may be required to ensure no
987single index block exceeds the file format&#8217;s max block size of
988<code>16777215</code> bytes (15.99 MiB). To achieve constant O(1) disk seeks for
989lookups the index must be a single level, which is permitted to exceed
990the file&#8217;s configured block size, but not the format&#8217;s max block size of
99115.99 MiB.</p>
992</div>
993<div class="paragraph">
994<p>If present, the ref index block(s) appears after the last ref block.</p>
995</div>
996<div class="paragraph">
997<p>If there are at least 4 ref blocks, a ref index block should be written
998to improve lookup times. Cold reads using the index require 2 disk reads
999(read index, read block), and binary searching &lt; 4 blocks also requires
1000&#8656; 2 reads. Omitting the index block from smaller files saves space.</p>
1001</div>
1002<div class="paragraph">
1003<p>If the file is unaligned and contains more than one ref block, the ref
1004index must be written.</p>
1005</div>
1006<div class="paragraph">
1007<p>Index block format:</p>
1008</div>
1009<div class="literalblock">
1010<div class="content">
1011<pre>'i'
1012uint24( block_len )
1013index_record+
1014uint24( restart_offset )+
1015uint16( restart_count )
1016
1017padding?</pre>
1018</div>
1019</div>
1020<div class="paragraph">
1021<p>The index blocks begin with <code>block_type = 'i'</code> and a 3-byte <code>block_len</code>
1022which encodes the number of bytes in the block, up to but not including
1023the optional <code>padding</code>.</p>
1024</div>
1025<div class="paragraph">
1026<p>The <code>restart_offset</code> and <code>restart_count</code> fields are identical in format,
1027meaning and usage as in ref blocks.</p>
1028</div>
1029<div class="paragraph">
1030<p>To reduce the number of reads required for random access in very large
1031files the index block may be larger than other blocks. However, readers
1032must hold the entire index in memory to benefit from this, so it&#8217;s a
1033time-space tradeoff in both file size and reader memory.</p>
1034</div>
1035<div class="paragraph">
1036<p>Increasing the file&#8217;s block size decreases the index size. Alternatively
1037a multi-level index may be used, keeping index blocks within the file&#8217;s
1038block size, but increasing the number of blocks that need to be
1039accessed.</p>
1040</div>
1041<div class="sect4">
1042<h5 id="_index_record">index record</h5>
1043<div class="paragraph">
1044<p>An index record describes the last entry in another block. Index records
1045are written as:</p>
1046</div>
1047<div class="literalblock">
1048<div class="content">
1049<pre>varint( prefix_length )
1050varint( (suffix_length &lt;&lt; 3) | 0 )
1051suffix
1052varint( block_position )</pre>
1053</div>
1054</div>
1055<div class="paragraph">
1056<p>Index records use prefix compression exactly like <code>ref_record</code>.</p>
1057</div>
1058<div class="paragraph">
1059<p>Index records store <code>block_position</code> after the suffix, specifying the
1060absolute position in bytes (from the start of the file) of the block
1061that ends with this reference. Readers can seek to <code>block_position</code> to
1062begin reading the block header.</p>
1063</div>
1064<div class="paragraph">
1065<p>Readers must examine the block header at <code>block_position</code> to determine
1066if the next block is another level index block, or the leaf-level ref
1067block.</p>
1068</div>
1069</div>
1070<div class="sect4">
1071<h5 id="_reading_the_index">Reading the index</h5>
1072<div class="paragraph">
1073<p>Readers loading the ref index must first read the footer (below) to
1074obtain <code>ref_index_position</code>. If not present, the position will be 0. The
1075<code>ref_index_position</code> is for the 1st level root of the ref index.</p>
1076</div>
1077</div>
1078</div>
1079<div class="sect3">
1080<h4 id="_obj_block_format">Obj block format</h4>
1081<div class="paragraph">
1082<p>Object blocks are optional. Writers may choose to omit object blocks,
1083especially if readers will not use the object name to ref mapping.</p>
1084</div>
1085<div class="paragraph">
1086<p>Object blocks use unique, abbreviated 2-31 byte object name keys, mapping to
1087ref blocks containing references pointing to that object directly, or as
1088the peeled value of an annotated tag. Like ref blocks, object blocks use
1089the file&#8217;s standard block size. The abbreviation length is available in
1090the footer as <code>obj_id_len</code>.</p>
1091</div>
1092<div class="paragraph">
1093<p>To save space in small files, object blocks may be omitted if the ref
1094index is not present, as brute force search will only need to read a few
1095ref blocks. When missing, readers should brute force a linear search of
1096all references to lookup by object name.</p>
1097</div>
1098<div class="paragraph">
1099<p>An object block is written as:</p>
1100</div>
1101<div class="literalblock">
1102<div class="content">
1103<pre>'o'
1104uint24( block_len )
1105obj_record+
1106uint24( restart_offset )+
1107uint16( restart_count )
1108
1109padding?</pre>
1110</div>
1111</div>
1112<div class="paragraph">
1113<p>Fields are identical to ref block. Binary search using the restart table
1114works the same as in reference blocks.</p>
1115</div>
1116<div class="paragraph">
1117<p>Because object names are abbreviated by writers to the shortest unique
1118abbreviation within the reftable, obj key lengths have a variable length. Their
1119length must be at least 2 bytes. Readers must compare only for common prefix
1120match within an obj block or obj index.</p>
1121</div>
1122<div class="sect4">
1123<h5 id="_obj_record">obj record</h5>
1124<div class="paragraph">
1125<p>An <code>obj_record</code> describes a single object abbreviation, and the blocks
1126containing references using that unique abbreviation:</p>
1127</div>
1128<div class="literalblock">
1129<div class="content">
1130<pre>varint( prefix_length )
1131varint( (suffix_length &lt;&lt; 3) | cnt_3 )
1132suffix
1133varint( cnt_large )?
1134varint( position_delta )*</pre>
1135</div>
1136</div>
1137<div class="paragraph">
1138<p>Like in reference blocks, abbreviations are prefix compressed within an
1139obj block. On large reftables with many unique objects, higher block
1140sizes (64k), and higher restart interval (128), a <code>prefix_length</code> of 2
1141or 3 and <code>suffix_length</code> of 3 may be common in obj records (unique
1142abbreviation of 5-6 raw bytes, 10-12 hex digits).</p>
1143</div>
1144<div class="paragraph">
1145<p>Each record contains <code>position_count</code> number of positions for matching
1146ref blocks. For 1-7 positions the count is stored in <code>cnt_3</code>. When
1147<code>cnt_3 = 0</code> the actual count follows in a varint, <code>cnt_large</code>.</p>
1148</div>
1149<div class="paragraph">
1150<p>The use of <code>cnt_3</code> bets most objects are pointed to by only a single
1151reference, some may be pointed to by a couple of references, and very
1152few (if any) are pointed to by more than 7 references.</p>
1153</div>
1154<div class="paragraph">
1155<p>A special case exists when <code>cnt_3 = 0</code> and <code>cnt_large = 0</code>: there are no
1156<code>position_delta</code>, but at least one reference starts with this
1157abbreviation. A reader that needs exact reference names must scan all
1158references to find which specific references have the desired object.
1159Writers should use this format when the <code>position_delta</code> list would have
1160overflowed the file&#8217;s block size due to a high number of references
1161pointing to the same object.</p>
1162</div>
1163<div class="paragraph">
1164<p>The first <code>position_delta</code> is the position from the start of the file.
1165Additional <code>position_delta</code> entries are sorted ascending and relative to
1166the prior entry, e.g. a reader would perform:</p>
1167</div>
1168<div class="literalblock">
1169<div class="content">
1170<pre>pos = position_delta[0]
1171prior = pos
1172for (j = 1; j &lt; position_count; j++) {
1173 pos = prior + position_delta[j]
1174 prior = pos
1175}</pre>
1176</div>
1177</div>
1178<div class="paragraph">
1179<p>With a position in hand, a reader must linearly scan the ref block,
1180starting from the first <code>ref_record</code>, testing each reference&#8217;s object names
1181(for <code>value_type = 0x1</code> or <code>0x2</code>) for full equality. Faster searching by
1182object name within a single ref block is not supported by the reftable format.
1183Smaller block sizes reduce the number of candidates this step must
1184consider.</p>
1185</div>
1186</div>
1187</div>
1188<div class="sect3">
1189<h4 id="_obj_index">Obj index</h4>
1190<div class="paragraph">
1191<p>The obj index stores the abbreviation from the last entry for every obj
1192block in the file, enabling reduced disk seeks for all lookups. It is
1193formatted exactly the same as the ref index, but refers to obj blocks.</p>
1194</div>
1195<div class="paragraph">
1196<p>The obj index should be present if obj blocks are present, as obj blocks
1197should only be written in larger files.</p>
1198</div>
1199<div class="paragraph">
1200<p>Readers loading the obj index must first read the footer (below) to
1201obtain <code>obj_index_position</code>. If not present, the position will be 0.</p>
1202</div>
1203</div>
1204<div class="sect3">
1205<h4 id="_log_block_format">Log block format</h4>
1206<div class="paragraph">
1207<p>Unlike ref and obj blocks, log blocks are always unaligned.</p>
1208</div>
1209<div class="paragraph">
1210<p>Log blocks are variable in size, and do not match the <code>block_size</code>
1211specified in the file header or footer. Writers should choose an
1212appropriate buffer size to prepare a log block for deflation, such as
1213<code>2 * block_size</code>.</p>
1214</div>
1215<div class="paragraph">
1216<p>A log block is written as:</p>
1217</div>
1218<div class="literalblock">
1219<div class="content">
1220<pre>'g'
1221uint24( block_len )
1222zlib_deflate {
1223 log_record+
1224 uint24( restart_offset )+
1225 uint16( restart_count )
1226}</pre>
1227</div>
1228</div>
1229<div class="paragraph">
1230<p>Log blocks look similar to ref blocks, except <code>block_type = 'g'</code>.</p>
1231</div>
1232<div class="paragraph">
1233<p>The 4-byte block header is followed by the deflated block contents using
1234zlib deflate. The <code>block_len</code> in the header is the inflated size
1235(including 4-byte block header), and should be used by readers to
1236preallocate the inflation output buffer. A log block&#8217;s <code>block_len</code> may
1237exceed the file&#8217;s block size.</p>
1238</div>
1239<div class="paragraph">
1240<p>Offsets within the log block (e.g. <code>restart_offset</code>) still include the
12414-byte header. Readers may prefer prefixing the inflation output buffer
1242with the 4-byte header.</p>
1243</div>
1244<div class="paragraph">
1245<p>Within the deflate container, a variable number of <code>log_record</code> describe
1246reference changes. The log record format is described below. See ref
1247block format (above) for a description of <code>restart_offset</code> and
1248<code>restart_count</code>.</p>
1249</div>
1250<div class="paragraph">
1251<p>Because log blocks have no alignment or padding between blocks, readers
1252must keep track of the bytes consumed by the inflater to know where the
1253next log block begins.</p>
1254</div>
1255<div class="sect4">
1256<h5 id="_log_record">log record</h5>
1257<div class="paragraph">
1258<p>Log record keys are structured as:</p>
1259</div>
1260<div class="literalblock">
1261<div class="content">
1262<pre>ref_name '\0' reverse_int64( update_index )</pre>
1263</div>
1264</div>
1265<div class="paragraph">
1266<p>where <code>update_index</code> is the unique transaction identifier. The
1267<code>update_index</code> field must be unique within the scope of a <code>ref_name</code>.
1268See the update transactions section below for further details.</p>
1269</div>
1270<div class="paragraph">
1271<p>The <code>reverse_int64</code> function inverses the value so lexicographical
1272ordering the network byte order encoding sorts the more recent records
1273with higher <code>update_index</code> values first:</p>
1274</div>
1275<div class="literalblock">
1276<div class="content">
1277<pre>reverse_int64(int64 t) {
1278 return 0xffffffffffffffff - t;
1279}</pre>
1280</div>
1281</div>
1282<div class="paragraph">
1283<p>Log records have a similar starting structure to ref and index records,
1284utilizing the same prefix compression scheme applied to the log record
1285key described above.</p>
1286</div>
1287<div class="literalblock">
1288<div class="content">
1289<pre> varint( prefix_length )
1290 varint( (suffix_length &lt;&lt; 3) | log_type )
1291 suffix
1292 log_data {
1293 old_id
1294 new_id
1295 varint( name_length ) name
1296 varint( email_length ) email
1297 varint( time_seconds )
1298 sint16( tz_offset )
1299 varint( message_length ) message
1300 }?</pre>
1301</div>
1302</div>
1303<div class="paragraph">
1304<p>Log record entries use <code>log_type</code> to indicate what follows:</p>
1305</div>
1306<div class="ulist">
1307<ul>
1308<li>
1309<p><code>0x0</code>: deletion; no log data.</p>
1310</li>
1311<li>
1312<p><code>0x1</code>: standard git reflog data using <code>log_data</code> above.</p>
1313</li>
1314</ul>
1315</div>
1316<div class="paragraph">
1317<p>The <code>log_type = 0x0</code> is mostly useful for <code>git stash drop</code>, removing an
1318entry from the reflog of <code>refs/stash</code> in a transaction file (below),
1319without needing to rewrite larger files. Readers reading a stack of
1320reflogs must treat this as a deletion.</p>
1321</div>
1322<div class="paragraph">
1323<p>For <code>log_type = 0x1</code>, the <code>log_data</code> section follows
1324<a href="../git-update-ref.html">git-update-ref(1)</a> logging and includes:</p>
1325</div>
1326<div class="ulist">
1327<ul>
1328<li>
1329<p>two object names (old id, new id)</p>
1330</li>
1331<li>
1332<p>varint string of committer&#8217;s name</p>
1333</li>
1334<li>
1335<p>varint string of committer&#8217;s email</p>
1336</li>
1337<li>
1338<p>varint time in seconds since epoch (Jan 1, 1970)</p>
1339</li>
1340<li>
1341<p>2-byte timezone offset in minutes (signed)</p>
1342</li>
1343<li>
1344<p>varint string of message</p>
1345</li>
1346</ul>
1347</div>
1348<div class="paragraph">
1349<p><code>tz_offset</code> is the absolute number of minutes from GMT the committer was
1350at the time of the update. For example <code>GMT-0800</code> is encoded in reftable
1351as <code>sint16(-480)</code> and <code>GMT+0230</code> is <code>sint16(150)</code>.</p>
1352</div>
1353<div class="paragraph">
1354<p>The committer email does not contain <code>&lt;</code> or <code>&gt;</code>, it&#8217;s the value normally
1355found between the <code>&lt;&gt;</code> in a git commit object header.</p>
1356</div>
1357<div class="paragraph">
1358<p>The <code>message_length</code> may be 0, in which case there was no message
1359supplied for the update.</p>
1360</div>
1361<div class="paragraph">
1362<p>Contrary to traditional reflog (which is a file), renames are encoded as
1363a combination of ref deletion and ref creation. A deletion is a log
1364record with a zero new_id, and a creation is a log record with a zero old_id.</p>
1365</div>
1366</div>
1367<div class="sect4">
1368<h5 id="_reading_the_log">Reading the log</h5>
1369<div class="paragraph">
1370<p>Readers accessing the log must first read the footer (below) to
1371determine the <code>log_position</code>. The first block of the log begins at
1372<code>log_position</code> bytes since the start of the file. The <code>log_position</code> is
1373not block aligned.</p>
1374</div>
1375</div>
1376<div class="sect4">
1377<h5 id="_importing_logs">Importing logs</h5>
1378<div class="paragraph">
1379<p>When importing from <code>$GIT_DIR/logs</code> writers should globally order all
1380log records roughly by timestamp while preserving file order, and assign
1381unique, increasing <code>update_index</code> values for each log line. Newer log
1382records get higher <code>update_index</code> values.</p>
1383</div>
1384<div class="paragraph">
1385<p>Although an import may write only a single reftable file, the reftable
1386file must span many unique <code>update_index</code>, as each log line requires its
1387own <code>update_index</code> to preserve semantics.</p>
1388</div>
1389</div>
1390</div>
1391<div class="sect3">
1392<h4 id="_log_index">Log index</h4>
1393<div class="paragraph">
1394<p>The log index stores the log key
1395(<code>refname \0 reverse_int64(update_index)</code>) for the last log record of
1396every log block in the file, supporting bounded-time lookup.</p>
1397</div>
1398<div class="paragraph">
1399<p>A log index block must be written if 2 or more log blocks are written to
1400the file. If present, the log index appears after the last log block.
1401There is no padding used to align the log index to block alignment.</p>
1402</div>
1403<div class="paragraph">
1404<p>Log index format is identical to ref index, except the keys are 9 bytes
1405longer to include <code>'\0'</code> and the 8-byte <code>reverse_int64(update_index)</code>.
1406Records use <code>block_position</code> to refer to the start of a log block.</p>
1407</div>
1408<div class="sect4">
1409<h5 id="_reading_the_index_2">Reading the index</h5>
1410<div class="paragraph">
1411<p>Readers loading the log index must first read the footer (below) to
1412obtain <code>log_index_position</code>. If not present, the position will be 0.</p>
1413</div>
1414</div>
1415</div>
1416<div class="sect3">
1417<h4 id="_footer">Footer</h4>
1418<div class="paragraph">
1419<p>After the last block of the file, a file footer is written. It begins
1420like the file header, but is extended with additional data.</p>
1421</div>
1422<div class="literalblock">
1423<div class="content">
1424<pre> HEADER
1425
1426 uint64( ref_index_position )
1427 uint64( (obj_position &lt;&lt; 5) | obj_id_len )
1428 uint64( obj_index_position )
1429
1430 uint64( log_position )
1431 uint64( log_index_position )
1432
1433 uint32( CRC-32 of above )</pre>
1434</div>
1435</div>
1436<div class="paragraph">
1437<p>If a section is missing (e.g. ref index) the corresponding position
1438field (e.g. <code>ref_index_position</code>) will be 0.</p>
1439</div>
1440<div class="ulist">
1441<ul>
1442<li>
1443<p><code>obj_position</code>: byte position for the first obj block.</p>
1444</li>
1445<li>
1446<p><code>obj_id_len</code>: number of bytes used to abbreviate object names in
1447obj blocks.</p>
1448</li>
1449<li>
1450<p><code>log_position</code>: byte position for the first log block.</p>
1451</li>
1452<li>
1453<p><code>ref_index_position</code>: byte position for the start of the ref index.</p>
1454</li>
1455<li>
1456<p><code>obj_index_position</code>: byte position for the start of the obj index.</p>
1457</li>
1458<li>
1459<p><code>log_index_position</code>: byte position for the start of the log index.</p>
1460</li>
1461</ul>
1462</div>
1463<div class="paragraph">
1464<p>The size of the footer is 68 bytes for version 1, and 72 bytes for
1465version 2.</p>
1466</div>
1467<div class="sect4">
1468<h5 id="_reading_the_footer">Reading the footer</h5>
1469<div class="paragraph">
1470<p>Readers must first read the file start to determine the version
1471number. Then they seek to <code>file_length - FOOTER_LENGTH</code> to access the
1472footer. A trusted external source (such as <code>stat(2)</code>) is necessary to
1473obtain <code>file_length</code>. When reading the footer, readers must verify:</p>
1474</div>
1475<div class="ulist">
1476<ul>
1477<li>
1478<p>4-byte magic is correct</p>
1479</li>
1480<li>
1481<p>1-byte version number is recognized</p>
1482</li>
1483<li>
1484<p>4-byte CRC-32 matches the other 64 bytes (including magic, and
1485version)</p>
1486</li>
1487</ul>
1488</div>
1489<div class="paragraph">
1490<p>Once verified, the other fields of the footer can be accessed.</p>
1491</div>
1492</div>
1493<div class="sect4">
1494<h5 id="_empty_tables">Empty tables</h5>
1495<div class="paragraph">
1496<p>A reftable may be empty. In this case, the file starts with a header
1497and is immediately followed by a footer.</p>
1498</div>
1499</div>
1500</div>
1501<div class="sect3">
1502<h4 id="_binary_search">Binary search</h4>
1503<div class="paragraph">
1504<p>Binary search within a block is supported by the <code>restart_offset</code> fields
1505at the end of the block. Readers can binary search through the restart
1506table to locate between which two restart points the sought reference or
1507key should appear.</p>
1508</div>
1509<div class="paragraph">
1510<p>Each record identified by a <code>restart_offset</code> stores the complete key in
1511the <code>suffix</code> field of the record, making the compare operation during
1512binary search straightforward.</p>
1513</div>
1514<div class="paragraph">
1515<p>Once a restart point lexicographically before the sought reference has
1516been identified, readers can linearly scan through the following record
1517entries to locate the sought record, terminating if the current record
1518sorts after (and therefore the sought key is not present).</p>
1519</div>
1520<div class="sect4">
1521<h5 id="_restart_point_selection">Restart point selection</h5>
1522<div class="paragraph">
1523<p>Writers determine the restart points at file creation. The process is
1524arbitrary, but every 16 or 64 records is recommended. Every 16 may be
1525more suitable for smaller block sizes (4k or 8k), every 64 for larger
1526block sizes (64k).</p>
1527</div>
1528<div class="paragraph">
1529<p>More frequent restart points reduces prefix compression and increases
1530space consumed by the restart table, both of which increase file size.</p>
1531</div>
1532<div class="paragraph">
1533<p>Less frequent restart points makes prefix compression more effective,
1534decreasing overall file size, with increased penalties for readers
1535walking through more records after the binary search step.</p>
1536</div>
1537<div class="paragraph">
1538<p>A maximum of <code>65535</code> restart points per block is supported.</p>
1539</div>
1540</div>
1541</div>
1542</div>
1543<div class="sect2">
1544<h3 id="_considerations">Considerations</h3>
1545<div class="sect3">
1546<h4 id="_lightweight_refs_dominate">Lightweight refs dominate</h4>
1547<div class="paragraph">
1548<p>The reftable format assumes the vast majority of references are single
1549object names valued with common prefixes, such as Gerrit Code Review&#8217;s
1550<code>refs/changes/</code> namespace, GitHub&#8217;s <code>refs/pulls/</code> namespace, or many
1551lightweight tags in the <code>refs/tags/</code> namespace.</p>
1552</div>
1553<div class="paragraph">
1554<p>Annotated tags storing the peeled object cost an additional object name per
1555reference.</p>
1556</div>
1557</div>
1558<div class="sect3">
1559<h4 id="_low_overhead">Low overhead</h4>
1560<div class="paragraph">
1561<p>A reftable with very few references (e.g. git.git with 5 heads) is 269
1562bytes for reftable, vs. 332 bytes for packed-refs. This supports
1563reftable scaling down for transaction logs (below).</p>
1564</div>
1565</div>
1566<div class="sect3">
1567<h4 id="_block_size_2">Block size</h4>
1568<div class="paragraph">
1569<p>For a Gerrit Code Review type repository with many change refs, larger
1570block sizes (64 KiB) and less frequent restart points (every 64) yield
1571better compression due to more references within the block compressing
1572against the prior reference.</p>
1573</div>
1574<div class="paragraph">
1575<p>Larger block sizes reduce the index size, as the reftable will require
1576fewer blocks to store the same number of references.</p>
1577</div>
1578</div>
1579<div class="sect3">
1580<h4 id="_minimal_disk_seeks">Minimal disk seeks</h4>
1581<div class="paragraph">
1582<p>Assuming the index block has been loaded into memory, binary searching
1583for any single reference requires exactly 1 disk seek to load the
1584containing block.</p>
1585</div>
1586</div>
1587<div class="sect3">
1588<h4 id="_scans_and_lookups_dominate">Scans and lookups dominate</h4>
1589<div class="paragraph">
1590<p>Scanning all references and lookup by name (or namespace such as
1591<code>refs/heads/</code>) are the most common activities performed on repositories.
1592Object names are stored directly with references to optimize this use case.</p>
1593</div>
1594</div>
1595<div class="sect3">
1596<h4 id="_logs_are_infrequently_read">Logs are infrequently read</h4>
1597<div class="paragraph">
1598<p>Logs are infrequently accessed, but can be large. Deflating log blocks
1599saves disk space, with some increased penalty at read time.</p>
1600</div>
1601<div class="paragraph">
1602<p>Logs are stored in an isolated section from refs, reducing the burden on
1603reference readers that want to ignore logs. Further, historical logs can
1604be isolated into log-only files.</p>
1605</div>
1606</div>
1607<div class="sect3">
1608<h4 id="_logs_are_read_backwards">Logs are read backwards</h4>
1609<div class="paragraph">
1610<p>Logs are frequently accessed backwards (most recent N records for master
1611to answer <code>master@{4}</code>), so log records are grouped by reference, and
1612sorted descending by update index.</p>
1613</div>
1614</div>
1615</div>
1616<div class="sect2">
1617<h3 id="_repository_format">Repository format</h3>
1618<div class="sect3">
1619<h4 id="_version_1">Version 1</h4>
1620<div class="paragraph">
1621<p>A repository must set its <code>$GIT_DIR/config</code> to configure reftable:</p>
1622</div>
1623<div class="literalblock">
1624<div class="content">
1625<pre>[core]
1626 repositoryformatversion = 1
1627[extensions]
1628 refStorage = reftable</pre>
1629</div>
1630</div>
1631</div>
1632<div class="sect3">
1633<h4 id="_layout">Layout</h4>
1634<div class="paragraph">
1635<p>A collection of reftable files are stored in the <code>$GIT_DIR/reftable/</code> directory.
1636Their names should have a random element, such that each filename is globally
1637unique; this helps avoid spurious failures on Windows, where open files cannot
1638be removed or overwritten. It suggested to use
1639<code>${min_update_index}-${max_update_index}-${random}.ref</code> as a naming convention.</p>
1640</div>
1641<div class="paragraph">
1642<p>Log-only files use the <code>.log</code> extension, while ref-only and mixed ref
1643and log files use <code>.ref</code>. extension.</p>
1644</div>
1645<div class="paragraph">
1646<p>The stack ordering file is <code>$GIT_DIR/reftable/tables.list</code> and lists the
1647current files, one per line, in order, from oldest (base) to newest
1648(most recent):</p>
1649</div>
1650<div class="literalblock">
1651<div class="content">
1652<pre>$ cat .git/reftable/tables.list
165300000001-00000001-RANDOM1.log
165400000002-00000002-RANDOM2.ref
165500000003-00000003-RANDOM3.ref</pre>
1656</div>
1657</div>
1658<div class="paragraph">
1659<p>Readers must read <code>$GIT_DIR/reftable/tables.list</code> to determine which
1660files are relevant right now, and search through the stack in reverse
1661order (last reftable is examined first).</p>
1662</div>
1663<div class="paragraph">
1664<p>Reftable files not listed in <code>tables.list</code> may be new (and about to be
1665added to the stack by the active writer), or ancient and ready to be
1666pruned.</p>
1667</div>
1668</div>
1669<div class="sect3">
1670<h4 id="_backward_compatibility">Backward compatibility</h4>
1671<div class="paragraph">
1672<p>Older clients should continue to recognize the directory as a git
1673repository so they don&#8217;t look for an enclosing repository in parent
1674directories. To this end, a reftable-enabled repository must contain the
1675following dummy files</p>
1676</div>
1677<div class="ulist">
1678<ul>
1679<li>
1680<p><code>.git/HEAD</code>, a regular file containing <code>ref: refs/heads/.invalid</code>.</p>
1681</li>
1682<li>
1683<p><code>.git/refs/</code>, a directory</p>
1684</li>
1685<li>
1686<p><code>.git/refs/heads</code>, a regular file</p>
1687</li>
1688</ul>
1689</div>
1690</div>
1691<div class="sect3">
1692<h4 id="_readers">Readers</h4>
1693<div class="paragraph">
1694<p>Readers can obtain a consistent snapshot of the reference space by
1695following:</p>
1696</div>
1697<div class="olist arabic">
1698<ol class="arabic">
1699<li>
1700<p>Open and read the <code>tables.list</code> file.</p>
1701</li>
1702<li>
1703<p>Open each of the reftable files that it mentions.</p>
1704</li>
1705<li>
1706<p>If any of the files is missing, goto 1.</p>
1707</li>
1708<li>
1709<p>Read from the now-open files as long as necessary.</p>
1710</li>
1711</ol>
1712</div>
1713</div>
1714<div class="sect3">
1715<h4 id="_update_transactions">Update transactions</h4>
1716<div class="paragraph">
1717<p>Although reftables are immutable, mutations are supported by writing a
1718new reftable and atomically appending it to the stack:</p>
1719</div>
1720<div class="olist arabic">
1721<ol class="arabic">
1722<li>
1723<p>Acquire <code>tables.list.lock</code>.</p>
1724</li>
1725<li>
1726<p>Read <code>tables.list</code> to determine current reftables.</p>
1727</li>
1728<li>
1729<p>Select <code>update_index</code> to be most recent file&#8217;s
1730<code>max_update_index + 1</code>.</p>
1731</li>
1732<li>
1733<p>Prepare temp reftable <code>tmp_XXXXXX</code>, including log entries.</p>
1734</li>
1735<li>
1736<p>Rename <code>tmp_XXXXXX</code> to <code>${update_index}-${update_index}-${random}.ref</code>.</p>
1737</li>
1738<li>
1739<p>Copy <code>tables.list</code> to <code>tables.list.lock</code>, appending file from (5).</p>
1740</li>
1741<li>
1742<p>Rename <code>tables.list.lock</code> to <code>tables.list</code>.</p>
1743</li>
1744</ol>
1745</div>
1746<div class="paragraph">
1747<p>During step 4 the new file&#8217;s <code>min_update_index</code> and <code>max_update_index</code>
1748are both set to the <code>update_index</code> selected by step 3. All log records
1749for the transaction use the same <code>update_index</code> in their keys. This
1750enables later correlation of which references were updated by the same
1751transaction.</p>
1752</div>
1753<div class="paragraph">
1754<p>Because a single <code>tables.list.lock</code> file is used to manage locking, the
1755repository is single-threaded for writers. Writers may have to busy-spin
1756(with backoff) around creating <code>tables.list.lock</code>, for up to an
1757acceptable wait period, aborting if the repository is too busy to
1758mutate. Application servers wrapped around repositories (e.g. Gerrit
1759Code Review) can layer their own lock/wait queue to improve fairness to
1760writers.</p>
1761</div>
1762</div>
1763<div class="sect3">
1764<h4 id="_reference_deletions">Reference deletions</h4>
1765<div class="paragraph">
1766<p>Deletion of any reference can be explicitly stored by setting the <code>type</code>
1767to <code>0x0</code> and omitting the <code>value</code> field of the <code>ref_record</code>. This serves
1768as a tombstone, overriding any assertions about the existence of the
1769reference from earlier files in the stack.</p>
1770</div>
1771</div>
1772<div class="sect3">
1773<h4 id="_compaction">Compaction</h4>
1774<div class="paragraph">
1775<p>A partial stack of reftables can be compacted by merging references
1776using a straightforward merge join across reftables, selecting the most
1777recent value for output, and omitting deleted references that do not
1778appear in remaining, lower reftables.</p>
1779</div>
1780<div class="paragraph">
1781<p>A compacted reftable should set its <code>min_update_index</code> to the smallest
1782of the input files' <code>min_update_index</code>, and its <code>max_update_index</code>
1783likewise to the largest input <code>max_update_index</code>.</p>
1784</div>
1785<div class="paragraph">
1786<p>For sake of illustration, assume the stack currently consists of
1787reftable files (from oldest to newest): A, B, C, and D. The compactor is
1788going to compact B and C, leaving A and D alone.</p>
1789</div>
1790<div class="olist arabic">
1791<ol class="arabic">
1792<li>
1793<p>Obtain lock <code>tables.list.lock</code> and read the <code>tables.list</code> file.</p>
1794</li>
1795<li>
1796<p>Obtain locks <code>B.lock</code> and <code>C.lock</code>. Ownership of these locks
1797prevents other processes from trying to compact these files.</p>
1798</li>
1799<li>
1800<p>Release <code>tables.list.lock</code>.</p>
1801</li>
1802<li>
1803<p>Compact <code>B</code> and <code>C</code> into a temp file
1804<code>${min_update_index}-${max_update_index}_XXXXXX</code>.</p>
1805</li>
1806<li>
1807<p>Reacquire lock <code>tables.list.lock</code>.</p>
1808</li>
1809<li>
1810<p>Verify that <code>B</code> and <code>C</code> are still in the stack, in that order. This
1811should always be the case, assuming that other processes are adhering to
1812the locking protocol.</p>
1813</li>
1814<li>
1815<p>Rename <code>${min_update_index}-${max_update_index}_XXXXXX</code> to
1816<code>${min_update_index}-${max_update_index}-${random}.ref</code>.</p>
1817</li>
1818<li>
1819<p>Write the new stack to <code>tables.list.lock</code>, replacing <code>B</code> and <code>C</code>
1820with the file from (4).</p>
1821</li>
1822<li>
1823<p>Rename <code>tables.list.lock</code> to <code>tables.list</code>.</p>
1824</li>
1825<li>
1826<p>Delete <code>B</code> and <code>C</code>, perhaps after a short sleep to avoid forcing
1827readers to backtrack.</p>
1828</li>
1829</ol>
1830</div>
1831<div class="paragraph">
1832<p>This strategy permits compactions to proceed independently of updates.</p>
1833</div>
1834<div class="paragraph">
1835<p>Each reftable (compacted or not) is uniquely identified by its name, so
1836open reftables can be cached by their name.</p>
1837</div>
1838</div>
1839<div class="sect3">
1840<h4 id="_windows">Windows</h4>
1841<div class="paragraph">
1842<p>On windows, and other systems that do not allow deleting or renaming to open
1843files, compaction may succeed, but other readers may prevent obsolete tables
1844from being deleted.</p>
1845</div>
1846<div class="paragraph">
1847<p>On these platforms, the following strategy can be followed: on closing a
1848reftable stack, reload <code>tables.list</code>, and delete any tables no longer mentioned
1849in <code>tables.list</code>.</p>
1850</div>
1851<div class="paragraph">
1852<p>Irregular program exit may still leave about unused files. In this case, a
1853cleanup operation should proceed as follows:</p>
1854</div>
1855<div class="ulist">
1856<ul>
1857<li>
1858<p>take a lock <code>tables.list.lock</code> to prevent concurrent modifications</p>
1859</li>
1860<li>
1861<p>refresh the reftable stack, by reading <code>tables.list</code></p>
1862</li>
1863<li>
1864<p>for each <code>*.ref</code> file, remove it if</p>
1865<div class="ulist">
1866<ul>
1867<li>
1868<p>it is not mentioned in <code>tables.list</code>, and</p>
1869</li>
1870<li>
1871<p>its max update_index is not beyond the max update_index of the stack</p>
1872</li>
1873</ul>
1874</div>
1875</li>
1876</ul>
1877</div>
1878</div>
1879</div>
1880<div class="sect2">
1881<h3 id="_alternatives_considered">Alternatives considered</h3>
1882<div class="sect3">
1883<h4 id="_bzip_packed_refs">bzip packed-refs</h4>
1884<div class="paragraph">
1885<p><code>bzip2</code> can significantly shrink a large packed-refs file (e.g. 62 MiB
1886compresses to 23 MiB, 37%). However the bzip format does not support
1887random access to a single reference. Readers must inflate and discard
1888while performing a linear scan.</p>
1889</div>
1890<div class="paragraph">
1891<p>Breaking packed-refs into chunks (individually compressing each chunk)
1892would reduce the amount of data a reader must inflate, but still leaves
1893the problem of indexing chunks to support readers efficiently locating
1894the correct chunk.</p>
1895</div>
1896<div class="paragraph">
1897<p>Given the compression achieved by reftable&#8217;s encoding, it does not seem
1898necessary to add the complexity of bzip/gzip/zlib.</p>
1899</div>
1900</div>
1901<div class="sect3">
1902<h4 id="_michael_haggertys_alternate_format">Michael Haggerty&#8217;s alternate format</h4>
1903<div class="paragraph">
1904<p>Michael Haggerty proposed
1905<a href="https://lore.kernel.org/git/CAMy9T_HCnyc1g8XWOOWhe7nN0aEFyyBskV2aOMb_fe%2BwGvEJ7A%40mail.gmail.com/">an
1906alternate</a> format to reftable on the Git mailing list. This format uses
1907smaller chunks, without the restart table, and avoids block alignment
1908with padding. Reflog entries immediately follow each ref, and are thus
1909interleaved between refs.</p>
1910</div>
1911<div class="paragraph">
1912<p>Performance testing indicates reftable is faster for lookups (51%
1913faster, 11.2 usec vs. 5.4 usec), although reftable produces a slightly
1914larger file (+ ~3.2%, 28.3M vs 29.2M):</p>
1915</div>
1916<table class="tableblock frame-all grid-all stretch">
1917<colgroup>
1918<col style="width: 25%;"/>
1919<col style="width: 25%;"/>
1920<col style="width: 25%;"/>
1921<col style="width: 25%;"/>
1922</colgroup>
1923<thead>
1924<tr>
1925<th class="tableblock halign-right valign-top">format</th>
1926<th class="tableblock halign-right valign-top">size</th>
1927<th class="tableblock halign-right valign-top">seek cold</th>
1928<th class="tableblock halign-right valign-top">seek hot</th>
1929</tr>
1930</thead>
1931<tbody>
1932<tr>
1933<td class="tableblock halign-right valign-top"><p class="tableblock">mh-alt</p></td>
1934<td class="tableblock halign-right valign-top"><p class="tableblock">28.3 M</p></td>
1935<td class="tableblock halign-right valign-top"><p class="tableblock">23.4 usec</p></td>
1936<td class="tableblock halign-right valign-top"><p class="tableblock">11.2 usec</p></td>
1937</tr>
1938<tr>
1939<td class="tableblock halign-right valign-top"><p class="tableblock">reftable</p></td>
1940<td class="tableblock halign-right valign-top"><p class="tableblock">29.2 M</p></td>
1941<td class="tableblock halign-right valign-top"><p class="tableblock">19.9 usec</p></td>
1942<td class="tableblock halign-right valign-top"><p class="tableblock">5.4 usec</p></td>
1943</tr>
1944</tbody>
1945</table>
1946</div>
1947<div class="sect3">
1948<h4 id="_jgit_ketch_reftree">JGit Ketch RefTree</h4>
1949<div class="paragraph">
1950<p><a href="https://dev.eclipse.org/mhonarc/lists/jgit-dev/msg03073.html">JGit Ketch</a>
1951proposed
1952<a href="https://lore.kernel.org/git/CAJo%3DhJvnAPNAdDcAAwAvU9C4RVeQdoS3Ev9WTguHx4fD0V_nOg%40mail.gmail.com/">RefTree</a>,
1953an encoding of references inside Git tree objects stored as part of the
1954repository&#8217;s object database.</p>
1955</div>
1956<div class="paragraph">
1957<p>The RefTree format adds additional load on the object database storage
1958layer (more loose objects, more objects in packs), and relies heavily on
1959the packer&#8217;s delta compression to save space. Namespaces which are flat
1960(e.g. thousands of tags in refs/tags) initially create very large loose
1961objects, and so RefTree does not address the problem of copying many
1962references to modify a handful.</p>
1963</div>
1964<div class="paragraph">
1965<p>Flat namespaces are not efficiently searchable in RefTree, as tree
1966objects in canonical formatting cannot be binary searched. This fails
1967the need to handle a large number of references in a single namespace,
1968such as GitHub&#8217;s <code>refs/pulls</code>, or a project with many tags.</p>
1969</div>
1970</div>
1971<div class="sect3">
1972<h4 id="_lmdb">LMDB</h4>
1973<div class="paragraph">
1974<p>David Turner proposed
1975<a href="https://lore.kernel.org/git/1455772670-21142-26-git-send-email-dturner@twopensource.com/">using
1976LMDB</a>, as LMDB is lightweight (64k of runtime code) and GPL-compatible
1977license.</p>
1978</div>
1979<div class="paragraph">
1980<p>A downside of LMDB is its reliance on a single C implementation. This
1981makes embedding inside JGit (a popular reimplementation of Git)
1982difficult, and hoisting onto virtual storage (for JGit DFS) virtually
1983impossible.</p>
1984</div>
1985<div class="paragraph">
1986<p>A common format that can be supported by all major Git implementations
1987(git-core, JGit, libgit2) is strongly preferred.</p>
1988</div>
1989</div>
1990</div>
1991</div>
1992</div>
1993</div>
1994<div id="footer">
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