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 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,
 449 rails 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
 451 single reference requires linearly scanning the file.</p>
 452 </div>
 453 <div class="paragraph">
 454 <p>Atomic pushes modifying multiple references require copying the entire
 455 packed-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
 460 blocks from the local file system, as each reference is its own file
 461 storing 41 bytes (and another file for the corresponding reflog). This
 462 negatively affects the number of inodes available when a large number of
 463 repositories are stored on the same filesystem. Readers can be penalized
 464 due 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
 474 repository 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
 478 one 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</code>(<code>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
 499 reference storage. References are sorted, enabling linear scans, binary
 500 search lookup, and range scans.</p>
 501 </div>
 502 <div class="paragraph">
 503 <p>Storage in the file is organized into variable sized blocks. Prefix
 504 compression is used within a single block to reduce disk space. Block
 505 size 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
 569 refs), 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
 657 to. 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
 683 within 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
 691 while (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
 712 confuse versions of Git using <code>$GIT_DIR/refs</code> directory tree to maintain
 713 references. 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 }
 731 ref_block*
 732 ref_index*
 733 obj_block*
 734 obj_index*
 735 log_block*
 736 log_index*
 737 footer</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 }
 749 log_block*
 750 log_index*
 751 footer</pre>
 752 </div>
 753 </div>
 754 <div class="paragraph">
 755 <p>In a log-only file, the first log block immediately follows the file
 756 header, 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
 763 not have to be a power of 2. The block size must be larger than the
 764 longest reference name or log entry used in the repository, as
 765 references cannot span blocks.</p>
 766 </div>
 767 <div class="paragraph">
 768 <p>Powers of two that are friendly to the virtual memory system or
 769 filesystem (such as 4k or 8k) are recommended. Larger sizes (64k) can
 770 yield better compression, with a possible increased cost incurred by
 771 readers 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
 781 including <code>padding</code> filled with NUL bytes at the end of a block to round
 782 out to the chosen alignment. When alignment is used, writers must
 783 specify 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
 787 set <code>block_size</code> <code>=</code> <code>0</code> in the file header, and omit <code>padding</code>. Unaligned
 788 files with more than one ref block must include the <a href="#Ref-index">ref
 789 index</a> to support fast lookup. Readers must be able to read both aligned
 790 and 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
 794 index 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'
 805 uint8( version_number = 1 )
 806 uint24( block_size )
 807 uint64( min_update_index )
 808 uint64( 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
 813 expected block alignment. Unaligned files must set <code>block_size</code> <code>=</code> <code>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
 818 used in a stack for <a href="#Update-transactions">transactions</a>, these
 819 fields can order the files such that the prior file&#8217;s
 820 <code>max_update_index</code> <code>+</code> <code>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'
 831 uint8( version_number = 2 )
 832 uint24( block_size )
 833 uint64( min_update_index )
 834 uint64( max_update_index )
 835 uint32( 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
 844 writing 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
 851 bytes smaller than all other blocks in the file. The first block
 852 immediately 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
 856 begins 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'
 867 uint24( block_len )
 868 ref_record+
 869 uint24( restart_offset )+
 870 uint16( restart_count )
 871
 872 padding?</pre>
 873 </div>
 874 </div>
 875 <div class="paragraph">
 876 <p>Blocks begin with <code>block_type</code> <code>=</code> 'r' and a 3-byte <code>block_len</code> which
 877 encodes the number of bytes in the block up to, but not including the
 878 optional <code>padding</code>. This is always less than or equal to the file&#8217;s
 879 block size. In the first ref block, <code>block_len</code> includes 24 bytes for
 880 the 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
 886 linear 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
 891 refer to the first byte of any <code>ref_record</code> whose name has not been
 892 prefix compressed. Entries in the <code>restart_offset</code> list must be sorted,
 893 ascending. 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,
 897 describing 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,
 901 all <code>restart_offset</code> in the first block are relative to the start of the
 902 file (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
 909 its value(s). Records are formatted as:</p>
 910 </div>
 911 <div class="literalblock">
 912 <div class="content">
 913 <pre>varint( prefix_length )
 914 varint( (suffix_length &lt;&lt; 3) | value_type )
 915 suffix
 916 varint( update_index_delta )
 917 value?</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
 922 reference record&#8217;s name should be copied to obtain this reference&#8217;s
 923 name. This must be 0 for the first reference in any block, and also must
 924 be 0 for any <code>ref_record</code> whose offset is listed in the <code>restart_offset</code>
 925 table 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
 941 adding <code>update_index_delta</code> to the <code>min_update_index</code> from the file
 942 header: <code>min_update_index</code> <code>+</code> <code>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
 946 the 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</code>( <code>target_len</code> ) <code>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
 966 reference target. No compression is applied to the target name.</p>
 967 </div>
 968 <div class="paragraph">
 969 <p>Types <code>0x4</code><code>..</code><code>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
 977 in the file, enabling reduced disk seeks for lookups. Any reference can
 978 be found by searching the index, identifying the containing block, and
 979 searching 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
 983 index block points to additional ref index blocks (2nd level), which may
 984 in turn point to either additional index blocks (e.g. 3rd level) or ref
 985 blocks (leaf level). Disk reads required to access a ref go up with
 986 higher index levels. Multi-level indexes may be required to ensure no
 987 single 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
 989 lookups the index must be a single level, which is permitted to exceed
 990 the file&#8217;s configured block size, but not the format&#8217;s max block size of
 991 15.99 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
 998 to 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
1004 index 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'
1012 uint24( block_len )
1013 index_record+
1014 uint24( restart_offset )+
1015 uint16( restart_count )
1016
1017 padding?</pre>
1018 </div>
1019 </div>
1020 <div class="paragraph">
1021 <p>The index blocks begin with <code>block_type</code> <code>=</code> 'i' and a 3-byte <code>block_len</code>
1022 which encodes the number of bytes in the block, up to but not including
1023 the 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,
1027 meaning 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
1031 files the index block may be larger than other blocks. However, readers
1032 must hold the entire index in memory to benefit from this, so it&#8217;s a
1033 time-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
1037 a multi-level index may be used, keeping index blocks within the file&#8217;s
1038 block size, but increasing the number of blocks that need to be
1039 accessed.</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
1045 are written as:</p>
1046 </div>
1047 <div class="literalblock">
1048 <div class="content">
1049 <pre>varint( prefix_length )
1050 varint( (suffix_length &lt;&lt; 3) | 0 )
1051 suffix
1052 varint( 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
1060 absolute position in bytes (from the start of the file) of the block
1061 that ends with this reference. Readers can seek to <code>block_position</code> to
1062 begin 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
1066 if the next block is another level index block, or the leaf-level ref
1067 block.</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
1074 obtain <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,
1083 especially 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
1087 ref blocks containing references pointing to that object directly, or as
1088 the peeled value of an annotated tag. Like ref blocks, object blocks use
1089 the file&#8217;s standard block size. The abbreviation length is available in
1090 the 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
1094 index is not present, as brute force search will only need to read a few
1095 ref blocks. When missing, readers should brute force a linear search of
1096 all 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'
1104 uint24( block_len )
1105 obj_record+
1106 uint24( restart_offset )+
1107 uint16( restart_count )
1108
1109 padding?</pre>
1110 </div>
1111 </div>
1112 <div class="paragraph">
1113 <p>Fields are identical to ref block. Binary search using the restart table
1114 works 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
1118 abbreviation within the reftable, obj key lengths have a variable length. Their
1119 length must be at least 2 bytes. Readers must compare only for common prefix
1120 match 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
1126 containing references using that unique abbreviation:</p>
1127 </div>
1128 <div class="literalblock">
1129 <div class="content">
1130 <pre>varint( prefix_length )
1131 varint( (suffix_length &lt;&lt; 3) | cnt_3 )
1132 suffix
1133 varint( cnt_large )?
1134 varint( position_delta )*</pre>
1135 </div>
1136 </div>
1137 <div class="paragraph">
1138 <p>Like in reference blocks, abbreviations are prefix compressed within an
1139 obj block. On large reftables with many unique objects, higher block
1140 sizes (64k), and higher restart interval (128), a <code>prefix_length</code> of 2
1141 or 3 and <code>suffix_length</code> of 3 may be common in obj records (unique
1142 abbreviation 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
1146 ref blocks. For 1-7 positions the count is stored in <code>cnt_3</code>. When
1147 <code>cnt_3</code> <code>=</code> <code>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
1151 reference, some may be pointed to by a couple of references, and very
1152 few (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</code> <code>=</code> <code>0</code> and <code>cnt_large</code> <code>=</code> <code>0</code>: there are no
1156 <code>position_delta</code>, but at least one reference starts with this
1157 abbreviation. A reader that needs exact reference names must scan all
1158 references to find which specific references have the desired object.
1159 Writers should use this format when the <code>position_delta</code> list would have
1160 overflowed the file&#8217;s block size due to a high number of references
1161 pointing 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.
1165 Additional <code>position_delta</code> entries are sorted ascending and relative to
1166 the 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]
1171 prior = pos
1172 for (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,
1180 starting from the first <code>ref_record</code>, testing each reference&#8217;s object names
1181 (for <code>value_type</code> <code>=</code> <code>0x1</code> or <code>0x2</code>) for full equality. Faster searching by
1182 object name within a single ref block is not supported by the reftable format.
1183 Smaller block sizes reduce the number of candidates this step must
1184 consider.</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
1192 block in the file, enabling reduced disk seeks for all lookups. It is
1193 formatted 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
1197 should 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
1201 obtain <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>
1211 specified in the file header or footer. Writers should choose an
1212 appropriate buffer size to prepare a log block for deflation, such as
1213 <code>2</code> * <code>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'
1221 uint24( block_len )
1222 zlib_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</code> <code>=</code> 'g'.</p>
1231 </div>
1232 <div class="paragraph">
1233 <p>The 4-byte block header is followed by the deflated block contents using
1234 zlib 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
1236 preallocate the inflation output buffer. A log block&#8217;s <code>block_len</code> may
1237 exceed 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
1241 4-byte header. Readers may prefer prefixing the inflation output buffer
1242 with 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
1246 reference changes. The log record format is described below. See ref
1247 block 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
1252 must keep track of the bytes consumed by the inflater to know where the
1253 next 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>.
1268 See 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
1272 ordering the network byte order encoding sorts the more recent records
1273 with 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,
1284 utilizing the same prefix compression scheme applied to the log record
1285 key 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</code> <code>=</code> <code>0x0</code> is mostly useful for <code>git</code> <code>stash</code> <code>drop</code>, removing an
1318 entry from the reflog of <code>refs/stash</code> in a transaction file (below),
1319 without needing to rewrite larger files. Readers reading a stack of
1320 reflogs must treat this as a deletion.</p>
1321 </div>
1322 <div class="paragraph">
1323 <p>For <code>log_type</code> <code>=</code> <code>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
1350 at the time of the update. For example <code>GMT-0800</code> is encoded in reftable
1351 as <code>sint16</code>(<code>-480</code>) and <code>GMT+0230</code> is <code>sint16</code>(<code>150</code>).</p>
1352 </div>
1353 <div class="paragraph">
1354 <p>The committer email does not contain &lt; or &gt;, it&#8217;s the value normally
1355 found between the &lt;&gt; 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
1359 supplied for the update.</p>
1360 </div>
1361 <div class="paragraph">
1362 <p>Contrary to traditional reflog (which is a file), renames are encoded as
1363 a combination of ref deletion and ref creation.  A deletion is a log
1364 record 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
1371 determine 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
1373 not 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
1380 log records roughly by timestamp while preserving file order, and assign
1381 unique, increasing <code>update_index</code> values for each log line. Newer log
1382 records 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
1386 file must span many unique <code>update_index</code>, as each log line requires its
1387 own <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</code> \0 <code>reverse_int64</code>(<code>update_index</code>)) for the last log record of
1396 every 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
1400 the file. If present, the log index appears after the last log block.
1401 There 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
1405 longer to include '\0' and the 8-byte <code>reverse_int64</code>(<code>update_index</code>).
1406 Records 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
1412 obtain <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
1420 like 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
1438 field (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
1447 obj 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
1465 version 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
1471 number. Then they seek to <code>file_length</code> <code>-</code> <code>FOOTER_LENGTH</code> to access the
1472 footer. A trusted external source (such as <code>stat</code>(<code>2</code>)) is necessary to
1473 obtain <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
1485 version)</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
1497 and 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
1505 at the end of the block. Readers can binary search through the restart
1506 table to locate between which two restart points the sought reference or
1507 key 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
1511 the <code>suffix</code> field of the record, making the compare operation during
1512 binary search straightforward.</p>
1513 </div>
1514 <div class="paragraph">
1515 <p>Once a restart point lexicographically before the sought reference has
1516 been identified, readers can linearly scan through the following record
1517 entries to locate the sought record, terminating if the current record
1518 sorts 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
1524 arbitrary, but every 16 or 64 records is recommended. Every 16 may be
1525 more suitable for smaller block sizes (4k or 8k), every 64 for larger
1526 block sizes (64k).</p>
1527 </div>
1528 <div class="paragraph">
1529 <p>More frequent restart points reduces prefix compression and increases
1530 space 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,
1534 decreasing overall file size, with increased penalties for readers
1535 walking 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
1549 object 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
1551 lightweight 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
1555 reference.</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
1562 bytes for reftable, vs. 332 bytes for packed-refs. This supports
1563 reftable 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
1570 block sizes (64 KiB) and less frequent restart points (every 64) yield
1571 better compression due to more references within the block compressing
1572 against the prior reference.</p>
1573 </div>
1574 <div class="paragraph">
1575 <p>Larger block sizes reduce the index size, as the reftable will require
1576 fewer 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
1583 for any single reference requires exactly 1 disk seek to load the
1584 containing 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.
1592 Object 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
1599 saves 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
1603 reference readers that want to ignore logs. Further, historical logs can
1604 be 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
1611 to answer <code>master@</code>{4}), so log records are grouped by reference, and
1612 sorted 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.
1636 Their names should have a random element, such that each filename is globally
1637 unique; this helps avoid spurious failures on Windows, where open files cannot
1638 be removed or overwritten. It suggested to use
1639 <code>$</code>{min_update_index}-${max_update_index}-${random}.<code>ref</code> as a naming convention.</p>
1640 </div>
1641 <div class="paragraph">
1642 <p>Log-only files use the .<code>log</code> extension, while ref-only and mixed ref
1643 and log files use .<code>ref</code>. extension.</p>
1644 </div>
1645 <div class="paragraph">
1646 <p>The stack ordering file is <code>$GIT_DIR/reftable/tables.list</code> and lists the
1647 current 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
1653 00000001-00000001-RANDOM1.log
1654 00000002-00000002-RANDOM2.ref
1655 00000003-00000003-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
1660 files are relevant right now, and search through the stack in reverse
1661 order (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
1665 added to the stack by the active writer), or ancient and ready to be
1666 pruned.</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
1673 repository so they don&#8217;t look for an enclosing repository in parent
1674 directories. To this end, a reftable-enabled repository must contain the
1675 following 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:</code> <code>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
1695 following:</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
1718 new 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</code> <code>+</code> <code>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>$</code>{update_index}-${update_index}-${random}.<code>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>
1748 are both set to the <code>update_index</code> selected by step 3. All log records
1749 for the transaction use the same <code>update_index</code> in their keys. This
1750 enables later correlation of which references were updated by the same
1751 transaction.</p>
1752 </div>
1753 <div class="paragraph">
1754 <p>Because a single <code>tables.list.lock</code> file is used to manage locking, the
1755 repository 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
1757 acceptable wait period, aborting if the repository is too busy to
1758 mutate. Application servers wrapped around repositories (e.g. Gerrit
1759 Code Review) can layer their own lock/wait queue to improve fairness to
1760 writers.</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>
1767 to <code>0x0</code> and omitting the <code>value</code> field of the <code>ref_record</code>. This serves
1768 as a tombstone, overriding any assertions about the existence of the
1769 reference 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
1776 using a straightforward merge join across reftables, selecting the most
1777 recent value for output, and omitting deleted references that do not
1778 appear 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
1782 of the input files' <code>min_update_index</code>, and its <code>max_update_index</code>
1783 likewise 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
1787 reftable files (from oldest to newest): A, B, C, and D. The compactor is
1788 going 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
1797 prevents 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>$</code>{min_update_index}-${max_update_index}_XXXXXX.</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
1811 should always be the case, assuming that other processes are adhering to
1812 the locking protocol.</p>
1813 </li>
1814 <li>
1815 <p>Rename <code>$</code>{min_update_index}-${max_update_index}_XXXXXX to
1816 <code>$</code>{min_update_index}-${max_update_index}-${random}.<code>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>
1820 with 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
1827 readers 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
1836 open 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
1843 files, compaction may succeed, but other readers may prevent obsolete tables
1844 from being deleted.</p>
1845 </div>
1846 <div class="paragraph">
1847 <p>On these platforms, the following strategy can be followed: on closing a
1848 reftable stack, reload <code>tables.list</code>, and delete any tables no longer mentioned
1849 in <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
1853 cleanup 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
1886 compresses to 23 MiB, 37%). However the bzip format does not support
1887 random access to a single reference. Readers must inflate and discard
1888 while performing a linear scan.</p>
1889 </div>
1890 <div class="paragraph">
1891 <p>Breaking packed-refs into chunks (individually compressing each chunk)
1892 would reduce the amount of data a reader must inflate, but still leaves
1893 the problem of indexing chunks to support readers efficiently locating
1894 the correct chunk.</p>
1895 </div>
1896 <div class="paragraph">
1897 <p>Given the compression achieved by reftable&#8217;s encoding, it does not seem
1898 necessary 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
1906 alternate</a> format to reftable on the Git mailing list. This format uses
1907 smaller chunks, without the restart table, and avoids block alignment
1908 with padding. Reflog entries immediately follow each ref, and are thus
1909 interleaved between refs.</p>
1910 </div>
1911 <div class="paragraph">
1912 <p>Performance testing indicates reftable is faster for lookups (51%
1913 faster, 11.2 usec vs. 5.4 usec), although reftable produces a slightly
1914 larger 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>
1951 proposed
1952 <a href="https://lore.kernel.org/git/CAJo%3DhJvnAPNAdDcAAwAvU9C4RVeQdoS3Ev9WTguHx4fD0V_nOg%40mail.gmail.com/">RefTree</a>,
1953 an encoding of references inside Git tree objects stored as part of the
1954 repository&#8217;s object database.</p>
1955 </div>
1956 <div class="paragraph">
1957 <p>The RefTree format adds additional load on the object database storage
1958 layer (more loose objects, more objects in packs), and relies heavily on
1959 the 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
1961 objects, and so RefTree does not address the problem of copying many
1962 references to modify a handful.</p>
1963 </div>
1964 <div class="paragraph">
1965 <p>Flat namespaces are not efficiently searchable in RefTree, as tree
1966 objects in canonical formatting cannot be binary searched. This fails
1967 the need to handle a large number of references in a single namespace,
1968 such 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
1976 LMDB</a>, as LMDB is lightweight (64k of runtime code) and GPL-compatible
1977 license.</p>
1978 </div>
1979 <div class="paragraph">
1980 <p>A downside of LMDB is its reliance on a single C implementation. This
1981 makes embedding inside JGit (a popular reimplementation of Git)
1982 difficult, and hoisting onto virtual storage (for JGit DFS) virtually
1983 impossible.</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">
1995 <div id="footer-text">
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