gitformat-pack.html

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   8 <title>gitformat-pack(5)</title>
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 441 <body class="manpage">
 442 <div id="header">
 443 <h1>gitformat-pack(5) Manual Page</h1>
 444 <h2 id="_name">NAME</h2>
 445 <div class="sectionbody">
 446 <p>gitformat-pack - Git pack format</p>
 447 </div>
 448 </div>
 449 <div id="content">
 450 <div class="sect1">
 451 <h2 id="_synopsis">SYNOPSIS</h2>
 452 <div class="sectionbody">
 453 <div class="verseblock">
 454 <pre class="content">$GIT_DIR/objects/pack/pack-<strong>.{pack,idx}
 455 $GIT_DIR/objects/pack/pack-</strong>.rev
 456 $GIT_DIR/objects/pack/pack-*.mtimes
 457 $GIT_DIR/objects/pack/multi-pack-index</pre>
 458 </div>
 459 </div>
 460 </div>
 461 <div class="sect1">
 462 <h2 id="_description">DESCRIPTION</h2>
 463 <div class="sectionbody">
 464 <div class="paragraph">
 465 <p>The Git pack format is how Git stores most of its primary repository
 466 data. Over the lifetime of a repository, loose objects (if any) and
 467 smaller packs are consolidated into larger pack(s). See
 468 <a href="git-gc.html">git-gc(1)</a> and <a href="git-pack-objects.html">git-pack-objects(1)</a>.</p>
 469 </div>
 470 <div class="paragraph">
 471 <p>The pack format is also used over-the-wire, see
 472 e.g. <a href="gitprotocol-v2.html">gitprotocol-v2(5)</a>, as well as being a part of
 473 other container formats in the case of <a href="gitformat-bundle.html">gitformat-bundle(5)</a>.</p>
 474 </div>
 475 </div>
 476 </div>
 477 <div class="sect1">
 478 <h2 id="_checksums_and_object_ids">Checksums and object IDs</h2>
 479 <div class="sectionbody">
 480 <div class="paragraph">
 481 <p>In a repository using the traditional SHA-1, pack checksums, index checksums,
 482 and object IDs (object names) mentioned below are all computed using SHA-1.
 483 Similarly, in SHA-256 repositories, these values are computed using SHA-256.</p>
 484 </div>
 485 </div>
 486 </div>
 487 <div class="sect1">
 488 <h2 id="_pack_pack_files_have_the_following_format">pack-*.pack files have the following format:</h2>
 489 <div class="sectionbody">
 490 <div class="ulist">
 491 <ul>
 492 <li>
 493 <p>A header appears at the beginning and consists of the following:</p>
 494 <div class="literalblock">
 495 <div class="content">
 496 <pre>4-byte signature:
 497     The signature is: {'P', 'A', 'C', 'K'}</pre>
 498 </div>
 499 </div>
 500 <div class="literalblock">
 501 <div class="content">
 502 <pre>   4-byte version number (network byte order):
 503 Git currently accepts version number 2 or 3 but
 504        generates version 2 only.</pre>
 505 </div>
 506 </div>
 507 <div class="literalblock">
 508 <div class="content">
 509 <pre>4-byte number of objects contained in the pack (network byte order)</pre>
 510 </div>
 511 </div>
 512 <div class="literalblock">
 513 <div class="content">
 514 <pre>Observation: we cannot have more than 4G versions ;-) and
 515 more than 4G objects in a pack.</pre>
 516 </div>
 517 </div>
 518 </li>
 519 <li>
 520 <p>The header is followed by a number of object entries, each of
 521 which looks like this:</p>
 522 <div class="literalblock">
 523 <div class="content">
 524 <pre>(undeltified representation)
 525 n-byte type and length (3-bit type, (n-1)*7+4-bit length)
 526 compressed data</pre>
 527 </div>
 528 </div>
 529 <div class="literalblock">
 530 <div class="content">
 531 <pre>   (deltified representation)
 532    n-byte type and length (3-bit type, (n-1)*7+4-bit length)
 533    base object name if OBJ_REF_DELTA or a negative relative
 534 offset from the delta object's position in the pack if this
 535 is an OBJ_OFS_DELTA object
 536    compressed delta data</pre>
 537 </div>
 538 </div>
 539 <div class="literalblock">
 540 <div class="content">
 541 <pre>Observation: the length of each object is encoded in a variable
 542 length format and is not constrained to 32-bit or anything.</pre>
 543 </div>
 544 </div>
 545 </li>
 546 <li>
 547 <p>The trailer records a pack checksum of all of the above.</p>
 548 </li>
 549 </ul>
 550 </div>
 551 <div class="sect2">
 552 <h3 id="_object_types">Object types</h3>
 553 <div class="paragraph">
 554 <p>Valid object types are:</p>
 555 </div>
 556 <div class="ulist">
 557 <ul>
 558 <li>
 559 <p>OBJ_COMMIT (1)</p>
 560 </li>
 561 <li>
 562 <p>OBJ_TREE (2)</p>
 563 </li>
 564 <li>
 565 <p>OBJ_BLOB (3)</p>
 566 </li>
 567 <li>
 568 <p>OBJ_TAG (4)</p>
 569 </li>
 570 <li>
 571 <p>OBJ_OFS_DELTA (6)</p>
 572 </li>
 573 <li>
 574 <p>OBJ_REF_DELTA (7)</p>
 575 </li>
 576 </ul>
 577 </div>
 578 <div class="paragraph">
 579 <p>Type 5 is reserved for future expansion. Type 0 is invalid.</p>
 580 </div>
 581 </div>
 582 <div class="sect2">
 583 <h3 id="_size_encoding">Size encoding</h3>
 584 <div class="paragraph">
 585 <p>This document uses the following "size encoding" of non-negative
 586 integers: From each byte, the seven least significant bits are
 587 used to form the resulting integer. As long as the most significant
 588 bit is 1, this process continues; the byte with MSB 0 provides the
 589 last seven bits.  The seven-bit chunks are concatenated. Later
 590 values are more significant.</p>
 591 </div>
 592 <div class="paragraph">
 593 <p>This size encoding should not be confused with the "offset encoding",
 594 which is also used in this document.</p>
 595 </div>
 596 </div>
 597 <div class="sect2">
 598 <h3 id="_deltified_representation">Deltified representation</h3>
 599 <div class="paragraph">
 600 <p>Conceptually there are only four object types: commit, tree, tag and
 601 blob. However to save space, an object could be stored as a "delta" of
 602 another "base" object. These representations are assigned new types
 603 ofs-delta and ref-delta, which is only valid in a pack file.</p>
 604 </div>
 605 <div class="paragraph">
 606 <p>Both ofs-delta and ref-delta store the "delta" to be applied to
 607 another object (called <em>base object</em>) to reconstruct the object. The
 608 difference between them is, ref-delta directly encodes base object
 609 name. If the base object is in the same pack, ofs-delta encodes
 610 the offset of the base object in the pack instead.</p>
 611 </div>
 612 <div class="paragraph">
 613 <p>The base object could also be deltified if it&#8217;s in the same pack.
 614 Ref-delta can also refer to an object outside the pack (i.e. the
 615 so-called "thin pack"). When stored on disk however, the pack should
 616 be self contained to avoid cyclic dependency.</p>
 617 </div>
 618 <div class="paragraph">
 619 <p>The delta data starts with the size of the base object and the
 620 size of the object to be reconstructed. These sizes are
 621 encoded using the size encoding from above.  The remainder of
 622 the delta data is a sequence of instructions to reconstruct the object
 623 from the base object. If the base object is deltified, it must be
 624 converted to canonical form first. Each instruction appends more and
 625 more data to the target object until it&#8217;s complete. There are two
 626 supported instructions so far: one for copying a byte range from the
 627 source object and one for inserting new data embedded in the
 628 instruction itself.</p>
 629 </div>
 630 <div class="paragraph">
 631 <p>Each instruction has variable length. Instruction type is determined
 632 by the seventh bit of the first octet. The following diagrams follow
 633 the convention in RFC 1951 (Deflate compressed data format).</p>
 634 </div>
 635 <div class="sect3">
 636 <h4 id="_instruction_to_copy_from_base_object">Instruction to copy from base object</h4>
 637 <div class="literalblock">
 638 <div class="content">
 639 <pre>+----------+---------+---------+---------+---------+-------+-------+-------+
 640 | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
 641 +----------+---------+---------+---------+---------+-------+-------+-------+</pre>
 642 </div>
 643 </div>
 644 <div class="paragraph">
 645 <p>This is the instruction format to copy a byte range from the source
 646 object. It encodes the offset to copy from and the number of bytes to
 647 copy. Offset and size are in little-endian order.</p>
 648 </div>
 649 <div class="paragraph">
 650 <p>All offset and size bytes are optional. This is to reduce the
 651 instruction size when encoding small offsets or sizes. The first seven
 652 bits in the first octet determine which of the next seven octets is
 653 present. If bit zero is set, offset1 is present. If bit one is set
 654 offset2 is present and so on.</p>
 655 </div>
 656 <div class="paragraph">
 657 <p>Note that a more compact instruction does not change offset and size
 658 encoding. For example, if only offset2 is omitted like below, offset3
 659 still contains bits 16-23. It does not become offset2 and contains
 660 bits 8-15 even if it&#8217;s right next to offset1.</p>
 661 </div>
 662 <div class="literalblock">
 663 <div class="content">
 664 <pre>+----------+---------+---------+
 665 | 10000101 | offset1 | offset3 |
 666 +----------+---------+---------+</pre>
 667 </div>
 668 </div>
 669 <div class="paragraph">
 670 <p>In its most compact form, this instruction only takes up one byte
 671 (0x80) with both offset and size omitted, which will have default
 672 values zero. There is another exception: size zero is automatically
 673 converted to 0x10000.</p>
 674 </div>
 675 </div>
 676 <div class="sect3">
 677 <h4 id="_instruction_to_add_new_data">Instruction to add new data</h4>
 678 <div class="literalblock">
 679 <div class="content">
 680 <pre>+----------+============+
 681 | 0xxxxxxx |    data    |
 682 +----------+============+</pre>
 683 </div>
 684 </div>
 685 <div class="paragraph">
 686 <p>This is the instruction to construct the target object without the base
 687 object. The following data is appended to the target object. The first
 688 seven bits of the first octet determine the size of data in
 689 bytes. The size must be non-zero.</p>
 690 </div>
 691 </div>
 692 <div class="sect3">
 693 <h4 id="_reserved_instruction">Reserved instruction</h4>
 694 <div class="literalblock">
 695 <div class="content">
 696 <pre>+----------+============
 697 | 00000000 |
 698 +----------+============</pre>
 699 </div>
 700 </div>
 701 <div class="paragraph">
 702 <p>This is the instruction reserved for future expansion.</p>
 703 </div>
 704 </div>
 705 </div>
 706 </div>
 707 </div>
 708 <div class="sect1">
 709 <h2 id="_original_version_1_pack_idx_files_have_the_following_format">Original (version 1) pack-*.idx files have the following format:</h2>
 710 <div class="sectionbody">
 711 <div class="ulist">
 712 <ul>
 713 <li>
 714 <p>The header consists of 256 4-byte network byte order
 715 integers.  N-th entry of this table records the number of
 716 objects in the corresponding pack, the first byte of whose
 717 object name is less than or equal to N.  This is called the
 718 <em>first-level fan-out</em> table.</p>
 719 </li>
 720 <li>
 721 <p>The header is followed by sorted 24-byte entries, one entry
 722 per object in the pack.  Each entry is:</p>
 723 <div class="literalblock">
 724 <div class="content">
 725 <pre>4-byte network byte order integer, recording where the
 726 object is stored in the packfile as the offset from the
 727 beginning.</pre>
 728 </div>
 729 </div>
 730 <div class="literalblock">
 731 <div class="content">
 732 <pre>one object name of the appropriate size.</pre>
 733 </div>
 734 </div>
 735 </li>
 736 <li>
 737 <p>The file is concluded with a trailer:</p>
 738 <div class="literalblock">
 739 <div class="content">
 740 <pre>A copy of the pack checksum at the end of the corresponding
 741 packfile.</pre>
 742 </div>
 743 </div>
 744 <div class="literalblock">
 745 <div class="content">
 746 <pre>Index checksum of all of the above.</pre>
 747 </div>
 748 </div>
 749 </li>
 750 </ul>
 751 </div>
 752 <div class="paragraph">
 753 <p>Pack Idx file:</p>
 754 </div>
 755 <div class="literalblock">
 756 <div class="content">
 757 <pre>   --  +--------------------------------+
 758 fanout      | fanout[0] = 2 (for example)    |-.
 759 table       +--------------------------------+ |
 760             | fanout[1]                      | |
 761             +--------------------------------+ |
 762             | fanout[2]                      | |
 763             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
 764             | fanout[255] = total objects    |---.
 765         --  +--------------------------------+ | |
 766 main        | offset                         | | |
 767 index       | object name 00XXXXXXXXXXXXXXXX | | |
 768 table       +--------------------------------+ | |
 769             | offset                         | | |
 770             | object name 00XXXXXXXXXXXXXXXX | | |
 771             +--------------------------------+&lt;+ |
 772           .-| offset                         |   |
 773           | | object name 01XXXXXXXXXXXXXXXX |   |
 774           | +--------------------------------+   |
 775           | | offset                         |   |
 776           | | object name 01XXXXXXXXXXXXXXXX |   |
 777           | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
 778           | | offset                         |   |
 779           | | object name FFXXXXXXXXXXXXXXXX |   |
 780         --| +--------------------------------+&lt;--+
 781 trailer   | | packfile checksum              |
 782           | +--------------------------------+
 783           | | idxfile checksum               |
 784           | +--------------------------------+
 785           .-------.
 786                   |
 787 Pack file entry: &lt;+</pre>
 788 </div>
 789 </div>
 790 <div class="literalblock">
 791 <div class="content">
 792 <pre>    packed object header:
 793 1-byte size extension bit (MSB)
 794        type (next 3 bit)
 795        size0 (lower 4-bit)
 796        n-byte sizeN (as long as MSB is set, each 7-bit)
 797         size0..sizeN form 4+7+7+..+7 bit integer, size0
 798         is the least significant part, and sizeN is the
 799         most significant part.
 800     packed object data:
 801        If it is not DELTA, then deflated bytes (the size above
 802         is the size before compression).
 803 If it is REF_DELTA, then
 804   base object name (the size above is the
 805         size of the delta data that follows).
 806          delta data, deflated.
 807 If it is OFS_DELTA, then
 808   n-byte offset (see below) interpreted as a negative
 809         offset from the type-byte of the header of the
 810         ofs-delta entry (the size above is the size of
 811         the delta data that follows).
 812   delta data, deflated.</pre>
 813 </div>
 814 </div>
 815 <div class="literalblock">
 816 <div class="content">
 817 <pre>  offset encoding:
 818 n bytes with MSB set in all but the last one.
 819 The offset is then the number constructed by
 820 concatenating the lower 7 bit of each byte, and
 821 for n &gt;= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
 822 to the result.</pre>
 823 </div>
 824 </div>
 825 </div>
 826 </div>
 827 <div class="sect1">
 828 <h2 id="_version_2_pack_idx_files_support_packs_larger_than_4_gib_and">Version 2 pack-*.idx files support packs larger than 4 GiB, and</h2>
 829 <div class="sectionbody">
 830 <div class="literalblock">
 831 <div class="content">
 832 <pre>have some other reorganizations.  They have the format:</pre>
 833 </div>
 834 </div>
 835 <div class="ulist">
 836 <ul>
 837 <li>
 838 <p>A 4-byte magic number <em>\377tOc</em> which is an unreasonable
 839 fanout[0] value.</p>
 840 </li>
 841 <li>
 842 <p>A 4-byte version number (= 2)</p>
 843 </li>
 844 <li>
 845 <p>A 256-entry fan-out table just like v1.</p>
 846 </li>
 847 <li>
 848 <p>A table of sorted object names.  These are packed together
 849 without offset values to reduce the cache footprint of the
 850 binary search for a specific object name.</p>
 851 </li>
 852 <li>
 853 <p>A table of 4-byte CRC32 values of the packed object data.
 854 This is new in v2 so compressed data can be copied directly
 855 from pack to pack during repacking without undetected
 856 data corruption.</p>
 857 </li>
 858 <li>
 859 <p>A table of 4-byte offset values (in network byte order).
 860 These are usually 31-bit pack file offsets, but large
 861 offsets are encoded as an index into the next table with
 862 the msbit set.</p>
 863 </li>
 864 <li>
 865 <p>A table of 8-byte offset entries (empty for pack files less
 866 than 2 GiB).  Pack files are organized with heavily used
 867 objects toward the front, so most object references should
 868 not need to refer to this table.</p>
 869 </li>
 870 <li>
 871 <p>The same trailer as a v1 pack file:</p>
 872 <div class="literalblock">
 873 <div class="content">
 874 <pre>A copy of the pack checksum at the end of the
 875 corresponding packfile.</pre>
 876 </div>
 877 </div>
 878 <div class="literalblock">
 879 <div class="content">
 880 <pre>Index checksum of all of the above.</pre>
 881 </div>
 882 </div>
 883 </li>
 884 </ul>
 885 </div>
 886 </div>
 887 </div>
 888 <div class="sect1">
 889 <h2 id="_pack_rev_files_have_the_format">pack-*.rev files have the format:</h2>
 890 <div class="sectionbody">
 891 <div class="ulist">
 892 <ul>
 893 <li>
 894 <p>A 4-byte magic number <em>0x52494458</em> (<em>RIDX</em>).</p>
 895 </li>
 896 <li>
 897 <p>A 4-byte version identifier (= 1).</p>
 898 </li>
 899 <li>
 900 <p>A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).</p>
 901 </li>
 902 <li>
 903 <p>A table of index positions (one per packed object, num_objects in
 904 total, each a 4-byte unsigned integer in network order), sorted by
 905 their corresponding offsets in the packfile.</p>
 906 </li>
 907 <li>
 908 <p>A trailer, containing a:</p>
 909 <div class="literalblock">
 910 <div class="content">
 911 <pre>checksum of the corresponding packfile, and</pre>
 912 </div>
 913 </div>
 914 <div class="literalblock">
 915 <div class="content">
 916 <pre>a checksum of all of the above.</pre>
 917 </div>
 918 </div>
 919 </li>
 920 </ul>
 921 </div>
 922 <div class="paragraph">
 923 <p>All 4-byte numbers are in network order.</p>
 924 </div>
 925 </div>
 926 </div>
 927 <div class="sect1">
 928 <h2 id="_pack_mtimes_files_have_the_format">pack-*.mtimes files have the format:</h2>
 929 <div class="sectionbody">
 930 <div class="paragraph">
 931 <p>All 4-byte numbers are in network byte order.</p>
 932 </div>
 933 <div class="ulist">
 934 <ul>
 935 <li>
 936 <p>A 4-byte magic number <em>0x4d544d45</em> (<em>MTME</em>).</p>
 937 </li>
 938 <li>
 939 <p>A 4-byte version identifier (= 1).</p>
 940 </li>
 941 <li>
 942 <p>A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).</p>
 943 </li>
 944 <li>
 945 <p>A table of 4-byte unsigned integers. The ith value is the
 946 modification time (mtime) of the ith object in the corresponding
 947 pack by lexicographic (index) order. The mtimes count standard
 948 epoch seconds.</p>
 949 </li>
 950 <li>
 951 <p>A trailer, containing a checksum of the corresponding packfile,
 952 and a checksum of all of the above (each having length according
 953 to the specified hash function).</p>
 954 </li>
 955 </ul>
 956 </div>
 957 </div>
 958 </div>
 959 <div class="sect1">
 960 <h2 id="_multi_pack_index_midx_files_have_the_following_format">multi-pack-index (MIDX) files have the following format:</h2>
 961 <div class="sectionbody">
 962 <div class="paragraph">
 963 <p>The multi-pack-index files refer to multiple pack-files and loose objects.</p>
 964 </div>
 965 <div class="paragraph">
 966 <p>In order to allow extensions that add extra data to the MIDX, we organize
 967 the body into "chunks" and provide a lookup table at the beginning of the
 968 body. The header includes certain length values, such as the number of packs,
 969 the number of base MIDX files, hash lengths and types.</p>
 970 </div>
 971 <div class="paragraph">
 972 <p>All 4-byte numbers are in network order.</p>
 973 </div>
 974 <div class="paragraph">
 975 <p>HEADER:</p>
 976 </div>
 977 <div class="literalblock">
 978 <div class="content">
 979 <pre>4-byte signature:
 980     The signature is: {'M', 'I', 'D', 'X'}</pre>
 981 </div>
 982 </div>
 983 <div class="literalblock">
 984 <div class="content">
 985 <pre>1-byte version number:
 986     Git only writes or recognizes version 1.</pre>
 987 </div>
 988 </div>
 989 <div class="literalblock">
 990 <div class="content">
 991 <pre>1-byte Object Id Version
 992     We infer the length of object IDs (OIDs) from this value:
 993         1 =&gt; SHA-1
 994         2 =&gt; SHA-256
 995     If the hash type does not match the repository's hash algorithm,
 996     the multi-pack-index file should be ignored with a warning
 997     presented to the user.</pre>
 998 </div>
 999 </div>
1000 <div class="literalblock">
1001 <div class="content">
1002 <pre>1-byte number of "chunks"</pre>
1003 </div>
1004 </div>
1005 <div class="literalblock">
1006 <div class="content">
1007 <pre>1-byte number of base multi-pack-index files:
1008     This value is currently always zero.</pre>
1009 </div>
1010 </div>
1011 <div class="literalblock">
1012 <div class="content">
1013 <pre>4-byte number of pack files</pre>
1014 </div>
1015 </div>
1016 <div class="paragraph">
1017 <p>CHUNK LOOKUP:</p>
1018 </div>
1019 <div class="literalblock">
1020 <div class="content">
1021 <pre>(C + 1) * 12 bytes providing the chunk offsets:
1022     First 4 bytes describe chunk id. Value 0 is a terminating label.
1023     Other 8 bytes provide offset in current file for chunk to start.
1024     (Chunks are provided in file-order, so you can infer the length
1025     using the next chunk position if necessary.)</pre>
1026 </div>
1027 </div>
1028 <div class="literalblock">
1029 <div class="content">
1030 <pre>The CHUNK LOOKUP matches the table of contents from
1031 the chunk-based file format, see linkgit:gitformat-chunk[5].</pre>
1032 </div>
1033 </div>
1034 <div class="literalblock">
1035 <div class="content">
1036 <pre>The remaining data in the body is described one chunk at a time, and
1037 these chunks may be given in any order. Chunks are required unless
1038 otherwise specified.</pre>
1039 </div>
1040 </div>
1041 <div class="paragraph">
1042 <p>CHUNK DATA:</p>
1043 </div>
1044 <div class="literalblock">
1045 <div class="content">
1046 <pre>Packfile Names (ID: {'P', 'N', 'A', 'M'})
1047     Store the names of packfiles as a sequence of NUL-terminated
1048     strings. There is no extra padding between the filenames,
1049     and they are listed in lexicographic order. The chunk itself
1050     is padded at the end with between 0 and 3 NUL bytes to make the
1051     chunk size a multiple of 4 bytes.</pre>
1052 </div>
1053 </div>
1054 <div class="literalblock">
1055 <div class="content">
1056 <pre>Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'})
1057     Stores a table of two 4-byte unsigned integers in network order.
1058     Each table entry corresponds to a single pack (in the order that
1059     they appear above in the `PNAM` chunk). The values for each table
1060     entry are as follows:
1061     - The first bit position (in pseudo-pack order, see below) to
1062       contain an object from that pack.
1063     - The number of bits whose objects are selected from that pack.</pre>
1064 </div>
1065 </div>
1066 <div class="literalblock">
1067 <div class="content">
1068 <pre>OID Fanout (ID: {'O', 'I', 'D', 'F'})
1069     The ith entry, F[i], stores the number of OIDs with first
1070     byte at most i. Thus F[255] stores the total
1071     number of objects.</pre>
1072 </div>
1073 </div>
1074 <div class="literalblock">
1075 <div class="content">
1076 <pre>OID Lookup (ID: {'O', 'I', 'D', 'L'})
1077     The OIDs for all objects in the MIDX are stored in lexicographic
1078     order in this chunk.</pre>
1079 </div>
1080 </div>
1081 <div class="literalblock">
1082 <div class="content">
1083 <pre>Object Offsets (ID: {'O', 'O', 'F', 'F'})
1084     Stores two 4-byte values for every object.
1085     1: The pack-int-id for the pack storing this object.
1086     2: The offset within the pack.
1087         If all offsets are less than 2^32, then the large offset chunk
1088         will not exist and offsets are stored as in IDX v1.
1089         If there is at least one offset value larger than 2^32-1, then
1090         the large offset chunk must exist, and offsets larger than
1091         2^31-1 must be stored in it instead. If the large offset chunk
1092         exists and the 31st bit is on, then removing that bit reveals
1093         the row in the large offsets containing the 8-byte offset of
1094         this object.</pre>
1095 </div>
1096 </div>
1097 <div class="literalblock">
1098 <div class="content">
1099 <pre>[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
1100     8-byte offsets into large packfiles.</pre>
1101 </div>
1102 </div>
1103 <div class="literalblock">
1104 <div class="content">
1105 <pre>[Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'})
1106     A list of MIDX positions (one per object in the MIDX, num_objects in
1107     total, each a 4-byte unsigned integer in network byte order), sorted
1108     according to their relative bitmap/pseudo-pack positions.</pre>
1109 </div>
1110 </div>
1111 <div class="paragraph">
1112 <p>TRAILER:</p>
1113 </div>
1114 <div class="literalblock">
1115 <div class="content">
1116 <pre>Index checksum of the above contents.</pre>
1117 </div>
1118 </div>
1119 </div>
1120 </div>
1121 <div class="sect1">
1122 <h2 id="_multi_pack_index_reverse_indexes">multi-pack-index reverse indexes</h2>
1123 <div class="sectionbody">
1124 <div class="paragraph">
1125 <p>Similar to the pack-based reverse index, the multi-pack index can also
1126 be used to generate a reverse index.</p>
1127 </div>
1128 <div class="paragraph">
1129 <p>Instead of mapping between offset, pack-, and index position, this
1130 reverse index maps between an object&#8217;s position within the MIDX, and
1131 that object&#8217;s position within a pseudo-pack that the MIDX describes
1132 (i.e., the ith entry of the multi-pack reverse index holds the MIDX
1133 position of ith object in pseudo-pack order).</p>
1134 </div>
1135 <div class="paragraph">
1136 <p>To clarify the difference between these orderings, consider a multi-pack
1137 reachability bitmap (which does not yet exist, but is what we are
1138 building towards here). Each bit needs to correspond to an object in the
1139 MIDX, and so we need an efficient mapping from bit position to MIDX
1140 position.</p>
1141 </div>
1142 <div class="paragraph">
1143 <p>One solution is to let bits occupy the same position in the oid-sorted
1144 index stored by the MIDX. But because oids are effectively random, their
1145 resulting reachability bitmaps would have no locality, and thus compress
1146 poorly. (This is the reason that single-pack bitmaps use the pack
1147 ordering, and not the .idx ordering, for the same purpose.)</p>
1148 </div>
1149 <div class="paragraph">
1150 <p>So we&#8217;d like to define an ordering for the whole MIDX based around
1151 pack ordering, which has far better locality (and thus compresses more
1152 efficiently). We can think of a pseudo-pack created by the concatenation
1153 of all of the packs in the MIDX. E.g., if we had a MIDX with three packs
1154 (a, b, c), with 10, 15, and 20 objects respectively, we can imagine an
1155 ordering of the objects like:</p>
1156 </div>
1157 <div class="literalblock">
1158 <div class="content">
1159 <pre>|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|</pre>
1160 </div>
1161 </div>
1162 <div class="paragraph">
1163 <p>where the ordering of the packs is defined by the MIDX&#8217;s pack list,
1164 and then the ordering of objects within each pack is the same as the
1165 order in the actual packfile.</p>
1166 </div>
1167 <div class="paragraph">
1168 <p>Given the list of packs and their counts of objects, you can
1169 naïvely reconstruct that pseudo-pack ordering (e.g., the object at
1170 position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
1171 slots). But there&#8217;s a catch. Objects may be duplicated between packs, in
1172 which case the MIDX only stores one pointer to the object (and thus we&#8217;d
1173 want only one slot in the bitmap).</p>
1174 </div>
1175 <div class="paragraph">
1176 <p>Callers could handle duplicates themselves by reading objects in order
1177 of their bit-position, but that&#8217;s linear in the number of objects, and
1178 much too expensive for ordinary bitmap lookups. Building a reverse index
1179 solves this, since it is the logical inverse of the index, and that
1180 index has already removed duplicates. But, building a reverse index on
1181 the fly can be expensive. Since we already have an on-disk format for
1182 pack-based reverse indexes, let&#8217;s reuse it for the MIDX&#8217;s pseudo-pack,
1183 too.</p>
1184 </div>
1185 <div class="paragraph">
1186 <p>Objects from the MIDX are ordered as follows to string together the
1187 pseudo-pack. Let <code>pack(o)</code> return the pack from which <code>o</code> was selected
1188 by the MIDX, and define an ordering of packs based on their numeric ID
1189 (as stored by the MIDX). Let <code>offset(o)</code> return the object offset of <code>o</code>
1190 within <code>pack(o)</code>. Then, compare <code>o1</code> and <code>o2</code> as follows:</p>
1191 </div>
1192 <div class="ulist">
1193 <ul>
1194 <li>
1195 <p>If one of <code>pack(o1)</code> and <code>pack(o2)</code> is preferred and the other
1196 is not, then the preferred one sorts first.</p>
1197 <div class="paragraph">
1198 <p>(This is a detail that allows the MIDX bitmap to determine which
1199 pack should be used by the pack-reuse mechanism, since it can ask
1200 the MIDX for the pack containing the object at bit position 0).</p>
1201 </div>
1202 </li>
1203 <li>
1204 <p>If <code>pack(o1) ≠ pack(o2)</code>, then sort the two objects in descending
1205 order based on the pack ID.</p>
1206 </li>
1207 <li>
1208 <p>Otherwise, <code>pack(o1) = pack(o2)</code>, and the objects are sorted in
1209 pack-order (i.e., <code>o1</code> sorts ahead of <code>o2</code> exactly when <code>offset(o1)
1210 &lt; offset(o2)</code>).</p>
1211 </li>
1212 </ul>
1213 </div>
1214 <div class="paragraph">
1215 <p>In short, a MIDX&#8217;s pseudo-pack is the de-duplicated concatenation of
1216 objects in packs stored by the MIDX, laid out in pack order, and the
1217 packs arranged in MIDX order (with the preferred pack coming first).</p>
1218 </div>
1219 <div class="paragraph">
1220 <p>The MIDX&#8217;s reverse index is stored in the optional <em>RIDX</em> chunk within
1221 the MIDX itself.</p>
1222 </div>
1223 <div class="sect2">
1224 <h3 id="_btmp_chunk"><code>BTMP</code> chunk</h3>
1225 <div class="paragraph">
1226 <p>The Bitmapped Packfiles (<code>BTMP</code>) chunk encodes additional information
1227 about the objects in the multi-pack index&#8217;s reachability bitmap. Recall
1228 that objects from the MIDX are arranged in "pseudo-pack" order (see
1229 above) for reachability bitmaps.</p>
1230 </div>
1231 <div class="paragraph">
1232 <p>From the example above, suppose we have packs "a", "b", and "c", with
1233 10, 15, and 20 objects, respectively. In pseudo-pack order, those would
1234 be arranged as follows:</p>
1235 </div>
1236 <div class="literalblock">
1237 <div class="content">
1238 <pre>|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|</pre>
1239 </div>
1240 </div>
1241 <div class="paragraph">
1242 <p>When working with single-pack bitmaps (or, equivalently, multi-pack
1243 reachability bitmaps with a preferred pack), <a href="git-pack-objects.html">git-pack-objects(1)</a>
1244 performs &#8220;verbatim&#8221; reuse, attempting to reuse chunks of the bitmapped
1245 or preferred packfile instead of adding objects to the packing list.</p>
1246 </div>
1247 <div class="paragraph">
1248 <p>When a chunk of bytes is reused from an existing pack, any objects
1249 contained therein do not need to be added to the packing list, saving
1250 memory and CPU time. But a chunk from an existing packfile can only be
1251 reused when the following conditions are met:</p>
1252 </div>
1253 <div class="ulist">
1254 <ul>
1255 <li>
1256 <p>The chunk contains only objects which were requested by the caller
1257 (i.e. does not contain any objects which the caller didn&#8217;t ask for
1258 explicitly or implicitly).</p>
1259 </li>
1260 <li>
1261 <p>All objects stored in non-thin packs as offset- or reference-deltas
1262 also include their base object in the resulting pack.</p>
1263 </li>
1264 </ul>
1265 </div>
1266 <div class="paragraph">
1267 <p>The <code>BTMP</code> chunk encodes the necessary information in order to implement
1268 multi-pack reuse over a set of packfiles as described above.
1269 Specifically, the <code>BTMP</code> chunk encodes three pieces of information (all
1270 32-bit unsigned integers in network byte-order) for each packfile <code>p</code>
1271 that is stored in the MIDX, as follows:</p>
1272 </div>
1273 <div class="dlist">
1274 <dl>
1275 <dt class="hdlist1"><code>bitmap_pos</code></dt>
1276 <dd>
1277 <p>The first bit position (in pseudo-pack order) in the
1278 multi-pack index&#8217;s reachability bitmap occupied by an object from <code>p</code>.</p>
1279 </dd>
1280 <dt class="hdlist1"><code>bitmap_nr</code></dt>
1281 <dd>
1282 <p>The number of bit positions (including the one at
1283 <code>bitmap_pos</code>) that encode objects from that pack <code>p</code>.</p>
1284 </dd>
1285 </dl>
1286 </div>
1287 <div class="paragraph">
1288 <p>For example, the <code>BTMP</code> chunk corresponding to the above example (with
1289 packs &#8220;a&#8221;, &#8220;b&#8221;, and &#8220;c&#8221;) would look like:</p>
1290 </div>
1291 <table class="tableblock frame-all grid-all stretch">
1292 <colgroup>
1293 <col style="width: 20%;"/>
1294 <col style="width: 40%;"/>
1295 <col style="width: 40%;"/>
1296 </colgroup>
1297 <thead>
1298 <tr>
1299 <th class="tableblock halign-left valign-top"></th>
1300 <th class="tableblock halign-left valign-top"><code>bitmap_pos</code></th>
1301 <th class="tableblock halign-left valign-top"><code>bitmap_nr</code></th>
1302 </tr>
1303 </thead>
1304 <tbody>
1305 <tr>
1306 <td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;a&#8221;</p></td>
1307 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>0</code></p></td>
1308 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>10</code></p></td>
1309 </tr>
1310 <tr>
1311 <td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;b&#8221;</p></td>
1312 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>10</code></p></td>
1313 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>15</code></p></td>
1314 </tr>
1315 <tr>
1316 <td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;c&#8221;</p></td>
1317 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>25</code></p></td>
1318 <td class="tableblock halign-left valign-top"><p class="tableblock"><code>20</code></p></td>
1319 </tr>
1320 </tbody>
1321 </table>
1322 <div class="paragraph">
1323 <p>With this information in place, we can treat each packfile as
1324 individually reusable in the same fashion as verbatim pack reuse is
1325 performed on individual packs prior to the implementation of the <code>BTMP</code>
1326 chunk.</p>
1327 </div>
1328 </div>
1329 </div>
1330 </div>
1331 <div class="sect1">
1332 <h2 id="_cruft_packs">cruft packs</h2>
1333 <div class="sectionbody">
1334 <div class="paragraph">
1335 <p>The cruft packs feature offer an alternative to Git&#8217;s traditional mechanism of
1336 removing unreachable objects. This document provides an overview of Git&#8217;s
1337 pruning mechanism, and how a cruft pack can be used instead to accomplish the
1338 same.</p>
1339 </div>
1340 <div class="sect2">
1341 <h3 id="_background">Background</h3>
1342 <div class="paragraph">
1343 <p>To remove unreachable objects from your repository, Git offers <code>git repack -Ad</code>
1344 (see <a href="git-repack.html">git-repack(1)</a>). Quoting from the documentation:</p>
1345 </div>
1346 <div class="listingblock">
1347 <div class="content">
1348 <pre>[...] unreachable objects in a previous pack become loose, unpacked objects,
1349 instead of being left in the old pack. [...] loose unreachable objects will be
1350 pruned according to normal expiry rules with the next 'git gc' invocation.</pre>
1351 </div>
1352 </div>
1353 <div class="paragraph">
1354 <p>Unreachable objects aren&#8217;t removed immediately, since doing so could race with
1355 an incoming push which may reference an object which is about to be deleted.
1356 Instead, those unreachable objects are stored as loose objects and stay that way
1357 until they are older than the expiration window, at which point they are removed
1358 by <a href="git-prune.html">git-prune(1)</a>.</p>
1359 </div>
1360 <div class="paragraph">
1361 <p>Git must store these unreachable objects loose in order to keep track of their
1362 per-object mtimes. If these unreachable objects were written into one big pack,
1363 then either freshening that pack (because an object contained within it was
1364 re-written) or creating a new pack of unreachable objects would cause the pack&#8217;s
1365 mtime to get updated, and the objects within it would never leave the expiration
1366 window. Instead, objects are stored loose in order to keep track of the
1367 individual object mtimes and avoid a situation where all cruft objects are
1368 freshened at once.</p>
1369 </div>
1370 <div class="paragraph">
1371 <p>This can lead to undesirable situations when a repository contains many
1372 unreachable objects which have not yet left the grace period. Having large
1373 directories in the shards of <code>.git/objects</code> can lead to decreased performance in
1374 the repository. But given enough unreachable objects, this can lead to inode
1375 starvation and degrade the performance of the whole system. Since we
1376 can never pack those objects, these repositories often take up a large amount of
1377 disk space, since we can only zlib compress them, but not store them in delta
1378 chains.</p>
1379 </div>
1380 </div>
1381 <div class="sect2">
1382 <h3 id="_cruft_packs_2">Cruft packs</h3>
1383 <div class="paragraph">
1384 <p>A cruft pack eliminates the need for storing unreachable objects in a loose
1385 state by including the per-object mtimes in a separate file alongside a single
1386 pack containing all loose objects.</p>
1387 </div>
1388 <div class="paragraph">
1389 <p>A cruft pack is written by <code>git repack --cruft</code> when generating a new pack.
1390 <a href="git-pack-objects.html">git-pack-objects(1)</a>'s <code>--cruft</code> option. Note that <code>git repack --cruft</code>
1391 is a classic all-into-one repack, meaning that everything in the resulting pack is
1392 reachable, and everything else is unreachable. Once written, the <code>--cruft</code>
1393 option instructs <code>git repack</code> to generate another pack containing only objects
1394 not packed in the previous step (which equates to packing all unreachable
1395 objects together). This progresses as follows:</p>
1396 </div>
1397 <div class="olist arabic">
1398 <ol class="arabic">
1399 <li>
1400 <p>Enumerate every object, marking any object which is (a) not contained in a
1401 kept-pack, and (b) whose mtime is within the grace period as a traversal
1402 tip.</p>
1403 </li>
1404 <li>
1405 <p>Perform a reachability traversal based on the tips gathered in the previous
1406 step, adding every object along the way to the pack.</p>
1407 </li>
1408 <li>
1409 <p>Write the pack out, along with a <code>.mtimes</code> file that records the per-object
1410 timestamps.</p>
1411 </li>
1412 </ol>
1413 </div>
1414 <div class="paragraph">
1415 <p>This mode is invoked internally by <a href="git-repack.html">git-repack(1)</a> when instructed to
1416 write a cruft pack. Crucially, the set of in-core kept packs is exactly the set
1417 of packs which will not be deleted by the repack; in other words, they contain
1418 all of the repository&#8217;s reachable objects.</p>
1419 </div>
1420 <div class="paragraph">
1421 <p>When a repository already has a cruft pack, <code>git repack --cruft</code> typically only
1422 adds objects to it. An exception to this is when <code>git repack</code> is given the
1423 <code>--cruft-expiration</code> option, which allows the generated cruft pack to omit
1424 expired objects instead of waiting for <a href="git-gc.html">git-gc(1)</a> to expire those objects
1425 later on.</p>
1426 </div>
1427 <div class="paragraph">
1428 <p>It is <a href="git-gc.html">git-gc(1)</a> that is typically responsible for removing expired
1429 unreachable objects.</p>
1430 </div>
1431 </div>
1432 <div class="sect2">
1433 <h3 id="_alternatives">Alternatives</h3>
1434 <div class="paragraph">
1435 <p>Notable alternatives to this design include:</p>
1436 </div>
1437 <div class="ulist">
1438 <ul>
1439 <li>
1440 <p>The location of the per-object mtime data.</p>
1441 </li>
1442 </ul>
1443 </div>
1444 <div class="paragraph">
1445 <p>On the location of mtime data, a new auxiliary file tied to the pack was chosen
1446 to avoid complicating the <code>.idx</code> format. If the <code>.idx</code> format were ever to gain
1447 support for optional chunks of data, it may make sense to consolidate the
1448 <code>.mtimes</code> format into the <code>.idx</code> itself.</p>
1449 </div>
1450 </div>
1451 </div>
1452 </div>
1453 <div class="sect1">
1454 <h2 id="_git">GIT</h2>
1455 <div class="sectionbody">
1456 <div class="paragraph">
1457 <p>Part of the <a href="git.html">git(1)</a> suite</p>
1458 </div>
1459 </div>
1460 </div>
1461 </div>
1462 <div id="footer">
1463 <div id="footer-text">
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