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 735 <body class="article">
 736 <div id="header">
 737 <h1>Multi-Pack-Index (MIDX) Design Notes</h1>
 738 <span id="revdate">2024-04-23</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>The Git object directory contains a <em>pack</em> directory containing
 744 packfiles (with suffix ".pack") and pack-indexes (with suffix
 745 ".idx"). The pack-indexes provide a way to lookup objects and
 746 navigate to their offset within the pack, but these must come
 747 in pairs with the packfiles. This pairing depends on the file
 748 names, as the pack-index differs only in suffix with its pack-
 749 file. While the pack-indexes provide fast lookup per packfile,
 750 this performance degrades as the number of packfiles increases,
 751 because abbreviations need to inspect every packfile and we are
 752 more likely to have a miss on our most-recently-used packfile.
 753 For some large repositories, repacking into a single packfile
 754 is not feasible due to storage space or excessive repack times.</p></div>
 755 <div class="paragraph"><p>The multi-pack-index (MIDX for short) stores a list of objects
 756 and their offsets into multiple packfiles. It contains:</p></div>
 757 <div class="ulist"><ul>
 758 <li>
 759 <p>
 760 A list of packfile names.
 761 </p>
 762 </li>
 763 <li>
 764 <p>
 765 A sorted list of object IDs.
 766 </p>
 767 </li>
 768 <li>
 769 <p>
 770 A list of metadata for the ith object ID including:
 771 </p>
 772 <div class="ulist"><ul>
 773 <li>
 774 <p>
 775 A value j referring to the jth packfile.
 776 </p>
 777 </li>
 778 <li>
 779 <p>
 780 An offset within the jth packfile for the object.
 781 </p>
 782 </li>
 783 </ul></div>
 784 </li>
 785 <li>
 786 <p>
 787 If large offsets are required, we use another list of large
 788   offsets similar to version 2 pack-indexes.
 789 </p>
 790 <div class="ulist"><ul>
 791 <li>
 792 <p>
 793 An optional list of objects in pseudo-pack order (used with MIDX bitmaps).
 794 </p>
 795 </li>
 796 </ul></div>
 797 </li>
 798 </ul></div>
 799 <div class="paragraph"><p>Thus, we can provide O(log N) lookup time for any number
 800 of packfiles.</p></div>
 801 </div>
 802 </div>
 803 <div class="sect1">
 804 <h2 id="_design_details">Design Details</h2>
 805 <div class="sectionbody">
 806 <div class="ulist"><ul>
 807 <li>
 808 <p>
 809 The MIDX is stored in a file named <em>multi-pack-index</em> in the
 810   .git/objects/pack directory. This could be stored in the pack
 811   directory of an alternate. It refers only to packfiles in that
 812   same directory.
 813 </p>
 814 </li>
 815 <li>
 816 <p>
 817 The core.multiPackIndex config setting must be on (which is the
 818   default) to consume MIDX files.  Setting it to <code>false</code> prevents
 819   Git from reading a MIDX file, even if one exists.
 820 </p>
 821 </li>
 822 <li>
 823 <p>
 824 The file format includes parameters for the object ID hash
 825   function, so a future change of hash algorithm does not require
 826   a change in format.
 827 </p>
 828 </li>
 829 <li>
 830 <p>
 831 The MIDX keeps only one record per object ID. If an object appears
 832   in multiple packfiles, then the MIDX selects the copy in the
 833   preferred packfile, otherwise selecting from the most-recently
 834   modified packfile.
 835 </p>
 836 </li>
 837 <li>
 838 <p>
 839 If there exist packfiles in the pack directory not registered in
 840   the MIDX, then those packfiles are loaded into the <code>packed_git</code>
 841   list and <code>packed_git_mru</code> cache.
 842 </p>
 843 </li>
 844 <li>
 845 <p>
 846 The pack-indexes (.idx files) remain in the pack directory so we
 847   can delete the MIDX file, set core.midx to false, or downgrade
 848   without any loss of information.
 849 </p>
 850 </li>
 851 <li>
 852 <p>
 853 The MIDX file format uses a chunk-based approach (similar to the
 854   commit-graph file) that allows optional data to be added.
 855 </p>
 856 </li>
 857 </ul></div>
 858 </div>
 859 </div>
 860 <div class="sect1">
 861 <h2 id="_future_work">Future Work</h2>
 862 <div class="sectionbody">
 863 <div class="ulist"><ul>
 864 <li>
 865 <p>
 866 The multi-pack-index allows many packfiles, especially in a context
 867   where repacking is expensive (such as a very large repo), or
 868   unexpected maintenance time is unacceptable (such as a high-demand
 869   build machine). However, the multi-pack-index needs to be rewritten
 870   in full every time. We can extend the format to be incremental, so
 871   writes are fast. By storing a small "tip" multi-pack-index that
 872   points to large "base" MIDX files, we can keep writes fast while
 873   still reducing the number of binary searches required for object
 874   lookups.
 875 </p>
 876 </li>
 877 <li>
 878 <p>
 879 If the multi-pack-index is extended to store a "stable object order"
 880   (a function Order(hash) = integer that is constant for a given hash,
 881   even as the multi-pack-index is updated) then MIDX bitmaps could be
 882   updated independently of the MIDX.
 883 </p>
 884 </li>
 885 <li>
 886 <p>
 887 Packfiles can be marked as "special" using empty files that share
 888   the initial name but replace ".pack" with ".keep" or ".promisor".
 889   We can add an optional chunk of data to the multi-pack-index that
 890   records flags of information about the packfiles. This allows new
 891   states, such as <em>repacked</em> or <em>redeltified</em>, that can help with
 892   pack maintenance in a multi-pack environment. It may also be
 893   helpful to organize packfiles by object type (commit, tree, blob,
 894   etc.) and use this metadata to help that maintenance.
 895 </p>
 896 </li>
 897 </ul></div>
 898 </div>
 899 </div>
 900 <div class="sect1">
 901 <h2 id="_related_links">Related Links</h2>
 902 <div class="sectionbody">
 903 <div class="paragraph"><p>[0] <a href="https://bugs.chromium.org/p/git/issues/detail?id=6">https://bugs.chromium.org/p/git/issues/detail?id=6</a>
 904     Chromium work item for: Multi-Pack Index (MIDX)</p></div>
 905 <div class="paragraph"><p>[1] <a href="https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/">https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/</a>
 906     An earlier RFC for the multi-pack-index feature</p></div>
 907 <div class="paragraph"><p>[2] <a href="https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/">https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/</a>
 908     Git Merge 2018 Contributor&#8217;s summit notes (includes discussion of MIDX)</p></div>
 909 </div>
 910 </div>
 911 </div>
 912 <div id="footnotes"><hr /></div>
 913 <div id="footer">
 914 <div id="footer-text">
 915 Last updated
 916  2022-02-16 17:29:08 PST
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