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chromium / external / gitster / git-htmldocs / 11f1537a82c84e620d742b3bc4256952defb28df / . / technical / multi-pack-index.html
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Junio C Hamano2f4318b2018-11-21 13:46:21[diff] [blame]1<?xml version="1.0" encoding="UTF-8"?>
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734</head>
735<body class="article">
736<div id="header">
737<h1>Multi-Pack-Index (MIDX) Design Notes</h1>
738</div>
739<div id="content">
740<div id="preamble">
741<div class="sectionbody">
742<div class="paragraph"><p>The Git object directory contains a <em>pack</em> directory containing
743packfiles (with suffix ".pack") and pack-indexes (with suffix
744".idx"). The pack-indexes provide a way to lookup objects and
745navigate to their offset within the pack, but these must come
746in pairs with the packfiles. This pairing depends on the file
747names, as the pack-index differs only in suffix with its pack-
748file. While the pack-indexes provide fast lookup per packfile,
749this performance degrades as the number of packfiles increases,
750because abbreviations need to inspect every packfile and we are
751more likely to have a miss on our most-recently-used packfile.
752For some large repositories, repacking into a single packfile
753is not feasible due to storage space or excessive repack times.</p></div>
754<div class="paragraph"><p>The multi-pack-index (MIDX for short) stores a list of objects
755and their offsets into multiple packfiles. It contains:</p></div>
756<div class="ulist"><ul>
757<li>
758<p>
759A list of packfile names.
760</p>
761</li>
762<li>
763<p>
764A sorted list of object IDs.
765</p>
766</li>
767<li>
768<p>
769A list of metadata for the ith object ID including:
770</p>
771</li>
772<li>
773<p>
774A value j referring to the jth packfile.
775</p>
776</li>
777<li>
778<p>
779An offset within the jth packfile for the object.
780</p>
781</li>
782<li>
783<p>
784If large offsets are required, we use another list of large
785 offsets similar to version 2 pack-indexes.
786</p>
787</li>
788</ul></div>
789<div class="paragraph"><p>Thus, we can provide O(log N) lookup time for any number
790of packfiles.</p></div>
791</div>
792</div>
793<div class="sect1">
794<h2 id="_design_details">Design Details</h2>
795<div class="sectionbody">
796<div class="ulist"><ul>
797<li>
798<p>
799The MIDX is stored in a file named <em>multi-pack-index</em> in the
800 .git/objects/pack directory. This could be stored in the pack
801 directory of an alternate. It refers only to packfiles in that
802 same directory.
803</p>
804</li>
805<li>
806<p>
807The pack.multiIndex config setting must be on to consume MIDX files.
808</p>
809</li>
810<li>
811<p>
812The file format includes parameters for the object ID hash
813 function, so a future change of hash algorithm does not require
814 a change in format.
815</p>
816</li>
817<li>
818<p>
819The MIDX keeps only one record per object ID. If an object appears
820 in multiple packfiles, then the MIDX selects the copy in the most-
821 recently modified packfile.
822</p>
823</li>
824<li>
825<p>
826If there exist packfiles in the pack directory not registered in
827 the MIDX, then those packfiles are loaded into the <code>packed_git</code>
828 list and <code>packed_git_mru</code> cache.
829</p>
830</li>
831<li>
832<p>
833The pack-indexes (.idx files) remain in the pack directory so we
834 can delete the MIDX file, set core.midx to false, or downgrade
835 without any loss of information.
836</p>
837</li>
838<li>
839<p>
840The MIDX file format uses a chunk-based approach (similar to the
841 commit-graph file) that allows optional data to be added.
842</p>
843</li>
844</ul></div>
845</div>
846</div>
847<div class="sect1">
848<h2 id="_future_work">Future Work</h2>
849<div class="sectionbody">
850<div class="ulist"><ul>
851<li>
852<p>
853Add a <em>verify</em> subcommand to the <em>git midx</em> builtin to verify the
854 contents of the multi-pack-index file match the offsets listed in
855 the corresponding pack-indexes.
856</p>
857</li>
858<li>
859<p>
860The multi-pack-index allows many packfiles, especially in a context
861 where repacking is expensive (such as a very large repo), or
862 unexpected maintenance time is unacceptable (such as a high-demand
863 build machine). However, the multi-pack-index needs to be rewritten
864 in full every time. We can extend the format to be incremental, so
865 writes are fast. By storing a small "tip" multi-pack-index that
866 points to large "base" MIDX files, we can keep writes fast while
867 still reducing the number of binary searches required for object
868 lookups.
869</p>
870</li>
871<li>
872<p>
873The reachability bitmap is currently paired directly with a single
874 packfile, using the pack-order as the object order to hopefully
875 compress the bitmaps well using run-length encoding. This could be
876 extended to pair a reachability bitmap with a multi-pack-index. If
877 the multi-pack-index is extended to store a "stable object order"
878 (a function Order(hash) = integer that is constant for a given hash,
879 even as the multi-pack-index is updated) then a reachability bitmap
880 could point to a multi-pack-index and be updated independently.
881</p>
882</li>
883<li>
884<p>
885Packfiles can be marked as "special" using empty files that share
886 the initial name but replace ".pack" with ".keep" or ".promisor".
887 We can add an optional chunk of data to the multi-pack-index that
888 records flags of information about the packfiles. This allows new
889 states, such as <em>repacked</em> or <em>redeltified</em>, that can help with
890 pack maintenance in a multi-pack environment. It may also be
891 helpful to organize packfiles by object type (commit, tree, blob,
892 etc.) and use this metadata to help that maintenance.
893</p>
894</li>
895<li>
896<p>
897The partial clone feature records special "promisor" packs that
898 may point to objects that are not stored locally, but available
899 on request to a server. The multi-pack-index does not currently
900 track these promisor packs.
901</p>
902</li>
903</ul></div>
904</div>
905</div>
906<div class="sect1">
907<h2 id="_related_links">Related Links</h2>
908<div class="sectionbody">
909<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>
910 Chromium work item for: Multi-Pack Index (MIDX)</p></div>
911<div class="paragraph"><p>[1] <a href="https://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/">https://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/</a>
912 An earlier RFC for the multi-pack-index feature</p></div>
913<div class="paragraph"><p>[2] <a href="https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/">https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/</a>
914 Git Merge 2018 Contributor&#8217;s summit notes (includes discussion of MIDX)</p></div>
915</div>
916</div>
917</div>
918<div id="footnotes"><hr /></div>
919<div id="footer">
920<div id="footer-text">
921Last updated
Junio C Hamanocf731002019-01-29 21:19:52[diff] [blame]922 2018-09-17 15:45:24 PDT
Junio C Hamano2f4318b2018-11-21 13:46:21[diff] [blame]923</div>
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