technical/partial-clone.html

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   8 <title>Partial Clone Design Notes</title>
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 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Partial Clone Design Notes</h1>
 439 <div class="details">
 440 <span id="revdate">2024-11-20</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div id="preamble">
 445 <div class="sectionbody">
 446 <div class="paragraph">
 447 <p>The "Partial Clone" feature is a performance optimization for Git that
 448 allows Git to function without having a complete copy of the repository.
 449 The goal of this work is to allow Git to better handle extremely large
 450 repositories.</p>
 451 </div>
 452 <div class="paragraph">
 453 <p>During clone and fetch operations, Git downloads the complete contents
 454 and history of the repository.  This includes all commits, trees, and
 455 blobs for the complete life of the repository.  For extremely large
 456 repositories, clones can take hours (or days) and consume 100+GiB of disk
 457 space.</p>
 458 </div>
 459 <div class="paragraph">
 460 <p>Often in these repositories there are many blobs and trees that the user
 461 does not need such as:</p>
 462 </div>
 463 <div class="olist arabic">
 464 <ol class="arabic">
 465 <li>
 466 <p>files outside of the user&#8217;s work area in the tree.  For example, in
 467 a repository with 500K directories and 3.5M files in every commit,
 468 we can avoid downloading many objects if the user only needs a
 469 narrow "cone" of the source tree.</p>
 470 </li>
 471 <li>
 472 <p>large binary assets.  For example, in a repository where large build
 473 artifacts are checked into the tree, we can avoid downloading all
 474 previous versions of these non-mergeable binary assets and only
 475 download versions that are actually referenced.</p>
 476 </li>
 477 </ol>
 478 </div>
 479 <div class="paragraph">
 480 <p>Partial clone allows us to avoid downloading such unneeded objects <strong>in
 481 advance</strong> during clone and fetch operations and thereby reduce download
 482 times and disk usage.  Missing objects can later be "demand fetched"
 483 if/when needed.</p>
 484 </div>
 485 <div class="paragraph">
 486 <p>A remote that can later provide the missing objects is called a
 487 promisor remote, as it promises to send the objects when
 488 requested. Initially Git supported only one promisor remote, the origin
 489 remote from which the user cloned and that was configured in the
 490 "extensions.partialClone" config option. Later support for more than
 491 one promisor remote has been implemented.</p>
 492 </div>
 493 <div class="paragraph">
 494 <p>Use of partial clone requires that the user be online and the origin
 495 remote or other promisor remotes be available for on-demand fetching
 496 of missing objects.  This may or may not be problematic for the user.
 497 For example, if the user can stay within the pre-selected subset of
 498 the source tree, they may not encounter any missing objects.
 499 Alternatively, the user could try to pre-fetch various objects if they
 500 know that they are going offline.</p>
 501 </div>
 502 </div>
 503 </div>
 504 <div class="sect1">
 505 <h2 id="_non_goals">Non-Goals</h2>
 506 <div class="sectionbody">
 507 <div class="paragraph">
 508 <p>Partial clone is a mechanism to limit the number of blobs and trees downloaded
 509 <strong>within</strong> a given range of commits&#8201;&#8212;&#8201;and is therefore independent of and not
 510 intended to conflict with existing DAG-level mechanisms to limit the set of
 511 requested commits (i.e. shallow clone, single branch, or fetch <em>&lt;refspec&gt;</em>).</p>
 512 </div>
 513 </div>
 514 </div>
 515 <div class="sect1">
 516 <h2 id="_design_overview">Design Overview</h2>
 517 <div class="sectionbody">
 518 <div class="paragraph">
 519 <p>Partial clone logically consists of the following parts:</p>
 520 </div>
 521 <div class="ulist">
 522 <ul>
 523 <li>
 524 <p>A mechanism for the client to describe unneeded or unwanted objects to
 525 the server.</p>
 526 </li>
 527 <li>
 528 <p>A mechanism for the server to omit such unwanted objects from packfiles
 529 sent to the client.</p>
 530 </li>
 531 <li>
 532 <p>A mechanism for the client to gracefully handle missing objects (that
 533 were previously omitted by the server).</p>
 534 </li>
 535 <li>
 536 <p>A mechanism for the client to backfill missing objects as needed.</p>
 537 </li>
 538 </ul>
 539 </div>
 540 </div>
 541 </div>
 542 <div class="sect1">
 543 <h2 id="_design_details">Design Details</h2>
 544 <div class="sectionbody">
 545 <div class="ulist">
 546 <ul>
 547 <li>
 548 <p>A new pack-protocol capability "filter" is added to the fetch-pack and
 549 upload-pack negotiation.</p>
 550 <div class="paragraph">
 551 <p>This uses the existing capability discovery mechanism.
 552 See "filter" in <a href="../gitprotocol-pack.html">gitprotocol-pack(5)</a>.</p>
 553 </div>
 554 </li>
 555 <li>
 556 <p>Clients pass a "filter-spec" to clone and fetch which is passed to the
 557 server to request filtering during packfile construction.</p>
 558 <div class="paragraph">
 559 <p>There are various filters available to accommodate different situations.
 560 See "--filter=&lt;filter-spec&gt;" in Documentation/rev-list-options.txt.</p>
 561 </div>
 562 </li>
 563 <li>
 564 <p>On the server pack-objects applies the requested filter-spec as it
 565 creates "filtered" packfiles for the client.</p>
 566 <div class="paragraph">
 567 <p>These filtered packfiles are <strong>incomplete</strong> in the traditional sense because
 568 they may contain objects that reference objects not contained in the
 569 packfile and that the client doesn&#8217;t already have.  For example, the
 570 filtered packfile may contain trees or tags that reference missing blobs
 571 or commits that reference missing trees.</p>
 572 </div>
 573 </li>
 574 <li>
 575 <p>On the client these incomplete packfiles are marked as "promisor packfiles"
 576 and treated differently by various commands.</p>
 577 </li>
 578 <li>
 579 <p>On the client a repository extension is added to the local config to
 580 prevent older versions of git from failing mid-operation because of
 581 missing objects that they cannot handle.
 582 See <code>extensions.partialClone</code> in <a href="../git-config.html">git-config(1)</a>.</p>
 583 </li>
 584 </ul>
 585 </div>
 586 </div>
 587 </div>
 588 <div class="sect1">
 589 <h2 id="_handling_missing_objects">Handling Missing Objects</h2>
 590 <div class="sectionbody">
 591 <div class="ulist">
 592 <ul>
 593 <li>
 594 <p>An object may be missing due to a partial clone or fetch, or missing
 595 due to repository corruption.  To differentiate these cases, the
 596 local repository specially indicates such filtered packfiles
 597 obtained from promisor remotes as "promisor packfiles".</p>
 598 <div class="paragraph">
 599 <p>These promisor packfiles consist of a "&lt;name&gt;.promisor" file with
 600 arbitrary contents (like the "&lt;name&gt;.keep" files), in addition to
 601 their "&lt;name&gt;.pack" and "&lt;name&gt;.idx" files.</p>
 602 </div>
 603 </li>
 604 <li>
 605 <p>The local repository considers a "promisor object" to be an object that
 606 it knows (to the best of its ability) that promisor remotes have promised
 607 that they have, either because the local repository has that object in one of
 608 its promisor packfiles, or because another promisor object refers to it.</p>
 609 <div class="paragraph">
 610 <p>When Git encounters a missing object, Git can see if it is a promisor object
 611 and handle it appropriately.  If not, Git can report a corruption.</p>
 612 </div>
 613 <div class="paragraph">
 614 <p>This means that there is no need for the client to explicitly maintain an
 615 expensive-to-modify list of missing objects.[a]</p>
 616 </div>
 617 </li>
 618 <li>
 619 <p>Since almost all Git code currently expects any referenced object to be
 620 present locally and because we do not want to force every command to do
 621 a dry-run first, a fallback mechanism is added to allow Git to attempt
 622 to dynamically fetch missing objects from promisor remotes.</p>
 623 <div class="paragraph">
 624 <p>When the normal object lookup fails to find an object, Git invokes
 625 promisor_remote_get_direct() to try to get the object from a promisor
 626 remote and then retry the object lookup.  This allows objects to be
 627 "faulted in" without complicated prediction algorithms.</p>
 628 </div>
 629 <div class="paragraph">
 630 <p>For efficiency reasons, no check as to whether the missing object is
 631 actually a promisor object is performed.</p>
 632 </div>
 633 <div class="paragraph">
 634 <p>Dynamic object fetching tends to be slow as objects are fetched one at
 635 a time.</p>
 636 </div>
 637 </li>
 638 <li>
 639 <p><code>checkout</code> (and any other command using <code>unpack-trees</code>) has been taught
 640 to bulk pre-fetch all required missing blobs in a single batch.</p>
 641 </li>
 642 <li>
 643 <p><code>rev-list</code> has been taught to print missing objects.</p>
 644 <div class="paragraph">
 645 <p>This can be used by other commands to bulk prefetch objects.
 646 For example, a "git log -p A..B" may internally want to first do
 647 something like "git rev-list --objects --quiet --missing=print A..B"
 648 and prefetch those objects in bulk.</p>
 649 </div>
 650 </li>
 651 <li>
 652 <p><code>fsck</code> has been updated to be fully aware of promisor objects.</p>
 653 </li>
 654 <li>
 655 <p><code>repack</code> in GC has been updated to not touch promisor packfiles at all,
 656 and to only repack other objects.</p>
 657 </li>
 658 <li>
 659 <p>The global variable "fetch_if_missing" is used to control whether an
 660 object lookup will attempt to dynamically fetch a missing object or
 661 report an error.</p>
 662 <div class="paragraph">
 663 <p>We are not happy with this global variable and would like to remove it,
 664 but that requires significant refactoring of the object code to pass an
 665 additional flag.</p>
 666 </div>
 667 </li>
 668 </ul>
 669 </div>
 670 </div>
 671 </div>
 672 <div class="sect1">
 673 <h2 id="_fetching_missing_objects">Fetching Missing Objects</h2>
 674 <div class="sectionbody">
 675 <div class="ulist">
 676 <ul>
 677 <li>
 678 <p>Fetching of objects is done by invoking a "git fetch" subprocess.</p>
 679 </li>
 680 <li>
 681 <p>The local repository sends a request with the hashes of all requested
 682 objects, and does not perform any packfile negotiation.
 683 It then receives a packfile.</p>
 684 </li>
 685 <li>
 686 <p>Because we are reusing the existing fetch mechanism, fetching
 687 currently fetches all objects referred to by the requested objects, even
 688 though they are not necessary.</p>
 689 </li>
 690 <li>
 691 <p>Fetching with <code>--refetch</code> will request a complete new filtered packfile from
 692 the remote, which can be used to change a filter without needing to
 693 dynamically fetch missing objects.</p>
 694 </li>
 695 </ul>
 696 </div>
 697 </div>
 698 </div>
 699 <div class="sect1">
 700 <h2 id="_using_many_promisor_remotes">Using many promisor remotes</h2>
 701 <div class="sectionbody">
 702 <div class="paragraph">
 703 <p>Many promisor remotes can be configured and used.</p>
 704 </div>
 705 <div class="paragraph">
 706 <p>This allows for example a user to have multiple geographically-close
 707 cache servers for fetching missing blobs while continuing to do
 708 filtered <code>git-fetch</code> commands from the central server.</p>
 709 </div>
 710 <div class="paragraph">
 711 <p>When fetching objects, promisor remotes are tried one after the other
 712 until all the objects have been fetched.</p>
 713 </div>
 714 <div class="paragraph">
 715 <p>Remotes that are considered "promisor" remotes are those specified by
 716 the following configuration variables:</p>
 717 </div>
 718 <div class="ulist">
 719 <ul>
 720 <li>
 721 <p><code>extensions.partialClone</code> <code>=</code> <em>&lt;name&gt;</em></p>
 722 </li>
 723 <li>
 724 <p><code>remote.</code><em>&lt;name&gt;</em><code>.promisor</code> <code>=</code> <code>true</code></p>
 725 </li>
 726 <li>
 727 <p><code>remote.</code><em>&lt;name&gt;</em><code>.partialCloneFilter</code> <code>=</code> ...</p>
 728 </li>
 729 </ul>
 730 </div>
 731 <div class="paragraph">
 732 <p>Only one promisor remote can be configured using the
 733 <code>extensions.partialClone</code> config variable. This promisor remote will
 734 be the last one tried when fetching objects.</p>
 735 </div>
 736 <div class="paragraph">
 737 <p>We decided to make it the last one we try, because it is likely that
 738 someone using many promisor remotes is doing so because the other
 739 promisor remotes are better for some reason (maybe they are closer or
 740 faster for some kind of objects) than the origin, and the origin is
 741 likely to be the remote specified by extensions.partialClone.</p>
 742 </div>
 743 <div class="paragraph">
 744 <p>This justification is not very strong, but one choice had to be made,
 745 and anyway the long term plan should be to make the order somehow
 746 fully configurable.</p>
 747 </div>
 748 <div class="paragraph">
 749 <p>For now though the other promisor remotes will be tried in the order
 750 they appear in the config file.</p>
 751 </div>
 752 </div>
 753 </div>
 754 <div class="sect1">
 755 <h2 id="_current_limitations">Current Limitations</h2>
 756 <div class="sectionbody">
 757 <div class="ulist">
 758 <ul>
 759 <li>
 760 <p>It is not possible to specify the order in which the promisor
 761 remotes are tried in other ways than the order in which they appear
 762 in the config file.</p>
 763 <div class="paragraph">
 764 <p>It is also not possible to specify an order to be used when fetching
 765 from one remote and a different order when fetching from another
 766 remote.</p>
 767 </div>
 768 </li>
 769 <li>
 770 <p>It is not possible to push only specific objects to a promisor
 771 remote.</p>
 772 <div class="paragraph">
 773 <p>It is not possible to push at the same time to multiple promisor
 774 remote in a specific order.</p>
 775 </div>
 776 </li>
 777 <li>
 778 <p>Dynamic object fetching will only ask promisor remotes for missing
 779 objects.  We assume that promisor remotes have a complete view of the
 780 repository and can satisfy all such requests.</p>
 781 </li>
 782 <li>
 783 <p>Repack essentially treats promisor and non-promisor packfiles as 2
 784 distinct partitions and does not mix them.</p>
 785 </li>
 786 <li>
 787 <p>Dynamic object fetching invokes fetch-pack once <strong>for each item</strong>
 788 because most algorithms stumble upon a missing object and need to have
 789 it resolved before continuing their work.  This may incur significant
 790 overhead&#8201;&#8212;&#8201;and multiple authentication requests&#8201;&#8212;&#8201;if many objects are
 791 needed.</p>
 792 </li>
 793 <li>
 794 <p>Dynamic object fetching currently uses the existing pack protocol V0
 795 which means that each object is requested via fetch-pack.  The server
 796 will send a full set of info/refs when the connection is established.
 797 If there are a large number of refs, this may incur significant overhead.</p>
 798 </li>
 799 </ul>
 800 </div>
 801 </div>
 802 </div>
 803 <div class="sect1">
 804 <h2 id="_future_work">Future Work</h2>
 805 <div class="sectionbody">
 806 <div class="ulist">
 807 <ul>
 808 <li>
 809 <p>Improve the way to specify the order in which promisor remotes are
 810 tried.</p>
 811 <div class="paragraph">
 812 <p>For example this could allow specifying explicitly something like:
 813 "When fetching from this remote, I want to use these promisor remotes
 814 in this order, though, when pushing or fetching to that remote, I want
 815 to use those promisor remotes in that order."</p>
 816 </div>
 817 </li>
 818 <li>
 819 <p>Allow pushing to promisor remotes.</p>
 820 <div class="paragraph">
 821 <p>The user might want to work in a triangular work flow with multiple
 822 promisor remotes that each have an incomplete view of the repository.</p>
 823 </div>
 824 </li>
 825 <li>
 826 <p>Allow non-pathname-based filters to make use of packfile bitmaps (when
 827 present).  This was just an omission during the initial implementation.</p>
 828 </li>
 829 <li>
 830 <p>Investigate use of a long-running process to dynamically fetch a series
 831 of objects, such as proposed in [5,6] to reduce process startup and
 832 overhead costs.</p>
 833 <div class="paragraph">
 834 <p>It would be nice if pack protocol V2 could allow that long-running
 835 process to make a series of requests over a single long-running
 836 connection.</p>
 837 </div>
 838 </li>
 839 <li>
 840 <p>Investigate pack protocol V2 to avoid the info/refs broadcast on
 841 each connection with the server to dynamically fetch missing objects.</p>
 842 </li>
 843 <li>
 844 <p>Investigate the need to handle loose promisor objects.</p>
 845 <div class="paragraph">
 846 <p>Objects in promisor packfiles are allowed to reference missing objects
 847 that can be dynamically fetched from the server.  An assumption was
 848 made that loose objects are only created locally and therefore should
 849 not reference a missing object.  We may need to revisit that assumption
 850 if, for example, we dynamically fetch a missing tree and store it as a
 851 loose object rather than a single object packfile.</p>
 852 </div>
 853 <div class="paragraph">
 854 <p>This does not necessarily mean we need to mark loose objects as promisor;
 855 it may be sufficient to relax the object lookup or is-promisor functions.</p>
 856 </div>
 857 </li>
 858 </ul>
 859 </div>
 860 </div>
 861 </div>
 862 <div class="sect1">
 863 <h2 id="_non_tasks">Non-Tasks</h2>
 864 <div class="sectionbody">
 865 <div class="ulist">
 866 <ul>
 867 <li>
 868 <p>Every time the subject of "demand loading blobs" comes up it seems
 869 that someone suggests that the server be allowed to "guess" and send
 870 additional objects that may be related to the requested objects.</p>
 871 <div class="paragraph">
 872 <p>No work has gone into actually doing that; we&#8217;re just documenting that
 873 it is a common suggestion.  We&#8217;re not sure how it would work and have
 874 no plans to work on it.</p>
 875 </div>
 876 <div class="paragraph">
 877 <p>It is valid for the server to send more objects than requested (even
 878 for a dynamic object fetch), but we are not building on that.</p>
 879 </div>
 880 </li>
 881 </ul>
 882 </div>
 883 </div>
 884 </div>
 885 <div class="sect1">
 886 <h2 id="_footnotes">Footnotes</h2>
 887 <div class="sectionbody">
 888 <div class="paragraph">
 889 <p>[a] expensive-to-modify list of missing objects:  Earlier in the design of
 890     partial clone we discussed the need for a single list of missing objects.
 891     This would essentially be a sorted linear list of OIDs that were
 892     omitted by the server during a clone or subsequent fetches.</p>
 893 </div>
 894 <div class="paragraph">
 895 <p>This file would need to be loaded into memory on every object lookup.
 896 It would need to be read, updated, and re-written (like the .git/index)
 897 on every explicit "git fetch" command <strong>and</strong> on any dynamic object fetch.</p>
 898 </div>
 899 <div class="paragraph">
 900 <p>The cost to read, update, and write this file could add significant
 901 overhead to every command if there are many missing objects.  For example,
 902 if there are 100M missing blobs, this file would be at least 2GiB on disk.</p>
 903 </div>
 904 <div class="paragraph">
 905 <p>With the "promisor" concept, we <strong>infer</strong> a missing object based upon the
 906 type of packfile that references it.</p>
 907 </div>
 908 </div>
 909 </div>
 910 <div class="sect1">
 911 <h2 id="_related_links">Related Links</h2>
 912 <div class="sectionbody">
 913 <div class="paragraph">
 914 <p>[0] <a href="https://crbug.com/git/2" class="bare">https://crbug.com/git/2</a>
 915     Bug#2: Partial Clone</p>
 916 </div>
 917 <div class="paragraph">
 918 <p>[1] <a href="https://lore.kernel.org/git/20170113155253.1644-1-benpeart@microsoft.com/" class="bare">https://lore.kernel.org/git/20170113155253.1644-1-benpeart@microsoft.com/</a><br/>
 919     Subject: [RFC] Add support for downloading blobs on demand<br/>
 920     Date: Fri, 13 Jan 2017 10:52:53 -0500</p>
 921 </div>
 922 <div class="paragraph">
 923 <p>[2] <a href="https://lore.kernel.org/git/cover.1506714999.git.jonathantanmy@google.com/" class="bare">https://lore.kernel.org/git/cover.1506714999.git.jonathantanmy@google.com/</a><br/>
 924     Subject: [PATCH 00/18] Partial clone (from clone to lazy fetch in 18 patches)<br/>
 925     Date: Fri, 29 Sep 2017 13:11:36 -0700</p>
 926 </div>
 927 <div class="paragraph">
 928 <p>[3] <a href="https://lore.kernel.org/git/20170426221346.25337-1-jonathantanmy@google.com/" class="bare">https://lore.kernel.org/git/20170426221346.25337-1-jonathantanmy@google.com/</a><br/>
 929     Subject: Proposal for missing blob support in Git repos<br/>
 930     Date: Wed, 26 Apr 2017 15:13:46 -0700</p>
 931 </div>
 932 <div class="paragraph">
 933 <p>[4] <a href="https://lore.kernel.org/git/1488999039-37631-1-git-send-email-git@jeffhostetler.com/" class="bare">https://lore.kernel.org/git/1488999039-37631-1-git-send-email-git@jeffhostetler.com/</a><br/>
 934     Subject: [PATCH 00/10] RFC Partial Clone and Fetch<br/>
 935     Date: Wed,  8 Mar 2017 18:50:29 +0000</p>
 936 </div>
 937 <div class="paragraph">
 938 <p>[5] <a href="https://lore.kernel.org/git/20170505152802.6724-1-benpeart@microsoft.com/" class="bare">https://lore.kernel.org/git/20170505152802.6724-1-benpeart@microsoft.com/</a><br/>
 939     Subject: [PATCH v7 00/10] refactor the filter process code into a reusable module<br/>
 940     Date: Fri,  5 May 2017 11:27:52 -0400</p>
 941 </div>
 942 <div class="paragraph">
 943 <p>[6] <a href="https://lore.kernel.org/git/20170714132651.170708-1-benpeart@microsoft.com/" class="bare">https://lore.kernel.org/git/20170714132651.170708-1-benpeart@microsoft.com/</a><br/>
 944     Subject: [RFC/PATCH v2 0/1] Add support for downloading blobs on demand<br/>
 945     Date: Fri, 14 Jul 2017 09:26:50 -0400</p>
 946 </div>
 947 </div>
 948 </div>
 949 </div>
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