technical/multi-pack-index.html

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   8 <title>Multi-Pack-Index (MIDX) Design Notes</title>
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 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Multi-Pack-Index (MIDX) Design Notes</h1>
 439 <div class="details">
 440 <span id="revdate">2024-11-22</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div id="preamble">
 445 <div class="sectionbody">
 446 <div class="paragraph">
 447 <p>The Git object directory contains a <em>pack</em> directory containing
 448 packfiles (with suffix ".pack") and pack-indexes (with suffix
 449 ".idx"). The pack-indexes provide a way to lookup objects and
 450 navigate to their offset within the pack, but these must come
 451 in pairs with the packfiles. This pairing depends on the file
 452 names, as the pack-index differs only in suffix with its pack-
 453 file. While the pack-indexes provide fast lookup per packfile,
 454 this performance degrades as the number of packfiles increases,
 455 because abbreviations need to inspect every packfile and we are
 456 more likely to have a miss on our most-recently-used packfile.
 457 For some large repositories, repacking into a single packfile
 458 is not feasible due to storage space or excessive repack times.</p>
 459 </div>
 460 <div class="paragraph">
 461 <p>The multi-pack-index (MIDX for short) stores a list of objects
 462 and their offsets into multiple packfiles. It contains:</p>
 463 </div>
 464 <div class="ulist">
 465 <ul>
 466 <li>
 467 <p>A list of packfile names.</p>
 468 </li>
 469 <li>
 470 <p>A sorted list of object IDs.</p>
 471 </li>
 472 <li>
 473 <p>A list of metadata for the ith object ID including:</p>
 474 <div class="ulist">
 475 <ul>
 476 <li>
 477 <p>A value j referring to the jth packfile.</p>
 478 </li>
 479 <li>
 480 <p>An offset within the jth packfile for the object.</p>
 481 </li>
 482 </ul>
 483 </div>
 484 </li>
 485 <li>
 486 <p>If large offsets are required, we use another list of large
 487 offsets similar to version 2 pack-indexes.</p>
 488 <div class="ulist">
 489 <ul>
 490 <li>
 491 <p>An optional list of objects in pseudo-pack order (used with MIDX bitmaps).</p>
 492 </li>
 493 </ul>
 494 </div>
 495 </li>
 496 </ul>
 497 </div>
 498 <div class="paragraph">
 499 <p>Thus, we can provide O(log N) lookup time for any number
 500 of packfiles.</p>
 501 </div>
 502 </div>
 503 </div>
 504 <div class="sect1">
 505 <h2 id="_design_details">Design Details</h2>
 506 <div class="sectionbody">
 507 <div class="ulist">
 508 <ul>
 509 <li>
 510 <p>The MIDX is stored in a file named <em>multi-pack-index</em> in the
 511 .git/objects/pack directory. This could be stored in the pack
 512 directory of an alternate. It refers only to packfiles in that
 513 same directory.</p>
 514 </li>
 515 <li>
 516 <p>The core.multiPackIndex config setting must be on (which is the
 517 default) to consume MIDX files.  Setting it to <code>false</code> prevents
 518 Git from reading a MIDX file, even if one exists.</p>
 519 </li>
 520 <li>
 521 <p>The file format includes parameters for the object ID hash
 522 function, so a future change of hash algorithm does not require
 523 a change in format.</p>
 524 </li>
 525 <li>
 526 <p>The MIDX keeps only one record per object ID. If an object appears
 527 in multiple packfiles, then the MIDX selects the copy in the
 528 preferred packfile, otherwise selecting from the most-recently
 529 modified packfile.</p>
 530 </li>
 531 <li>
 532 <p>If there exist packfiles in the pack directory not registered in
 533 the MIDX, then those packfiles are loaded into the <code>packed_git</code>
 534 list and <code>packed_git_mru</code> cache.</p>
 535 </li>
 536 <li>
 537 <p>The pack-indexes (.idx files) remain in the pack directory so we
 538 can delete the MIDX file, set core.midx to false, or downgrade
 539 without any loss of information.</p>
 540 </li>
 541 <li>
 542 <p>The MIDX file format uses a chunk-based approach (similar to the
 543 commit-graph file) that allows optional data to be added.</p>
 544 </li>
 545 </ul>
 546 </div>
 547 </div>
 548 </div>
 549 <div class="sect1">
 550 <h2 id="_incremental_multi_pack_indexes">Incremental multi-pack indexes</h2>
 551 <div class="sectionbody">
 552 <div class="paragraph">
 553 <p>As repositories grow in size, it becomes more expensive to write a
 554 multi-pack index (MIDX) that includes all packfiles. To accommodate
 555 this, the "incremental multi-pack indexes" feature allows for combining
 556 a "chain" of multi-pack indexes.</p>
 557 </div>
 558 <div class="paragraph">
 559 <p>Each individual component of the chain need only contain a small number
 560 of packfiles. Appending to the chain does not invalidate earlier parts
 561 of the chain, so repositories can control how much time is spent
 562 updating the MIDX chain by determining the number of packs in each layer
 563 of the MIDX chain.</p>
 564 </div>
 565 <div class="sect2">
 566 <h3 id="_design_state">Design state</h3>
 567 <div class="paragraph">
 568 <p>At present, the incremental multi-pack indexes feature is missing two
 569 important components:</p>
 570 </div>
 571 <div class="ulist">
 572 <ul>
 573 <li>
 574 <p>The ability to rewrite earlier portions of the MIDX chain (i.e., to
 575 "compact" some collection of adjacent MIDX layers into a single
 576 MIDX). At present the only supported way of shrinking a MIDX chain
 577 is to rewrite the entire chain from scratch without the <code>--split</code>
 578 flag.</p>
 579 <div class="paragraph">
 580 <p>There are no fundamental limitations that stand in the way of being able
 581 to implement this feature. It is omitted from the initial implementation
 582 in order to reduce the complexity, but will be added later.</p>
 583 </div>
 584 </li>
 585 <li>
 586 <p>Support for reachability bitmaps. The classic single MIDX
 587 implementation does support reachability bitmaps (see the section
 588 titled "multi-pack-index reverse indexes" in
 589 <a href="../gitformat-pack.html">gitformat-pack(5)</a> for more details).</p>
 590 <div class="paragraph">
 591 <p>As above, there are no fundamental limitations that stand in the way of
 592 extending the incremental MIDX format to support reachability bitmaps.
 593 The design below specifically takes this into account, and support for
 594 reachability bitmaps will be added in a future patch series. It is
 595 omitted from the current implementation for the same reason as above.</p>
 596 </div>
 597 <div class="paragraph">
 598 <p>In brief, to support reachability bitmaps with the incremental MIDX
 599 feature, the concept of the pseudo-pack order is extended across each
 600 layer of the incremental MIDX chain to form a concatenated pseudo-pack
 601 order. This concatenation takes place in the same order as the chain
 602 itself (in other words, the concatenated pseudo-pack order for a chain
 603 {$H1, <code>$H2,</code> <code>$H3</code>} would be the pseudo-pack order for <code>$H1</code>, followed by
 604 the pseudo-pack order for <code>$H2</code>, followed by the pseudo-pack order for
 605 <code>$H3</code>).</p>
 606 </div>
 607 <div class="paragraph">
 608 <p>The layout will then be extended so that each layer of the incremental
 609 MIDX chain can write a *.<code>bitmap</code>. The objects in each layer&#8217;s bitmap
 610 are offset by the number of objects in the previous layers of the chain.</p>
 611 </div>
 612 </li>
 613 </ul>
 614 </div>
 615 </div>
 616 <div class="sect2">
 617 <h3 id="_file_layout">File layout</h3>
 618 <div class="paragraph">
 619 <p>Instead of storing a single <code>multi-pack-index</code> file (with an optional
 620 .<code>rev</code> and .<code>bitmap</code> extension) in <code>$GIT_DIR/objects/pack</code>, incremental
 621 MIDXs are stored in the following layout:</p>
 622 </div>
 623 <div class="listingblock">
 624 <div class="content">
 625 <pre>$GIT_DIR/objects/pack/multi-pack-index.d/
 626 $GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-chain
 627 $GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H1.midx
 628 $GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H2.midx
 629 $GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H3.midx</pre>
 630 </div>
 631 </div>
 632 <div class="paragraph">
 633 <p>The <code>multi-pack-index-chain</code> file contains a list of the incremental
 634 MIDX files in the chain, in order. The above example shows a chain whose
 635 <code>multi-pack-index-chain</code> file would contain the following lines:</p>
 636 </div>
 637 <div class="listingblock">
 638 <div class="content">
 639 <pre>$H1
 640 $H2
 641 $H3</pre>
 642 </div>
 643 </div>
 644 <div class="paragraph">
 645 <p>The <code>multi-pack-index-$H1.midx</code> file contains the first layer of the
 646 multi-pack-index chain. The <code>multi-pack-index-$H2.midx</code> file contains
 647 the second layer of the chain, and so on.</p>
 648 </div>
 649 <div class="paragraph">
 650 <p>When both an incremental- and non-incremental MIDX are present, the
 651 non-incremental MIDX is always read first.</p>
 652 </div>
 653 </div>
 654 <div class="sect2">
 655 <h3 id="_object_positions_for_incremental_midxs">Object positions for incremental MIDXs</h3>
 656 <div class="paragraph">
 657 <p>In the original multi-pack-index design, we refer to objects via their
 658 lexicographic position (by object IDs) within the repository&#8217;s singular
 659 multi-pack-index. In the incremental multi-pack-index design, we refer
 660 to objects via their index into a concatenated lexicographic ordering
 661 among each component in the MIDX chain.</p>
 662 </div>
 663 <div class="paragraph">
 664 <p>If <code>objects_nr</code>() is a function that returns the number of objects in a
 665 given MIDX layer, then the index of an object at lexicographic position
 666 <code>i</code> within, say, $H3 is defined as:</p>
 667 </div>
 668 <div class="listingblock">
 669 <div class="content">
 670 <pre>objects_nr($H2) + objects_nr($H1) + i</pre>
 671 </div>
 672 </div>
 673 <div class="paragraph">
 674 <p>(in the C implementation, this is often computed as <code>i</code> <code>+</code>
 675 <code>m-</code>&gt;<code>num_objects_in_base</code>).</p>
 676 </div>
 677 </div>
 678 </div>
 679 </div>
 680 <div class="sect1">
 681 <h2 id="_future_work">Future Work</h2>
 682 <div class="sectionbody">
 683 <div class="ulist">
 684 <ul>
 685 <li>
 686 <p>The multi-pack-index allows many packfiles, especially in a context
 687 where repacking is expensive (such as a very large repo), or
 688 unexpected maintenance time is unacceptable (such as a high-demand
 689 build machine). However, the multi-pack-index needs to be rewritten
 690 in full every time. We can extend the format to be incremental, so
 691 writes are fast. By storing a small "tip" multi-pack-index that
 692 points to large "base" MIDX files, we can keep writes fast while
 693 still reducing the number of binary searches required for object
 694 lookups.</p>
 695 </li>
 696 <li>
 697 <p>If the multi-pack-index is extended to store a "stable object order"
 698 (a function Order(hash) = integer that is constant for a given hash,
 699 even as the multi-pack-index is updated) then MIDX bitmaps could be
 700 updated independently of the MIDX.</p>
 701 </li>
 702 <li>
 703 <p>Packfiles can be marked as "special" using empty files that share
 704 the initial name but replace ".pack" with ".keep" or ".promisor".
 705 We can add an optional chunk of data to the multi-pack-index that
 706 records flags of information about the packfiles. This allows new
 707 states, such as <em>repacked</em> or <em>redeltified</em>, that can help with
 708 pack maintenance in a multi-pack environment. It may also be
 709 helpful to organize packfiles by object type (commit, tree, blob,
 710 etc.) and use this metadata to help that maintenance.</p>
 711 </li>
 712 </ul>
 713 </div>
 714 </div>
 715 </div>
 716 <div class="sect1">
 717 <h2 id="_related_links">Related Links</h2>
 718 <div class="sectionbody">
 719 <div class="paragraph">
 720 <p>[0] <a href="https://bugs.chromium.org/p/git/issues/detail?id=6" class="bare">https://bugs.chromium.org/p/git/issues/detail?id=6</a>
 721     Chromium work item for: Multi-Pack Index (MIDX)</p>
 722 </div>
 723 <div class="paragraph">
 724 <p>[1] <a href="https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/" class="bare">https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/</a>
 725     An earlier RFC for the multi-pack-index feature</p>
 726 </div>
 727 <div class="paragraph">
 728 <p>[2] <a href="https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/" class="bare">https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/</a>
 729     Git Merge 2018 Contributor&#8217;s summit notes (includes discussion of MIDX)</p>
 730 </div>
 731 </div>
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 736 Last updated 2024-08-20 04:00:09 +0900
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