diff options
| author | Junio C Hamano <gitster@pobox.com> | 2022-06-03 15:26:23 -0700 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2022-06-03 15:26:23 -0700 |
| commit | c926f62d061511274a5bfe2b3653a1f729b97a04 (patch) | |
| tree | 11b6cecfee8dcaeeb11a6fba485f60001e103778 /technical | |
| parent | 5e833e00b1d044e2b5830a3fb639718bcc4c1811 (diff) | |
| download | git-htmldocs-c926f62d061511274a5bfe2b3653a1f729b97a04.tar.gz | |
Autogenerated HTML docs for v2.36.1-299-gab336
Diffstat (limited to 'technical')
| -rw-r--r-- | technical/cruft-packs.html | 900 | ||||
| -rw-r--r-- | technical/cruft-packs.txt | 123 | ||||
| -rw-r--r-- | technical/pack-format.html | 40 | ||||
| -rw-r--r-- | technical/pack-format.txt | 19 |
4 files changed, 1081 insertions, 1 deletions
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+
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+ padding-top: 0.5em;
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+ // Function that scans the DOM tree for header elements (the DOM2
+ // nodeIterator API would be a better technique but not supported by all
+ // browsers).
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+ if (entry.nodeName.toLowerCase() == 'div'
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+ }
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+ toc.removeChild(tocEntriesToRemove[i]);
+ }
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+ for (var i = 0; i < entries.length; ++i) {
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+ var a = document.createElement("a");
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+ a.appendChild(document.createTextNode(entry.text));
+ var div = document.createElement("div");
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+ toc.appendChild(div);
+ }
+ if (entries.length == 0)
+ toc.parentNode.removeChild(toc);
+},
+
+
+/////////////////////////////////////////////////////////////////////
+// Footnotes generator
+/////////////////////////////////////////////////////////////////////
+
+/* Based on footnote generation code from:
+ * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html
+ */
+
+footnotes: function () {
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+ }
+
+ // Rebuild footnote entries.
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+ "' title='View footnote' class='footnote'>" + n + "</a>]";
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+
+install: function(toclevels) {
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+ function reinstall() {
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+ if (toclevels) {
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+ reinstall();
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+ if (document.addEventListener)
+ document.addEventListener("DOMContentLoaded", reinstallAndRemoveTimer, false);
+ else
+ window.onload = reinstallAndRemoveTimer;
+}
+
+}
+asciidoc.install();
+/*]]>*/
+</script>
+</head>
+<body class="article">
+<div id="header">
+<h1>Cruft packs</h1>
+</div>
+<div id="content">
+<div id="preamble">
+<div class="sectionbody">
+<div class="paragraph"><p>The cruft packs feature offer an alternative to Git’s traditional mechanism of
+removing unreachable objects. This document provides an overview of Git’s
+pruning mechanism, and how a cruft pack can be used instead to accomplish the
+same.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_background">Background</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>To remove unreachable objects from your repository, Git offers <code>git repack -Ad</code>
+(see <a href="../git-repack.html">git-repack(1)</a>). Quoting from the documentation:</p></div>
+<div class="quoteblock">
+<div class="content">[…] unreachable objects in a previous pack become loose, unpacked objects,
+instead of being left in the old pack. […] loose unreachable objects will be
+pruned according to normal expiry rules with the next <em>git gc</em> invocation.</div>
+<div class="attribution">
+</div></div>
+<div class="paragraph"><p>Unreachable objects aren’t removed immediately, since doing so could race with
+an incoming push which may reference an object which is about to be deleted.
+Instead, those unreachable objects are stored as loose objects and stay that way
+until they are older than the expiration window, at which point they are removed
+by <a href="../git-prune.html">git-prune(1)</a>.</p></div>
+<div class="paragraph"><p>Git must store these unreachable objects loose in order to keep track of their
+per-object mtimes. If these unreachable objects were written into one big pack,
+then either freshening that pack (because an object contained within it was
+re-written) or creating a new pack of unreachable objects would cause the pack’s
+mtime to get updated, and the objects within it would never leave the expiration
+window. Instead, objects are stored loose in order to keep track of the
+individual object mtimes and avoid a situation where all cruft objects are
+freshened at once.</p></div>
+<div class="paragraph"><p>This can lead to undesirable situations when a repository contains many
+unreachable objects which have not yet left the grace period. Having large
+directories in the shards of <code>.git/objects</code> can lead to decreased performance in
+the repository. But given enough unreachable objects, this can lead to inode
+starvation and degrade the performance of the whole system. Since we
+can never pack those objects, these repositories often take up a large amount of
+disk space, since we can only zlib compress them, but not store them in delta
+chains.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_cruft_packs">Cruft packs</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>A cruft pack eliminates the need for storing unreachable objects in a loose
+state by including the per-object mtimes in a separate file alongside a single
+pack containing all loose objects.</p></div>
+<div class="paragraph"><p>A cruft pack is written by <code>git repack --cruft</code> when generating a new pack.
+<a href="../git-pack-objects.html">git-pack-objects(1)</a>'s <code>--cruft</code> option. Note that <code>git repack --cruft</code>
+is a classic all-into-one repack, meaning that everything in the resulting pack is
+reachable, and everything else is unreachable. Once written, the <code>--cruft</code>
+option instructs <code>git repack</code> to generate another pack containing only objects
+not packed in the previous step (which equates to packing all unreachable
+objects together). This progresses as follows:</p></div>
+<div class="olist arabic"><ol class="arabic">
+<li>
+<p>
+Enumerate every object, marking any object which is (a) not contained in a
+ kept-pack, and (b) whose mtime is within the grace period as a traversal
+ tip.
+</p>
+</li>
+<li>
+<p>
+Perform a reachability traversal based on the tips gathered in the previous
+ step, adding every object along the way to the pack.
+</p>
+</li>
+<li>
+<p>
+Write the pack out, along with a <code>.mtimes</code> file that records the per-object
+ timestamps.
+</p>
+</li>
+</ol></div>
+<div class="paragraph"><p>This mode is invoked internally by <a href="../git-repack.html">git-repack(1)</a> when instructed to
+write a cruft pack. Crucially, the set of in-core kept packs is exactly the set
+of packs which will not be deleted by the repack; in other words, they contain
+all of the repository’s reachable objects.</p></div>
+<div class="paragraph"><p>When a repository already has a cruft pack, <code>git repack --cruft</code> typically only
+adds objects to it. An exception to this is when <code>git repack</code> is given the
+<code>--cruft-expiration</code> option, which allows the generated cruft pack to omit
+expired objects instead of waiting for <a href="../git-gc.html">git-gc(1)</a> to expire those objects
+later on.</p></div>
+<div class="paragraph"><p>It is <a href="../git-gc.html">git-gc(1)</a> that is typically responsible for removing expired
+unreachable objects.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_caution_for_mixed_version_environments">Caution for mixed-version environments</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>Repositories that have cruft packs in them will continue to work with any older
+version of Git. Note, however, that previous versions of Git which do not
+understand the <code>.mtimes</code> file will use the cruft pack’s mtime as the mtime for
+all of the objects in it. In other words, do not expect older (pre-cruft pack)
+versions of Git to interpret or even read the contents of the <code>.mtimes</code> file.</p></div>
+<div class="paragraph"><p>Note that having mixed versions of Git GC-ing the same repository can lead to
+unreachable objects never being completely pruned. This can happen under the
+following circumstances:</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+An older version of Git running GC explodes the contents of an existing
+ cruft pack loose, using the cruft pack’s mtime.
+</p>
+</li>
+<li>
+<p>
+A newer version running GC collects those loose objects into a cruft pack,
+ where the .mtime file reflects the loose object’s actual mtimes, but the
+ cruft pack mtime is "now".
+</p>
+</li>
+</ul></div>
+<div class="paragraph"><p>Repeating this process will lead to unreachable objects not getting pruned as a
+result of repeatedly resetting the objects' mtimes to the present time.</p></div>
+<div class="paragraph"><p>If you are GC-ing repositories in a mixed version environment, consider omitting
+the <code>--cruft</code> option when using <a href="../git-repack.html">git-repack(1)</a> and <a href="../git-gc.html">git-gc(1)</a>, and
+leaving the <code>gc.cruftPacks</code> configuration unset until all writers understand
+cruft packs.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_alternatives">Alternatives</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>Notable alternatives to this design include:</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+The location of the per-object mtime data, and
+</p>
+</li>
+<li>
+<p>
+Storing unreachable objects in multiple cruft packs.
+</p>
+</li>
+</ul></div>
+<div class="paragraph"><p>On the location of mtime data, a new auxiliary file tied to the pack was chosen
+to avoid complicating the <code>.idx</code> format. If the <code>.idx</code> format were ever to gain
+support for optional chunks of data, it may make sense to consolidate the
+<code>.mtimes</code> format into the <code>.idx</code> itself.</p></div>
+<div class="paragraph"><p>Storing unreachable objects among multiple cruft packs (e.g., creating a new
+cruft pack during each repacking operation including only unreachable objects
+which aren’t already stored in an earlier cruft pack) is significantly more
+complicated to construct, and so aren’t pursued here. The obvious drawback to
+the current implementation is that the entire cruft pack must be re-written from
+scratch.</p></div>
+</div>
+</div>
+</div>
+<div id="footnotes"><hr /></div>
+<div id="footer">
+<div id="footer-text">
+Last updated
+ 2022-06-03 15:24:31 PDT
+</div>
+</div>
+</body>
+</html>
diff --git a/technical/cruft-packs.txt b/technical/cruft-packs.txt new file mode 100644 index 000000000..d81f3a898 --- /dev/null +++ b/technical/cruft-packs.txt @@ -0,0 +1,123 @@ += Cruft packs + +The cruft packs feature offer an alternative to Git's traditional mechanism of +removing unreachable objects. This document provides an overview of Git's +pruning mechanism, and how a cruft pack can be used instead to accomplish the +same. + +== Background + +To remove unreachable objects from your repository, Git offers `git repack -Ad` +(see linkgit:git-repack[1]). Quoting from the documentation: + +[quote] +[...] unreachable objects in a previous pack become loose, unpacked objects, +instead of being left in the old pack. [...] loose unreachable objects will be +pruned according to normal expiry rules with the next 'git gc' invocation. + +Unreachable objects aren't removed immediately, since doing so could race with +an incoming push which may reference an object which is about to be deleted. +Instead, those unreachable objects are stored as loose objects and stay that way +until they are older than the expiration window, at which point they are removed +by linkgit:git-prune[1]. + +Git must store these unreachable objects loose in order to keep track of their +per-object mtimes. If these unreachable objects were written into one big pack, +then either freshening that pack (because an object contained within it was +re-written) or creating a new pack of unreachable objects would cause the pack's +mtime to get updated, and the objects within it would never leave the expiration +window. Instead, objects are stored loose in order to keep track of the +individual object mtimes and avoid a situation where all cruft objects are +freshened at once. + +This can lead to undesirable situations when a repository contains many +unreachable objects which have not yet left the grace period. Having large +directories in the shards of `.git/objects` can lead to decreased performance in +the repository. But given enough unreachable objects, this can lead to inode +starvation and degrade the performance of the whole system. Since we +can never pack those objects, these repositories often take up a large amount of +disk space, since we can only zlib compress them, but not store them in delta +chains. + +== Cruft packs + +A cruft pack eliminates the need for storing unreachable objects in a loose +state by including the per-object mtimes in a separate file alongside a single +pack containing all loose objects. + +A cruft pack is written by `git repack --cruft` when generating a new pack. +linkgit:git-pack-objects[1]'s `--cruft` option. Note that `git repack --cruft` +is a classic all-into-one repack, meaning that everything in the resulting pack is +reachable, and everything else is unreachable. Once written, the `--cruft` +option instructs `git repack` to generate another pack containing only objects +not packed in the previous step (which equates to packing all unreachable +objects together). This progresses as follows: + + 1. Enumerate every object, marking any object which is (a) not contained in a + kept-pack, and (b) whose mtime is within the grace period as a traversal + tip. + + 2. Perform a reachability traversal based on the tips gathered in the previous + step, adding every object along the way to the pack. + + 3. Write the pack out, along with a `.mtimes` file that records the per-object + timestamps. + +This mode is invoked internally by linkgit:git-repack[1] when instructed to +write a cruft pack. Crucially, the set of in-core kept packs is exactly the set +of packs which will not be deleted by the repack; in other words, they contain +all of the repository's reachable objects. + +When a repository already has a cruft pack, `git repack --cruft` typically only +adds objects to it. An exception to this is when `git repack` is given the +`--cruft-expiration` option, which allows the generated cruft pack to omit +expired objects instead of waiting for linkgit:git-gc[1] to expire those objects +later on. + +It is linkgit:git-gc[1] that is typically responsible for removing expired +unreachable objects. + +== Caution for mixed-version environments + +Repositories that have cruft packs in them will continue to work with any older +version of Git. Note, however, that previous versions of Git which do not +understand the `.mtimes` file will use the cruft pack's mtime as the mtime for +all of the objects in it. In other words, do not expect older (pre-cruft pack) +versions of Git to interpret or even read the contents of the `.mtimes` file. + +Note that having mixed versions of Git GC-ing the same repository can lead to +unreachable objects never being completely pruned. This can happen under the +following circumstances: + + - An older version of Git running GC explodes the contents of an existing + cruft pack loose, using the cruft pack's mtime. + - A newer version running GC collects those loose objects into a cruft pack, + where the .mtime file reflects the loose object's actual mtimes, but the + cruft pack mtime is "now". + +Repeating this process will lead to unreachable objects not getting pruned as a +result of repeatedly resetting the objects' mtimes to the present time. + +If you are GC-ing repositories in a mixed version environment, consider omitting +the `--cruft` option when using linkgit:git-repack[1] and linkgit:git-gc[1], and +leaving the `gc.cruftPacks` configuration unset until all writers understand +cruft packs. + +== Alternatives + +Notable alternatives to this design include: + + - The location of the per-object mtime data, and + - Storing unreachable objects in multiple cruft packs. + +On the location of mtime data, a new auxiliary file tied to the pack was chosen +to avoid complicating the `.idx` format. If the `.idx` format were ever to gain +support for optional chunks of data, it may make sense to consolidate the +`.mtimes` format into the `.idx` itself. + +Storing unreachable objects among multiple cruft packs (e.g., creating a new +cruft pack during each repacking operation including only unreachable objects +which aren't already stored in an earlier cruft pack) is significantly more +complicated to construct, and so aren't pursued here. The obvious drawback to +the current implementation is that the entire cruft pack must be re-written from +scratch. diff --git a/technical/pack-format.html b/technical/pack-format.html index eae23ff1b..2149c9ddb 100644 --- a/technical/pack-format.html +++ b/technical/pack-format.html @@ -1169,6 +1169,44 @@ A trailer, containing a: </div>
</div>
<div class="sect1">
+<h2 id="_pack_mtimes_files_have_the_format">pack-*.mtimes files have the format:</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>All 4-byte numbers are in network byte order.</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+A 4-byte magic number <em>0x4d544d45</em> (<em>MTME</em>).
+</p>
+</li>
+<li>
+<p>
+A 4-byte version identifier (= 1).
+</p>
+</li>
+<li>
+<p>
+A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
+</p>
+</li>
+<li>
+<p>
+A table of 4-byte unsigned integers. The ith value is the
+ modification time (mtime) of the ith object in the corresponding
+ pack by lexicographic (index) order. The mtimes count standard
+ epoch seconds.
+</p>
+</li>
+<li>
+<p>
+A trailer, containing a checksum of the corresponding packfile,
+ and a checksum of all of the above (each having length according
+ to the specified hash function).
+</p>
+</li>
+</ul></div>
+</div>
+</div>
+<div class="sect1">
<h2 id="_multi_pack_index_midx_files_have_the_following_format">multi-pack-index (MIDX) files have the following format:</h2>
<div class="sectionbody">
<div class="paragraph"><p>The multi-pack-index files refer to multiple pack-files and loose objects.</p></div>
@@ -1375,7 +1413,7 @@ the MIDX itself.</p></div> <div id="footer">
<div id="footer-text">
Last updated
- 2022-02-16 17:29:08 PST
+ 2022-06-03 15:24:31 PDT
</div>
</div>
</body>
diff --git a/technical/pack-format.txt b/technical/pack-format.txt index 6d3efb7d1..b520aa9c4 100644 --- a/technical/pack-format.txt +++ b/technical/pack-format.txt @@ -294,6 +294,25 @@ Pack file entry: <+ All 4-byte numbers are in network order. +== pack-*.mtimes files have the format: + +All 4-byte numbers are in network byte order. + + - A 4-byte magic number '0x4d544d45' ('MTME'). + + - A 4-byte version identifier (= 1). + + - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256). + + - A table of 4-byte unsigned integers. The ith value is the + modification time (mtime) of the ith object in the corresponding + pack by lexicographic (index) order. The mtimes count standard + epoch seconds. + + - A trailer, containing a checksum of the corresponding packfile, + and a checksum of all of the above (each having length according + to the specified hash function). + == multi-pack-index (MIDX) files have the following format: The multi-pack-index files refer to multiple pack-files and loose objects. |