From 28cb5e66ddaed47aa8789fa326fec0339831b80b Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:31 +0000 Subject: maintenance: add prefetch task When working with very large repositories, an incremental 'git fetch' command can download a large amount of data. If there are many other users pushing to a common repo, then this data can rival the initial pack-file size of a 'git clone' of a medium-size repo. Users may want to keep the data on their local repos as close as possible to the data on the remote repos by fetching periodically in the background. This can break up a large daily fetch into several smaller hourly fetches. The task is called "prefetch" because it is work done in advance of a foreground fetch to make that 'git fetch' command much faster. However, if we simply ran 'git fetch ' in the background, then the user running a foreground 'git fetch ' would lose some important feedback when a new branch appears or an existing branch updates. This is especially true if a remote branch is force-updated and this isn't noticed by the user because it occurred in the background. Further, the functionality of 'git push --force-with-lease' becomes suspect. When running 'git fetch ' in the background, use the following options for careful updating: 1. --no-tags prevents getting a new tag when a user wants to see the new tags appear in their foreground fetches. 2. --refmap= removes the configured refspec which usually updates refs/remotes//* with the refs advertised by the remote. While this looks confusing, this was documented and tested by b40a50264ac (fetch: document and test --refmap="", 2020-01-21), including this sentence in the documentation: Providing an empty `` to the `--refmap` option causes Git to ignore the configured refspecs and rely entirely on the refspecs supplied as command-line arguments. 3. By adding a new refspec "+refs/heads/*:refs/prefetch//*" we can ensure that we actually load the new values somewhere in our refspace while not updating refs/heads or refs/remotes. By storing these refs here, the commit-graph job will update the commit-graph with the commits from these hidden refs. 4. --prune will delete the refs/prefetch/ refs that no longer appear on the remote. 5. --no-write-fetch-head prevents updating FETCH_HEAD. We've been using this step as a critical background job in Scalar [1] (and VFS for Git). This solved a pain point that was showing up in user reports: fetching was a pain! Users do not like waiting to download the data that was created while they were away from their machines. After implementing background fetch, the foreground fetch commands sped up significantly because they mostly just update refs and download a small amount of new data. The effect is especially dramatic when paried with --no-show-forced-udpates (through fetch.showForcedUpdates=false). [1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/FetchStep.cs Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/git-maintenance.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'Documentation') diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt index 6abcb8255a..12668fccf7 100644 --- a/Documentation/git-maintenance.txt +++ b/Documentation/git-maintenance.txt @@ -47,6 +47,21 @@ commit-graph:: `commit-graph-chain` file. They will be deleted by a later run based on the expiration delay. +prefetch:: + The `prefetch` task updates the object directory with the latest + objects from all registered remotes. For each remote, a `git fetch` + command is run. The refmap is custom to avoid updating local or remote + branches (those in `refs/heads` or `refs/remotes`). Instead, the + remote refs are stored in `refs/prefetch//`. Also, tags are + not updated. ++ +This is done to avoid disrupting the remote-tracking branches. The end users +expect these refs to stay unmoved unless they initiate a fetch. With prefetch +task, however, the objects necessary to complete a later real fetch would +already be obtained, so the real fetch would go faster. In the ideal case, +it will just become an update to bunch of remote-tracking branches without +any object transfer. + gc:: Clean up unnecessary files and optimize the local repository. "GC" stands for "garbage collection," but this task performs many -- cgit v1.2.3 From 252cfb7cb86a33f7740c799748ee6c586931c7bc Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:32 +0000 Subject: maintenance: add loose-objects task One goal of background maintenance jobs is to allow a user to disable auto-gc (gc.auto=0) but keep their repository in a clean state. Without any cleanup, loose objects will clutter the object database and slow operations. In addition, the loose objects will take up extra space because they are not stored with deltas against similar objects. Create a 'loose-objects' task for the 'git maintenance run' command. This helps clean up loose objects without disrupting concurrent Git commands using the following sequence of events: 1. Run 'git prune-packed' to delete any loose objects that exist in a pack-file. Concurrent commands will prefer the packed version of the object to the loose version. (Of course, there are exceptions for commands that specifically care about the location of an object. These are rare for a user to run on purpose, and we hope a user that has selected background maintenance will not be trying to do foreground maintenance.) 2. Run 'git pack-objects' on a batch of loose objects. These objects are grouped by scanning the loose object directories in lexicographic order until listing all loose objects -or- reaching 50,000 objects. This is more than enough if the loose objects are created only by a user doing normal development. We noticed users with _millions_ of loose objects because VFS for Git downloads blobs on-demand when a file read operation requires populating a virtual file. This step is based on a similar step in Scalar [1] and VFS for Git. [1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/LooseObjectsStep.cs Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/git-maintenance.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'Documentation') diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt index 12668fccf7..fc95eb594f 100644 --- a/Documentation/git-maintenance.txt +++ b/Documentation/git-maintenance.txt @@ -70,6 +70,21 @@ gc:: be disruptive in some situations, as it deletes stale data. See linkgit:git-gc[1] for more details on garbage collection in Git. +loose-objects:: + The `loose-objects` job cleans up loose objects and places them into + pack-files. In order to prevent race conditions with concurrent Git + commands, it follows a two-step process. First, it deletes any loose + objects that already exist in a pack-file; concurrent Git processes + will examine the pack-file for the object data instead of the loose + object. Second, it creates a new pack-file (starting with "loose-") + containing a batch of loose objects. The batch size is limited to 50 + thousand objects to prevent the job from taking too long on a + repository with many loose objects. The `gc` task writes unreachable + objects as loose objects to be cleaned up by a later step only if + they are not re-added to a pack-file; for this reason it is not + advisable to enable both the `loose-objects` and `gc` tasks at the + same time. + OPTIONS ------- --auto:: -- cgit v1.2.3 From 3e220e60696ebc27c719b83adc6f734d6857521f Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:33 +0000 Subject: maintenance: create auto condition for loose-objects The loose-objects task deletes loose objects that already exist in a pack-file, then place the remaining loose objects into a new pack-file. If this step runs all the time, then we risk creating pack-files with very few objects with every 'git commit' process. To prevent overwhelming the packs directory with small pack-files, place a minimum number of objects to justify the task. The 'maintenance.loose-objects.auto' config option specifies a minimum number of loose objects to justify the task to run under the '--auto' option. This defaults to 100 loose objects. Setting the value to zero will prevent the step from running under '--auto' while a negative value will force it to run every time. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/config/maintenance.txt | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'Documentation') diff --git a/Documentation/config/maintenance.txt b/Documentation/config/maintenance.txt index 7cc6700d57..c31613be62 100644 --- a/Documentation/config/maintenance.txt +++ b/Documentation/config/maintenance.txt @@ -14,3 +14,12 @@ maintenance.commit-graph.auto:: reachable commits that are not in the commit-graph file is at least the value of `maintenance.commit-graph.auto`. The default value is 100. + +maintenance.loose-objects.auto:: + This integer config option controls how often the `loose-objects` task + should be run as part of `git maintenance run --auto`. If zero, then + the `loose-objects` task will not run with the `--auto` option. A + negative value will force the task to run every time. Otherwise, a + positive value implies the command should run when the number of + loose objects is at least the value of `maintenance.loose-objects.auto`. + The default value is 100. -- cgit v1.2.3 From 18e449f86b74bab35b150549c8342d252fe7ae00 Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:34 +0000 Subject: midx: enable core.multiPackIndex by default The core.multiPackIndex setting has been around since c4d25228ebb (config: create core.multiPackIndex setting, 2018-07-12), but has been disabled by default. If a user wishes to use the multi-pack-index feature, then they must enable this config and run 'git multi-pack-index write'. The multi-pack-index feature is relatively stable now, so make the config option true by default. For users that do not use a multi-pack-index, the only extra cost will be a file lookup to see if a multi-pack-index file exists (once per process, per object directory). Also, this config option will be referenced by an upcoming "incremental-repack" task in the maintenance builtin, so move the config option into the repository settings struct. Note that if GIT_TEST_MULTI_PACK_INDEX=1, then we want to ignore the config option and treat core.multiPackIndex as enabled. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/config/core.txt | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'Documentation') diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt index 74619a9c03..86c91d5381 100644 --- a/Documentation/config/core.txt +++ b/Documentation/config/core.txt @@ -606,8 +606,8 @@ core.useReplaceRefs:: core.multiPackIndex:: Use the multi-pack-index file to track multiple packfiles using a - single index. See link:technical/multi-pack-index.html[the - multi-pack-index design document]. + single index. See linkgit:git-multi-pack-index[1] for more + information. Defaults to true. core.sparseCheckout:: Enable "sparse checkout" feature. See linkgit:git-sparse-checkout[1] -- cgit v1.2.3 From 52fe41ff1cd6e1f0b67d4e864e718d949e225f30 Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:36 +0000 Subject: maintenance: add incremental-repack task The previous change cleaned up loose objects using the 'loose-objects' that can be run safely in the background. Add a similar job that performs similar cleanups for pack-files. One issue with running 'git repack' is that it is designed to repack all pack-files into a single pack-file. While this is the most space-efficient way to store object data, it is not time or memory efficient. This becomes extremely important if the repo is so large that a user struggles to store two copies of the pack on their disk. Instead, perform an "incremental" repack by collecting a few small pack-files into a new pack-file. The multi-pack-index facilitates this process ever since 'git multi-pack-index expire' was added in 19575c7 (multi-pack-index: implement 'expire' subcommand, 2019-06-10) and 'git multi-pack-index repack' was added in ce1e4a1 (midx: implement midx_repack(), 2019-06-10). The 'incremental-repack' task runs the following steps: 1. 'git multi-pack-index write' creates a multi-pack-index file if one did not exist, and otherwise will update the multi-pack-index with any new pack-files that appeared since the last write. This is particularly relevant with the background fetch job. When the multi-pack-index sees two copies of the same object, it stores the offset data into the newer pack-file. This means that some old pack-files could become "unreferenced" which I will use to mean "a pack-file that is in the pack-file list of the multi-pack-index but none of the objects in the multi-pack-index reference a location inside that pack-file." 2. 'git multi-pack-index expire' deletes any unreferenced pack-files and updaes the multi-pack-index to drop those pack-files from the list. This is safe to do as concurrent Git processes will see the multi-pack-index and not open those packs when looking for object contents. (Similar to the 'loose-objects' job, there are some Git commands that open pack-files regardless of the multi-pack-index, but they are rarely used. Further, a user that self-selects to use background operations would likely refrain from using those commands.) 3. 'git multi-pack-index repack --bacth-size=' collects a set of pack-files that are listed in the multi-pack-index and creates a new pack-file containing the objects whose offsets are listed by the multi-pack-index to be in those objects. The set of pack- files is selected greedily by sorting the pack-files by modified time and adding a pack-file to the set if its "expected size" is smaller than the batch size until the total expected size of the selected pack-files is at least the batch size. The "expected size" is calculated by taking the size of the pack-file divided by the number of objects in the pack-file and multiplied by the number of objects from the multi-pack-index with offset in that pack-file. The expected size approximates how much data from that pack-file will contribute to the resulting pack-file size. The intention is that the resulting pack-file will be close in size to the provided batch size. The next run of the incremental-repack task will delete these repacked pack-files during the 'expire' step. In this version, the batch size is set to "0" which ignores the size restrictions when selecting the pack-files. It instead selects all pack-files and repacks all packed objects into a single pack-file. This will be updated in the next change, but it requires doing some calculations that are better isolated to a separate change. These steps are based on a similar background maintenance step in Scalar (and VFS for Git) [1]. This was incredibly effective for users of the Windows OS repository. After using the same VFS for Git repository for over a year, some users had _thousands_ of pack-files that combined to up to 250 GB of data. We noticed a few users were running into the open file descriptor limits (due in part to a bug in the multi-pack-index fixed by af96fe3 (midx: add packs to packed_git linked list, 2019-04-29). These pack-files were mostly small since they contained the commits and trees that were pushed to the origin in a given hour. The GVFS protocol includes a "prefetch" step that asks for pre-computed pack- files containing commits and trees by timestamp. These pack-files were grouped into "daily" pack-files once a day for up to 30 days. If a user did not request prefetch packs for over 30 days, then they would get the entire history of commits and trees in a new, large pack-file. This led to a large number of pack-files that had poor delta compression. By running this pack-file maintenance step once per day, these repos with thousands of packs spanning 200+ GB dropped to dozens of pack- files spanning 30-50 GB. This was done all without removing objects from the system and using a constant batch size of two gigabytes. Once the work was done to reduce the pack-files to small sizes, the batch size of two gigabytes means that not every run triggers a repack operation, so the following run will not expire a pack-file. This has kept these repos in a "clean" state. [1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/PackfileMaintenanceStep.cs Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/git-maintenance.txt | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'Documentation') diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt index fc95eb594f..3f5d8946b4 100644 --- a/Documentation/git-maintenance.txt +++ b/Documentation/git-maintenance.txt @@ -85,6 +85,24 @@ loose-objects:: advisable to enable both the `loose-objects` and `gc` tasks at the same time. +incremental-repack:: + The `incremental-repack` job repacks the object directory + using the `multi-pack-index` feature. In order to prevent race + conditions with concurrent Git commands, it follows a two-step + process. First, it calls `git multi-pack-index expire` to delete + pack-files unreferenced by the `multi-pack-index` file. Second, it + calls `git multi-pack-index repack` to select several small + pack-files and repack them into a bigger one, and then update the + `multi-pack-index` entries that refer to the small pack-files to + refer to the new pack-file. This prepares those small pack-files + for deletion upon the next run of `git multi-pack-index expire`. + The selection of the small pack-files is such that the expected + size of the big pack-file is at least the batch size; see the + `--batch-size` option for the `repack` subcommand in + linkgit:git-multi-pack-index[1]. The default batch-size is zero, + which is a special case that attempts to repack all pack-files + into a single pack-file. + OPTIONS ------- --auto:: -- cgit v1.2.3 From e841a79a131d8ce491cf04d0ca3e24f139a10b82 Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:38 +0000 Subject: maintenance: add incremental-repack auto condition The incremental-repack task updates the multi-pack-index by deleting pack- files that have been replaced with new packs, then repacking a batch of small pack-files into a larger pack-file. This incremental repack is faster than rewriting all object data, but is slower than some other maintenance activities. The 'maintenance.incremental-repack.auto' config option specifies how many pack-files should exist outside of the multi-pack-index before running the step. These pack-files could be created by 'git fetch' commands or by the loose-objects task. The default value is 10. Setting the option to zero disables the task with the '--auto' option, and a negative value makes the task run every time. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/config/maintenance.txt | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'Documentation') diff --git a/Documentation/config/maintenance.txt b/Documentation/config/maintenance.txt index c31613be62..a0706d8f09 100644 --- a/Documentation/config/maintenance.txt +++ b/Documentation/config/maintenance.txt @@ -23,3 +23,12 @@ maintenance.loose-objects.auto:: positive value implies the command should run when the number of loose objects is at least the value of `maintenance.loose-objects.auto`. The default value is 100. + +maintenance.incremental-repack.auto:: + This integer config option controls how often the `incremental-repack` + task should be run as part of `git maintenance run --auto`. If zero, + then the `incremental-repack` task will not run with the `--auto` + option. A negative value will force the task to run every time. + Otherwise, a positive value implies the command should run when the + number of pack-files not in the multi-pack-index is at least the value + of `maintenance.incremental-repack.auto`. The default value is 10. -- cgit v1.2.3