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 --- t/t7900-maintenance.sh | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) (limited to 't') diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index 53c883531e..045524e6ad 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -62,4 +62,30 @@ test_expect_success 'run --task duplicate' ' test_i18ngrep "cannot be selected multiple times" err ' +test_expect_success 'run --task=prefetch with no remotes' ' + git maintenance run --task=prefetch 2>err && + test_must_be_empty err +' + +test_expect_success 'prefetch multiple remotes' ' + git clone . clone1 && + git clone . clone2 && + git remote add remote1 "file://$(pwd)/clone1" && + git remote add remote2 "file://$(pwd)/clone2" && + git -C clone1 switch -c one && + git -C clone2 switch -c two && + test_commit -C clone1 one && + test_commit -C clone2 two && + GIT_TRACE2_EVENT="$(pwd)/run-prefetch.txt" git maintenance run --task=prefetch 2>/dev/null && + fetchargs="--prune --no-tags --no-write-fetch-head --recurse-submodules=no --refmap= --quiet" && + test_subcommand git fetch remote1 $fetchargs +refs/heads/\\*:refs/prefetch/remote1/\\* 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 --- t/t7900-maintenance.sh | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) (limited to 't') diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index 045524e6ad..b3fc7c8670 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -88,4 +88,43 @@ test_expect_success 'prefetch multiple remotes' ' test_cmp_rev refs/remotes/remote2/two refs/prefetch/remote2/two ' +test_expect_success 'loose-objects task' ' + # Repack everything so we know the state of the object dir + git repack -adk && + + # Hack to stop maintenance from running during "git commit" + echo in use >.git/objects/maintenance.lock && + + # Assuming that "git commit" creates at least one loose object + test_commit create-loose-object && + rm .git/objects/maintenance.lock && + + ls .git/objects >obj-dir-before && + test_file_not_empty obj-dir-before && + ls .git/objects/pack/*.pack >packs-before && + test_line_count = 1 packs-before && + + # The first run creates a pack-file + # but does not delete loose objects. + git maintenance run --task=loose-objects && + ls .git/objects >obj-dir-between && + test_cmp obj-dir-before obj-dir-between && + ls .git/objects/pack/*.pack >packs-between && + test_line_count = 2 packs-between && + ls .git/objects/pack/loose-*.pack >loose-packs && + test_line_count = 1 loose-packs && + + # The second run deletes loose objects + # but does not create a pack-file. + git maintenance run --task=loose-objects && + ls .git/objects >obj-dir-after && + cat >expect <<-\EOF && + info + pack + EOF + test_cmp expect obj-dir-after && + ls .git/objects/pack/*.pack >packs-after && + test_cmp packs-between packs-after +' + test_done -- 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 --- t/t7900-maintenance.sh | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) (limited to 't') diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index b3fc7c8670..27565c55a2 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -127,4 +127,26 @@ test_expect_success 'loose-objects task' ' test_cmp packs-between packs-after ' +test_expect_success 'maintenance.loose-objects.auto' ' + git repack -adk && + GIT_TRACE2_EVENT="$(pwd)/trace-lo1.txt" \ + git -c maintenance.loose-objects.auto=1 maintenance \ + run --auto --task=loose-objects 2>/dev/null && + test_subcommand ! git prune-packed --quiet /dev/null && + test_subcommand ! git prune-packed --quiet /dev/null && + test_subcommand git prune-packed --quiet /dev/null && + test_subcommand git prune-packed --quiet Date: Fri, 25 Sep 2020 12:33:35 +0000 Subject: midx: use start_delayed_progress() Now that the multi-pack-index may be written as part of auto maintenance at the end of a command, reduce the progress output when the operations are quick. Use start_delayed_progress() instead of start_progress(). Update t5319-multi-pack-index.sh to use GIT_PROGRESS_DELAY=0 now that the progress indicators are conditional. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- t/t5319-multi-pack-index.sh | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) (limited to 't') diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index 7dfff0f8f4..ec87f616c6 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -171,12 +171,12 @@ test_expect_success 'write progress off for redirected stderr' ' ' test_expect_success 'write force progress on for stderr' ' - git multi-pack-index --object-dir=$objdir --progress write 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir --progress write 2>err && test_file_not_empty err ' test_expect_success 'write with the --no-progress option' ' - git multi-pack-index --object-dir=$objdir --no-progress write 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir --no-progress write 2>err && test_line_count = 0 err ' @@ -333,17 +333,17 @@ test_expect_success 'git-fsck incorrect offset' ' ' test_expect_success 'repack progress off for redirected stderr' ' - git multi-pack-index --object-dir=$objdir repack 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir repack 2>err && test_line_count = 0 err ' test_expect_success 'repack force progress on for stderr' ' - git multi-pack-index --object-dir=$objdir --progress repack 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir --progress repack 2>err && test_file_not_empty err ' test_expect_success 'repack with the --no-progress option' ' - git multi-pack-index --object-dir=$objdir --no-progress repack 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir --no-progress repack 2>err && test_line_count = 0 err ' @@ -487,7 +487,7 @@ test_expect_success 'expire progress off for redirected stderr' ' test_expect_success 'expire force progress on for stderr' ' ( cd dup && - git multi-pack-index --progress expire 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --progress expire 2>err && test_file_not_empty err ) ' @@ -495,7 +495,7 @@ test_expect_success 'expire force progress on for stderr' ' test_expect_success 'expire with the --no-progress option' ' ( cd dup && - git multi-pack-index --no-progress expire 2>err && + GIT_PROGRESS_DELAY=0 git multi-pack-index --no-progress expire 2>err && test_line_count = 0 err ) ' -- 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 --- t/t5319-multi-pack-index.sh | 1 + t/t7900-maintenance.sh | 38 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 39 insertions(+) (limited to 't') diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index ec87f616c6..2f942ee1fa 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -3,6 +3,7 @@ test_description='multi-pack-indexes' . ./test-lib.sh +GIT_TEST_MULTI_PACK_INDEX=0 objdir=.git/objects midx_read_expect () { diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index 27565c55a2..a2db2291b0 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -5,6 +5,7 @@ test_description='git maintenance builtin' . ./test-lib.sh GIT_TEST_COMMIT_GRAPH=0 +GIT_TEST_MULTI_PACK_INDEX=0 test_expect_success 'help text' ' test_expect_code 129 git maintenance -h 2>err && @@ -149,4 +150,41 @@ test_expect_success 'maintenance.loose-objects.auto' ' test_subcommand git prune-packed --quiet packs-before && + test_line_count = 3 packs-before && + + # the job repacks the two into a new pack, but does not + # delete the old ones. + git maintenance run --task=incremental-repack && + ls $packDir/*.pack >packs-between && + test_line_count = 4 packs-between && + + # the job deletes the two old packs, and does not write + # a new one because only one pack remains. + git maintenance run --task=incremental-repack && + ls .git/objects/pack/*.pack >packs-after && + test_line_count = 1 packs-after +' + test_done -- cgit v1.2.3 From a13e3d0ec87c9d12b93700375fceacdfb2c6885f Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 25 Sep 2020 12:33:37 +0000 Subject: maintenance: auto-size incremental-repack batch When repacking during the 'incremental-repack' task, we use the --batch-size option in 'git multi-pack-index repack'. The initial setting used --batch-size=0 to repack everything into a single pack-file. This is not sustainable for a large repository. The amount of work required is also likely to use too many system resources for a background job. Update the 'incremental-repack' task by dynamically computing a --batch-size option based on the current pack-file structure. The dynamic default size is computed with this idea in mind for a client repository that was cloned from a very large remote: there is likely one "big" pack-file that was created at clone time. Thus, do not try repacking it as it is likely packed efficiently by the server. Instead, we select the second-largest pack-file, and create a batch size that is one larger than that pack-file. If there are three or more pack-files, then this guarantees that at least two will be combined into a new pack-file. Of course, this means that the second-largest pack-file size is likely to grow over time and may eventually surpass the initially-cloned pack-file. Recall that the pack-file batch is selected in a greedy manner: the packs are considered from oldest to newest and are selected if they have size smaller than the batch size until the total selected size is larger than the batch size. Thus, that oldest "clone" pack will be first to repack after the new data creates a pack larger than that. We also want to place some limits on how large these pack-files become, in order to bound the amount of time spent repacking. A maximum batch-size of two gigabytes means that large repositories will never be packed into a single pack-file using this job, but also that repack is rather expensive. This is a trade-off that is valuable to have if the maintenance is being run automatically or in the background. Users who truly want to optimize for space and performance (and are willing to pay the upfront cost of a full repack) can use the 'gc' task to do so. Create a test for this two gigabyte limit by creating an EXPENSIVE test that generates two pack-files of roughly 2.5 gigabytes in size, then performs an incremental repack. Check that the --batch-size argument in the subcommand uses the hard-coded maximum. Helped-by: Chris Torek Reported-by: Son Luong Ngoc Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- t/t7900-maintenance.sh | 36 ++++++++++++++++++++++++++++++++++-- 1 file changed, 34 insertions(+), 2 deletions(-) (limited to 't') diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index a2db2291b0..9e6ea23f35 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -181,10 +181,42 @@ test_expect_success 'incremental-repack task' ' test_line_count = 4 packs-between && # the job deletes the two old packs, and does not write - # a new one because only one pack remains. + # a new one because the batch size is not high enough to + # pack the largest pack-file. git maintenance run --task=incremental-repack && ls .git/objects/pack/*.pack >packs-after && - test_line_count = 1 packs-after + test_line_count = 2 packs-after +' + +test_expect_success EXPENSIVE 'incremental-repack 2g limit' ' + for i in $(test_seq 1 5) + do + test-tool genrandom foo$i $((512 * 1024 * 1024 + 1)) >>big || + return 1 + done && + git add big && + git commit -m "Add big file (1)" && + + # ensure any possible loose objects are in a pack-file + git maintenance run --task=loose-objects && + + rm big && + for i in $(test_seq 6 10) + do + test-tool genrandom foo$i $((512 * 1024 * 1024 + 1)) >>big || + return 1 + done && + git add big && + git commit -m "Add big file (2)" && + + # ensure any possible loose objects are in a pack-file + git maintenance run --task=loose-objects && + + # Now run the incremental-repack task and check the batch-size + GIT_TRACE2_EVENT="$(pwd)/run-2g.txt" git maintenance run \ + --task=incremental-repack 2>/dev/null && + test_subcommand git multi-pack-index repack \ + --no-progress --batch-size=2147483647 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 --- t/t7900-maintenance.sh | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) (limited to 't') diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index 9e6ea23f35..55116c2f04 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -219,4 +219,32 @@ test_expect_success EXPENSIVE 'incremental-repack 2g limit' ' --no-progress --batch-size=2147483647 /dev/null && + test_subcommand ! git multi-pack-index write --no-progress /dev/null && + test_subcommand ! git multi-pack-index write --no-progress /dev/null && + test_subcommand git multi-pack-index write --no-progress