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NAME
git-maintenance - Run tasks to optimize Git repository data
SYNOPSIS
git maintenance run [<options>]
DESCRIPTION
Run tasks to optimize Git repository data, speeding up other Git commands and reducing
storage requirements for the repository.
Git commands that add repository data, such as git add or git fetch, are optimized for a
responsive user experience. These commands do not take time to optimize the Git data,
since such optimizations scale with the full size of the repository while these user
commands each perform a relatively small action.
The git maintenance command provides flexibility for how to optimize the Git repository.
SUBCOMMANDS
register
Initialize Git config values so any scheduled maintenance will start running on this
repository. This adds the repository to the maintenance.repo config variable in the
current user’s global config and enables some recommended configuration values for
maintenance.<task>.schedule. The tasks that are enabled are safe for running in the
background without disrupting foreground processes.
The register subcommand will also set the maintenance.strategy config value to
incremental, if this value is not previously set. The incremental strategy uses the
following schedule for each maintenance task:
• gc: disabled.
• commit-graph: hourly.
• prefetch: hourly.
• loose-objects: daily.
• incremental-repack: daily.
git maintenance register will also disable foreground maintenance by setting
maintenance.auto = false in the current repository. This config setting will remain
after a git maintenance unregister command.
run
Run one or more maintenance tasks. If one or more --task options are specified, then
those tasks are run in that order. Otherwise, the tasks are determined by which
maintenance.<task>.enabled config options are true. By default, only
maintenance.gc.enabled is true.
start
Start running maintenance on the current repository. This performs the same config
updates as the register subcommand, then updates the background scheduler to run git
maintenance run --scheduled on an hourly basis.
stop
Halt the background maintenance schedule. The current repository is not removed from
the list of maintained repositories, in case the background maintenance is restarted
later.
unregister
Remove the current repository from background maintenance. This only removes the
repository from the configured list. It does not stop the background maintenance
processes from running.
TASKS
commit-graph
The commit-graph job updates the commit-graph files incrementally, then verifies that
the written data is correct. The incremental write is safe to run alongside concurrent
Git processes since it will not expire .graph files that were in the previous
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 configured
refspec is modified to place all requested refs within 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 a
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 smaller tasks. This task can be expensive for
large repositories, as it repacks all Git objects into a single pack-file. It can also
be disruptive in some situations, as it deletes stale data. See 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.
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 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.
pack-refs
The pack-refs task collects the loose reference files and collects them into a single
file. This speeds up operations that need to iterate across many references. See git-
pack-refs(1) for more information.
OPTIONS
--auto
When combined with the run subcommand, run maintenance tasks only if certain
thresholds are met. For example, the gc task runs when the number of loose objects
exceeds the number stored in the gc.auto config setting, or when the number of
pack-files exceeds the gc.autoPackLimit config setting. Not compatible with the
--schedule option.
--schedule
When combined with the run subcommand, run maintenance tasks only if certain time
conditions are met, as specified by the maintenance.<task>.schedule config value for
each <task>. This config value specifies a number of seconds since the last time that
task ran, according to the maintenance.<task>.lastRun config value. The tasks that are
tested are those provided by the --task=<task> option(s) or those with
maintenance.<task>.enabled set to true.
--quiet
Do not report progress or other information over stderr.
--task=<task>
If this option is specified one or more times, then only run the specified tasks in
the specified order. If no --task=<task> arguments are specified, then only the tasks
with maintenance.<task>.enabled configured as true are considered. See the TASKS
section for the list of accepted <task> values.
--scheduler=auto|crontab|systemd-timer|launchctl|schtasks
When combined with the start subcommand, specify the scheduler for running the hourly,
daily and weekly executions of git maintenance run. Possible values for <scheduler>
are auto, crontab (POSIX), systemd-timer (Linux), launchctl (macOS), and schtasks
(Windows). When auto is specified, the appropriate platform-specific scheduler is
used; on Linux, systemd-timer is used if available, otherwise crontab. Default is
auto.
TROUBLESHOOTING
The git maintenance command is designed to simplify the repository maintenance patterns
while minimizing user wait time during Git commands. A variety of configuration options
are available to allow customizing this process. The default maintenance options focus on
operations that complete quickly, even on large repositories.
Users may find some cases where scheduled maintenance tasks do not run as frequently as
intended. Each git maintenance run command takes a lock on the repository’s object
database, and this prevents other concurrent git maintenance run commands from running on
the same repository. Without this safeguard, competing processes could leave the
repository in an unpredictable state.
The background maintenance schedule runs git maintenance run processes on an hourly basis.
Each run executes the "hourly" tasks. At midnight, that process also executes the "daily"
tasks. At midnight on the first day of the week, that process also executes the "weekly"
tasks. A single process iterates over each registered repository, performing the scheduled
tasks for that frequency. Depending on the number of registered repositories and their
sizes, this process may take longer than an hour. In this case, multiple git maintenance
run commands may run on the same repository at the same time, colliding on the object
database lock. This results in one of the two tasks not running.
If you find that some maintenance windows are taking longer than one hour to complete,
then consider reducing the complexity of your maintenance tasks. For example, the gc task
is much slower than the incremental-repack task. However, this comes at a cost of a
slightly larger object database. Consider moving more expensive tasks to be run less
frequently.
Expert users may consider scheduling their own maintenance tasks using a different
schedule than is available through git maintenance start and Git configuration options.
These users should be aware of the object database lock and how concurrent git maintenance
run commands behave. Further, the git gc command should not be combined with git
maintenance run commands. git gc modifies the object database but does not take the lock
in the same way as git maintenance run. If possible, use git maintenance run --task=gc
instead of git gc.
The following sections describe the mechanisms put in place to run background maintenance
by git maintenance start and how to customize them.
BACKGROUND MAINTENANCE ON POSIX SYSTEMS
The standard mechanism for scheduling background tasks on POSIX systems is cron(8). This
tool executes commands based on a given schedule. The current list of user-scheduled tasks
can be found by running crontab -l. The schedule written by git maintenance start is
similar to this:
# BEGIN GIT MAINTENANCE SCHEDULE
# The following schedule was created by Git
# Any edits made in this region might be
# replaced in the future by a Git command.
0 1-23 * * * "/<path>/git" --exec-path="/<path>" for-each-repo --config=maintenance.repo maintenance run --schedule=hourly
0 0 * * 1-6 "/<path>/git" --exec-path="/<path>" for-each-repo --config=maintenance.repo maintenance run --schedule=daily
0 0 * * 0 "/<path>/git" --exec-path="/<path>" for-each-repo --config=maintenance.repo maintenance run --schedule=weekly
# END GIT MAINTENANCE SCHEDULE
The comments are used as a region to mark the schedule as written by Git. Any
modifications within this region will be completely deleted by git maintenance stop or
overwritten by git maintenance start.
The crontab entry specifies the full path of the git executable to ensure that the
executed git command is the same one with which git maintenance start was issued
independent of PATH. If the same user runs git maintenance start with multiple Git
executables, then only the latest executable is used.
These commands use git for-each-repo --config=maintenance.repo to run git maintenance run
--schedule=<frequency> on each repository listed in the multi-valued maintenance.repo
config option. These are typically loaded from the user-specific global config. The git
maintenance process then determines which maintenance tasks are configured to run on each
repository with each <frequency> using the maintenance.<task>.schedule config options.
These values are loaded from the global or repository config values.
If the config values are insufficient to achieve your desired background maintenance
schedule, then you can create your own schedule. If you run crontab -e, then an editor
will load with your user-specific cron schedule. In that editor, you can add your own
schedule lines. You could start by adapting the default schedule listed earlier, or you
could read the crontab(5) documentation for advanced scheduling techniques. Please do use
the full path and --exec-path techniques from the default schedule to ensure you are
executing the correct binaries in your schedule.
BACKGROUND MAINTENANCE ON LINUX SYSTEMD SYSTEMS
While Linux supports cron, depending on the distribution, cron may be an optional package
not necessarily installed. On modern Linux distributions, systemd timers are superseding
it.
If user systemd timers are available, they will be used as a replacement of cron.
In this case, git maintenance start will create user systemd timer units and start the
timers. The current list of user-scheduled tasks can be found by running systemctl --user
list-timers. The timers written by git maintenance start are similar to this:
$ systemctl --user list-timers
NEXT LEFT LAST PASSED UNIT ACTIVATES
Thu 2021-04-29 19:00:00 CEST 42min left Thu 2021-04-29 18:00:11 CEST 17min ago git-maintenance@hourly.timer git-maintenance@hourly.service
Fri 2021-04-30 00:00:00 CEST 5h 42min left Thu 2021-04-29 00:00:11 CEST 18h ago git-maintenance@daily.timer git-maintenance@daily.service
Mon 2021-05-03 00:00:00 CEST 3 days left Mon 2021-04-26 00:00:11 CEST 3 days ago git-maintenance@weekly.timer git-maintenance@weekly.service
One timer is registered for each --schedule=<frequency> option.
The definition of the systemd units can be inspected in the following files:
~/.config/systemd/user/git-maintenance@.timer
~/.config/systemd/user/git-maintenance@.service
~/.config/systemd/user/timers.target.wants/git-maintenance@hourly.timer
~/.config/systemd/user/timers.target.wants/git-maintenance@daily.timer
~/.config/systemd/user/timers.target.wants/git-maintenance@weekly.timer
git maintenance start will overwrite these files and start the timer again with systemctl
--user, so any customization should be done by creating a drop-in file, i.e. a .conf
suffixed file in the ~/.config/systemd/user/git-maintenance@.service.d directory.
git maintenance stop will stop the user systemd timers and delete the above mentioned
files.
For more details, see systemd.timer(5).
BACKGROUND MAINTENANCE ON MACOS SYSTEMS
While macOS technically supports cron, using crontab -e requires elevated privileges and
the executed process does not have a full user context. Without a full user context, Git
and its credential helpers cannot access stored credentials, so some maintenance tasks are
not functional.
Instead, git maintenance start interacts with the launchctl tool, which is the recommended
way to schedule timed jobs in macOS. Scheduling maintenance through git maintenance
(start|stop) requires some launchctl features available only in macOS 10.11 or later.
Your user-specific scheduled tasks are stored as XML-formatted .plist files in
~/Library/LaunchAgents/. You can see the currently-registered tasks using the following
command:
$ ls ~/Library/LaunchAgents/org.git-scm.git*
org.git-scm.git.daily.plist
org.git-scm.git.hourly.plist
org.git-scm.git.weekly.plist
One task is registered for each --schedule=<frequency> option. To inspect how the XML
format describes each schedule, open one of these .plist files in an editor and inspect
the <array> element following the <key>StartCalendarInterval</key> element.
git maintenance start will overwrite these files and register the tasks again with
launchctl, so any customizations should be done by creating your own .plist files with
distinct names. Similarly, the git maintenance stop command will unregister the tasks with
launchctl and delete the .plist files.
To create more advanced customizations to your background tasks, see launchctl.plist(5)
for more information.
BACKGROUND MAINTENANCE ON WINDOWS SYSTEMS
Windows does not support cron and instead has its own system for scheduling background
tasks. The git maintenance start command uses the schtasks command to submit tasks to this
system. You can inspect all background tasks using the Task Scheduler application. The
tasks added by Git have names of the form Git Maintenance (<frequency>). The Task
Scheduler GUI has ways to inspect these tasks, but you can also export the tasks to XML
files and view the details there.
Note that since Git is a console application, these background tasks create a console
window visible to the current user. This can be changed manually by selecting the "Run
whether user is logged in or not" option in Task Scheduler. This change requires a
password input, which is why git maintenance start does not select it by default.
If you want to customize the background tasks, please rename the tasks so future calls to
git maintenance (start|stop) do not overwrite your custom tasks.
GIT
Part of the git(1) suite