git-maintenance - Run tasks to optimize Git repository data

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.

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.

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:

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.

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:

One timer is registered for each --schedule=<frequency> option.

The definition of the systemd units can be inspected in the following files:

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).

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:

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.

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.

Part of the git[1] suite