maintenance: add troubleshooting guide to docs

The 'git maintenance run' subcommand takes a lock on the object database
to prevent concurrent processes from competing for resources. This is an
important safety measure to prevent possible repository corruption and
data loss.

This feature can lead to confusing behavior if a user is not aware of
it. Add a TROUBLESHOOTING section to the 'git maintenance' builtin
documentation that discusses these tradeoffs. The short version of this
section is that Git will not corrupt your repository, but if the list of
scheduled tasks takes longer than an hour then some scheduled tasks may
be dropped due to this object database collision. For example, a
long-running "daily" task at midnight might prevent an "hourly" task
from running at 1AM.

The opposite is also possible, but less likely as long as the "hourly"
tasks are much faster than the "daily" and "weekly" tasks.

Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

Documentation/git-maintenance.txt

@@ -175,6 +175,50 @@ OPTIONS
 	`maintenance.<task>.enabled` configured as `true` are considered.
 	See the 'TASKS' section for the list of accepted `<task>` values.
 
+
+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`.
+
+
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