Parallezing do_unmount

[anthonyryan1]

I've been exploring shutdown times on some OpenRC servers with up to 100 hard drives connected to each machine.

I found that one of the primary bottlenecks is the sequential nature of do_unmount in rc-mount.sh. On a machine with hundreds of mount points, it will iterate through them sequentially one after another.

My first idea was to run each unmount command in the background with &. But that would break assumptions about unmount order, potentially creating unmount errors for people with filesystems mounted inside racing their parent to unmount.

I'm thinking the correct way to make do_unmount run in parallel would be to use fstabinfo --passno '' to get all the pass numbers, then iterate from the largest passno down to 0 using fstabinfo --passno =N to decide which drives can be unmounted in parallel during each batch.

I'm currently looking for feedback and concerns before creating a pull request.

[vapier]

i don't think passno works. that assumes all mounts are in fstab to begin with, and that's def not a requirement.

this should be a compiled program so that it can actually build a graph and then process it as such using threads.

i could see there being a benefit to tickling the underlying devices in parallel in case they went to sleep -- waking back up to unmount is def slow. i'm not sure how much overhead there actually is though if all the drives are active and we just ran umount syscall on them in serial.

[Tommimon]

I'd like to give this a try and implement a c program that uses mountinfo output to build a dependency graph. The program output will contain a dependency list for each mount point, those lists can of course be nested. The shell script will then run the unmount operation in parallel starting from the leaves lists and proceeding to other elements when all dependencies are met.

Do you think there's a better suitable approach for this specific issue?

[vapier]

the entire tool should be written in C, not a mix of C & shell

[Tommimon]

To be sure I understood correctly, you suggest a unmount_all.c which, when executed once, will do:

1. retrieve data from mountinfo.c.
2. create a dependency graph on all the mount points present.
3. unmount them in parallel when possible.

If this is what you suggest, I think is a good Idea to avoid mixing C with shell when is not required.

[vapier]

correct

[doot-64]

We have a proposed solution here if anyone wants to take a look.