Handle cgroups when subvolumes/snapshots are deleted when quota support is enabled.

[friederschueler]

Hello,

I have enabled quota support and btrfs creates a qgroup with id 0/{subvolid} whenever a subvolume is created. But this qgroup is not deleted when the subvolume is deleted. Could you add an option to delete the correct qgroup when the subvolume is deleted?

[digint]

First of all, let me say that I don't have much experience with btrfs qgroups, so I'll need your help here.

As far as I see from the documentations, qgroups are deleted like this:

btrfs qgroup destroy <qgroupid> <path>

For a subvolume with ID=123 ("Subvolume ID" from btrfs sub show) of a btrfs filesystem mounted on /mnt/btr_pool, the command to be run should be something like this:

btrfs qgroup destroy "0/123" /mnt/btr_pool/

Can you tell me what exactly the <path> is here? Does it have to be the mount-point of a volume mounted with subvolid=0, or can it be "just anywhere within a btrfs filesystem", e.g. the <target-directory> from the btrbk config line: target send-receive <target-directory>, or <volume-directory> from volume <volume-directory>?

[friederschueler]

Thanks for the quick response. I am not that expierenced using qgroups either and it seems there is not a lot of documentation out there.
On my pure btrfs system:

/etc/btrbk # btrfs subvolume list -a .
ID 257 gen 18326 top level 5 path <FS_TREE>/root
ID 259 gen 18326 top level 5 path <FS_TREE>/snapshots
ID 264 gen 18109 top level 257 path root/home
ID 271 gen 67 top level 257 path root/var/lib/machines

any path, e.g. "." works. I guess it can be just anywhere within a btrfs filesystem.

[digint]

I added a qgroup branch, with preliminary experimental support for destroying qgroups:

New configuration options:

• snapshot_qgroup_destroy yes|no: destroys qgroup associated to the source snapshot 0/<subvolume_id> before deleting the snapshot
• target_qgroup_destroy yes|no: : destroys qgroup associated to the target subvolume 0/<subvolume_id> before deleting the target subvolume

This is not yet tested.

@friederschueler: could you please look at https://github.com/digint/btrbk/blob/qgroup/btrbk, run it with btrbk -l debug dryrun, find and double-check the lines that look like this:

...
[btrfs] qgroup delete
[btrfs]   qgroup_id: 0/331
[btrfs]   subvolume: /tmp/btrbk_test/mnt_source/svol.20150930
Destroy qgroup "0/331" for: /tmp/btrbk_test/mnt_source/svol.20150930
### (dryrun) 'btrfs' 'qgroup' 'destroy' '0/331' '/tmp/btrbk_test/mnt_source/svol.20150930'
...

and tell me what happens when you execute the btrfs qgroup destroy commands?
Does it delete the correct qgroup?

[friederschueler]

Works like a charm! Thanks for the quick added feature.

I tested with dryrun and debug on and checked the commands. They looked good and the normal run deleted all the unnecessary groups without any problems.

I only tested snapshot_qgroup_destroy.

[digint]

Great!

I will not merge this branch to master unless I have the time to experiment with btrfs quotas myself, since I simply cannot tell if this feature could possibly destroy unwanted things for people making heavy use of quotas.

Can you tell me if a qgroup "0/id" is also created on a btrfs receive operation? I suspect it is, as soon as quota support is enabled for the receiving subvolume (target-directory in btrbk). If not, target_qgroup_destroy would not make much sense.

[TomJohnZ]

Hello,

I've been using the qgroup branch successfully for months now; it's really helped keep my subvolume list cleaner. However, it looks like that branch hasn't received update merges in a while. I was wondering if support has been dropped for qgroups, or alternatively if it might get merged into master at some point?

Thanks a lot for all your great work!

[digint]

Thanks for reminding me, this one got forgotten on last release. Sorry about that.

I merged newest tag v0.24.0 into the qgroup branch (a6cc808), and removed some deprecated calls to run_cmd() in 2ae4557.

Should work as expected.

@TomJohnZ can you please confirm it working on a productive machine?

[TomJohnZ]

Thanks a lot for the merge! I've been running it for a few days and it looks like it everything's working as expected. I appreciate the help.

[digint]

Merged "qgroup" branch (now obsolete) to master, with some adaptions: e9a517f

[digint]

included in btrbk-0.26.0