Re: freezes during snapshot creation/deletion -- to be expected? (Was: Re: btrfs based backup?)

[Oliver Freyermuth <o.freyermuth@googlemail.com>]

Am 21.11.19 um 17:44 schrieb Christian Pernegger:
> No to deduplication, maybe to quotas. It's possible that Timeshift
> enables them, how can I check?

You can test with:
 $ btrfs qgroup show /
 ERROR: can't list qgroups: quotas not enabled
but none of the tools you are using should activate qgroups I think
(at least btrbk does not). 

> Just had another episode:
> 2019-11-21T17:17:01+0100 startup v0.26.0 - - - # btrbk command line
> client, version 0.26.0
> 2019-11-21T17:17:01+0100 snapshot starting
> /mnt/timeshift/backup/btrbk-snapshots/@.20191121T171701+0100
> /mnt/timeshift/backup/@ - -
> 2019-11-21T17:17:01+0100 snapshot success
> /mnt/timeshift/backup/btrbk-snapshots/@.20191121T171701+0100
> /mnt/timeshift/backup/@ - -
> 2019-11-21T17:17:01+0100 snapshot starting
> /mnt/timeshift/backup/btrbk-snapshots/@home.20191121T171701+0100
> /mnt/timeshift/backup/@home - -
> 2019-11-21T17:17:01+0100 snapshot success
> /mnt/timeshift/backup/btrbk-snapshots/@home.20191121T171701+0100
> /mnt/timeshift/backup/@home - -
> 2019-11-21T17:17:01+0100 delete_snapshot starting
> /mnt/timeshift/backup/btrbk-snapshots/@.20191119T161701+0100 - - -
> 2019-11-21T17:17:01+0100 delete_snapshot success
> /mnt/timeshift/backup/btrbk-snapshots/@.20191119T161701+0100 - - -
> 2019-11-21T17:17:01+0100 delete_snapshot starting
> /mnt/timeshift/backup/btrbk-snapshots/@home.20191119T161701+0100 - - -
> 2019-11-21T17:17:01+0100 delete_snapshot success
> /mnt/timeshift/backup/btrbk-snapshots/@home.20191119T161701+0100 - - -
> 2019-11-21T17:17:01+0100 delete_snapshot starting
> /mnt/timeshift/backup/btrbk-snapshots/@home-chris-.steam.20191119T161701+0100
> - - -
> 2019-11-21T17:17:01+0100 delete_snapshot success
> /mnt/timeshift/backup/btrbk-snapshots/@home-chris-.steam.20191119T161701+0100
> - - -
> 2019-11-21T17:17:01+0100 finished success - - - -
> 
> I had a tail on the log, these came out in one go, no larger pauses.
> At first I thought, just my luck, here I am lying in wait and of
> course everything works, then the mini-freeze happened. CPU usage in
> one core spiked during the freeze, but I couldn't switch tabs from the
> graphs to the process list in gnome-system-monitor. Top it is, next
> time.

This is an interesting observation. I believe this means it is happening when the snapshot deletes are actually going to the storage,
which usually happens only _after_ btrbk is finished (in case you catch it with top, a kernel thread "btrfs-cleaner" should be doing this job). 
Another interesting test could be to adjust btrbk configuration to:
btrfs_commit_delete = each
which will ensure the delete_snapshot operations are flushed to disk one by one, so the freeze should then correlate to the log
(and might be converted from one longer freeze to multiple, contiguous smaller freezes). 

Sadly, I have no idea on why this would freeze for you (well, it's the only actual I/O-heavy part when you don't do the transfers at this point in time). 
But maybe Qu will have a good idea. 

Cheers,
	Oliver