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

[Marc Joliet <marcec@gmx.de>]

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Am Samstag, 23. November 2019, 00:21:18 CET schrieben Sie:
> Am Freitag, 22. November 2019, 02:36:56 CET schrieb Chris Murphy:
> > On Thu, Nov 21, 2019 at 3:39 PM Marc Joliet <marcec@gmx.de> wrote:
> > > On a side note, I am also really annoyed by the lockups caused by
> > > qgroups.
> > > On my Gentoo systems (which use btrbk) I have it disabled for that
> > > reason, but I left it on on my openSUSE laptop (a Dell XPS 13 9360),
> > > which locks up for about 15-30 minutes while cleaning up snapshots a few
> > > times a week (usually after reboots or after "zypper dup").
> >
> > 15 seconds is not at all acceptable on a desktop system, 15 minutes is
> > atrocious. A computer that appears to hang for 15 seconds, it is
> > completely reasonable for ordinary users to consider has totally
> > faceplanted, will not recover, and to force power off. The
> > distribution really needs to do something about that kind of negative
> > user experience.
>
> Sadly, I can't say if it's better without snapshotting /home, because I
> hadn't accumulated many / snapshots at that point in time.  It might have
> gotten worse even with only / being snapshotted.  But like I said, I'll
> experiment with configuring snapper before blaming SUSE.  I believe the
> installation even recommends against snapshotting /home, but hey, I wanted
> to do it anyway :-) .
>
> But to be precise, it's not locked up continuously during snapshot deletion.
> Occasionally I'll be able to operate my desktop for a few seconds, and if I
> leave top running in a GUI terminal (in my case konsole), I'll see it
> updating (almost) the entire time.  My guess (emphasis on *guess*) is that
> the qgroups update is holding some lock that is preventing other I/O from
> finishing, thus locking up any application that wants to write to disk and
> isn't doing so concurrently (maybe Plasma is blocking on fsync() at the
> time?).

So just to follow up on this, reducing the total number of snapshots and
increasing the time between their creation from hourly to once every six hours
did help a *little* bit.  However, about a week ago I decided to try an
experiment and added the "autodefrag" mount option (which I don't usually do
on SSDs), and that helped *massively*.  Ever since, snapper-cleanup.service
runs without me noticing at all!

[ What made me try it was that booting the laptop and logging in started
getting really slow and top was showing several btrfs-endio threads hogging
the CPU, *before* snapper-cleanup.service or anything else specific to btrfs
was running (their activity usually coincided with KDE Baloo activity), i.e.,
general I/O was performing badly. ]

Greetings
--
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup

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