Re: commands like "du", "df", and "btrfs fs sync" hang

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 9 May 2016 13:13:53 -0400
schrieb Nicholas D Steeves <nsteeves@gmail.com>:

> On May 7, 2016 7:43 AM, "Kai Krakow" <hurikhan77@gmail.com> wrote:
> >
> > Am Thu, 5 May 2016 08:35:37 +0200
> > schrieb Kai Krakow <hurikhan77@gmail.com>:
> >  
> > > Am Tue, 3 May 2016 08:48:14 +0200
> > > schrieb Kai Krakow <hurikhan77@gmail.com>:
> > >  
>  [...]  
>  [...]  
> > >  [...]  
>  [...]  
>  [...]  
> > >
> > > With the snapshot and sync related bits disabled in my script, I
> > > no longer experience freezing df/du/... commands.  
> >
> > Ah well, I still do - it just triggers much less often.
> >
> > But I can track it down to the automounter now: As soon as I stop
> > the automounter of my backup device, du/df/etc no longer hang.
> > Still I'm not sure if the problem is originating from btrfs or
> > autofs itself. 
> 
> Hi Kai,
> 
> If it's anything like what I've encountered it can be worked around
> with careful use of sleep, sync, btrfs sub sync and/or btrfs fi sync.
> Also, if it's a similar issue to my own then from what I've read on
> this list it's a timing or race issue (or maybe a locking issue).
> There's a Debian bug where someone is testing out different
> values/strategies.  The values/strategy I use can be found in
> btrfs-borg on github.  If I'm correct, you'll probably also need to
> script in delays to things time to settle with your automounter.
> 
> Of course, ideally someone will one day spend the time debugging
> what's actually going on.

Thanks for the pointer... Maybe I could just remove the idle timeout of
the automount unit, so it never tries to automatically unmount once
mounted. Tho, I prefer if it did that. I don't want the backup FS being
online all of the time.

Maybe such a race results from btrfs still doing background work (like
removing snapshots) while there is no longer a user of the mount point
running (processes terminated) - and now, when systemd's automounter
tries to unmount, it doesn't work and autofs hangs.

If I know experience hanging processes, I can simply "systemctl restart
mnt-usbdevice.automount" and the hanging processes start to work again.

-- 
Regards,
Kai

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