Snapshot usage guidelines (to avoid stability issues)

[Kostik Belousov]

On Thu, Jun 21, 2007 at 12:32:40PM +0300, Cristian KLEIN wrote:
> Kostik Belousov wrote:
> > On Thu, Jun 21, 2007 at 01:45:32AM +0300, Cristian KLEIN wrote:
> >> Kostik Belousov wrote:
> >>> On Wed, Jun 20, 2007 at 06:26:26PM +0200, Espen Skoglund wrote:
> >>>> Hi,
> >>>>
> >>>> A couple of days ago I decided to start using snapshots on my system.
> >>>> Knowing that the functionality has been available for quite some time
> >>>> now I (apparently wrongly) assumed that it was ready for production
> >>>> use.  My bad.
> >>>>
> >>>> My system, a 6-STABLE from week and a half ago, uses two 250GB
> >>>> gstriped disks and has about 4-5 snapshots on one 215GB UFS partiton.
> >>>> This morning I learned that things had gone terribly wrong during some
> >>>> nightly cronjobs, hanging the whole system.  Suspecting that snapshots
> >>>> were the culprit I soon learned after some investigation that the
> >>>> snapshot functionality wasn't nearly as stable as I had hoped for.
> >>>>
> >>>> Looking at PRs and mailing lists there seems to be mainly two
> >>>> outstanding stability issues with snapshots: a) snapshotted fs running
> >>>> full, and b) deleting large amount of files on an fs with multiple
> >>>> snapshots.  The former issue, while certainly annoying, one could be
> >>>> able to work around.  The latter issue, on the other hand, seems more
> >>>> like a definite show stopper.
> >>>>
> >>>> Am I right in inferring that the two above cases are main issues with
> >>>> snaphots at this time, or are there other known gotchas that I have to
> >>>> look out for.
> >>> About the issue b). Are you system sources before or after 2007-06-11
> >>> 10:53:48 UTC ? Or simply show me the version of sys/ufs/ffs/ffs_snapshot.c.
> >>>
> >>> If it is less then 1.103.2.24, see developers handbook for instruction on
> >>> reporting deadlocks.
> >>>
> >> Do you think that 1.103.2.24 might solve this
> >> http://lists.freebsd.org/pipermail/freebsd-fs/2007-May/003161.html ?
> > I very much doubt it. AFAIR, I asked at least ddb backtrace for the crash,
> > and you did not answered.
> 
> I'm really sorry, but I wasn't able to reproduce the problem on another
> system. I tried taking / deleting many snapshots while running bonnie++,
> but the panic would not occur.
> 
> I have 1,7mil inodes (4% utilization) on the server where the crash
> occured. Might this be a prerequisite of the panic?

It may be related to inode/cyl groups density ratio.
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