8.1R possible zfs snapshot livelock?

[Andriy Gapon]

on 17/05/2011 14:29 Jeremy Chadwick said the following:
> On Tue, May 17, 2011 at 01:48:04PM +0300, Andriy Gapon wrote:
>> on 17/05/2011 10:30 Jeremy Chadwick said the following:
>>> On Tue, May 17, 2011 at 02:43:44AM -0400, Charles Sprickman wrote:
>>>> Does this sound familiar to anyone running ZFS with snapshots?
>>>
>>> Yes, and is exactly why I don't use them.  :-)
>>
>> You put a smiley, but is this an attempt at FUD?
> 
> I wish it were.  

The reason I asked is that I could have easily answered "No, that's why I use them
all the time".  And I am sure many people would join me on this.
So the way you originally described the issue was sufficiently non-specific and
strong.

> I experienced similar behaviour to Charles during the
> early 8.x days (possibly 8.1-RELEASE, I forget; I may be thinking of
> 8.0?) where ZFS snapshots would occasionally result in the kernel
> deadlocking on ZFS-bound I/O.  The kernel was alive/responsive to some
> degree but ZFS I/O would just indefinitely stall at that point,
> requiring a full system reset.  No disk or controller problems (same
> hardware I'm using today actually!).
> 
> I believe there were commits and improvements for snapshotting committed
> between 8.1-RELEASE and 8.2-RELEASE, but I haven't bothered to test
> them.  The experience left a very bad taste in my mouth and as such I
> have avoided ZFS snapshots since.
> 
> I'd be willing to try them again assuming someone can at least confirm
> that there were commits done to address snapshot concerns during the
> past year or so.  But...
> 
> There are still some outstanding incidents that directly pertain to ZFS
> snapshots, or are "related" to ZFS snapshots (meaning things like
> send/recv which are commonly used alongside snapshots), which I remember
> reading about but really saw no answer to:
> 
> * ZFS send | ssh zfs recv results in ZFS subsystem hanging; 8.1-RELEASE;
>   February 2011:
>   http://lists.freebsd.org/pipermail/freebsd-fs/2011-February/010602.html
> 
> * Kernel panic during heavy disk I/O while "zfs recv" being used
>   simultaneously; CURRENT (so ZFS v28?); April 2011:
>   http://lists.freebsd.org/pipermail/freebsd-fs/2011-April/011155.html
> 
> * ZFS snapshots taking an extremely long time to be deleted; RELENG_8_1;
>   February 2011:
>   http://lists.freebsd.org/pipermail/freebsd-fs/2011-February/010797.html
> 
> * "zfs destroy -r" not working on filesystem-level snapshots but works
>   on pool-level snapshots; RELENG_8 with ZFS v28 patch (and is specific
>   to ZFS v28 given the info); May 2011:
>   http://lists.freebsd.org/pipermail/freebsd-fs/2011-May/011412.html
> 
> Sorry to just rattle off a bunch of URLs and issues at once; it's not my
> intention to slander work on ZFS or anything even remotely like that.
> 
> I'm just wondering given the number of problem reports that seem to come
> in about snapshot or snapshot-related ZFS stuff, where we stand on
> these?  This is mainly for Charles' benefit and not so much mine (our
> rsnapshot/rsync-based backups work great for us at this time, sans the
> stomping of atime).
> 

Problem reports are always over-represented on the mailing lists.
People rarely write that e.g. ZFS snapshot has flawlessly worked for them for the
millionth time again today.  I am not aware of any known-but-not-fixed issues in
this area.  Each problem report should be properly investigated individually.

-- 
Andriy Gapon