8.1R possible zfs snapshot livelock?

[Luke Marsden]

Hi all,

On Tue, 2011-05-17 at 04:29 -0700, Jeremy Chadwick wrote:
> 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

As the original author of this post I wanted to chime in to say that our
problem was mis-diagnosed here as being related to snapshots and zfs
send/receive.  Instead, it was a bug [1] relating to force-unmounting a
ZFS filesystem which has active child nullfs mounts and active special
devices (FIFO).  There is a related kernel panic [1] which suggests that
this is a problem area.  I've been meaning to collect enough information
to submit a proper bug report -- I can at least reliably reproduce the
issue -- but have been rather too busy with the 1.0 release of our
application, and was put off by one response: "IMO this is expected".

[1] http://lists.freebsd.org/pipermail/freebsd-fs/2011-March/010983.html

Our application -- see HCFS at http://www.hybrid-cluster.com/tech/ --
makes very heavy use of ZFS snapshots and ZFS send/receive on FreeBSD
(currently 8.1), and since we engineered it so that it never attempts
foolish force-unmounts on "busy" filesystems we've seen no kernel hangs
over the course of hundreds of thousands of snapshot and zfs replication
events in testing.

I'm interested to know whether the OP's problem is fixed in 8.2 or
8-STABLE, since it could affect us.  Also, thanks for the links to the
backports for 8.2, Jeremy, I'll include those in our next system image.

-- 
Best Regards,
Luke Marsden
CTO, Hybrid Logic Ltd.

Web: http://www.hybrid-cluster.com/
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