8.1R possible zfs snapshot livelock?

[Luke Marsden]

On Wed, 2011-05-18 at 14:05 +0200, Borja Marcos wrote: 
> On May 17, 2011, at 1:29 PM, Jeremy Chadwick wrote:
> 
> > * 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
> 
> I found a reproducible deadlock condition actually. If you keep some
> I/O activity on a dataset on which you are receiving a ZFS incremental
> snapshot at the same time, it can deadlock.
> 
> Imagine this situation: Two servers, A and B. A dataset on server A is
> replicated at regular intervals to B, so that you keep a reasonably up
> to date copy.
> 
> Something like:
> 
> (Runnning on server A):
> 
> zfs snapshot thepool/thedataset at thistime
> zfs send -Ri thepooll/thedataser at previoustime
> hepool/thedataset at thistime | ssh serverB zfs receive -d thepool
> 
> It works, but I suffered a deadlock when one of the periodic "daily"
> scripts was running. Doing some tests, I saw that ZFS  can deadlock if
> you do a zfs receive onto a dataset which has some read activity.
> Disabling atime didn't help either.
> 
> But if you make sure *not* to access the replicated dataset it works,
> I haven´t seen it failing otherwise. 
> 
> If  you wish to reproduce it, try creating a dataset for /usr/obj,
> running make buildworld on it, replicating at, say, 30 or 60 second
> intervals, and keep several scripts (or rsync) reading the target
> dataset files and just copying them to another place in the usual,
> "classic" way. (example: tar cf - . | ( cd /destination && tar xf -)
> 

Is there a PR for this?  I'd like to see it addressed, since read-only
I/O on a dataset which is being updated by `zfs recv` is an important
part of what we plan to do with ZFS on FreeBSD.

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

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