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Date:      Wed, 13 Oct 2010 13:08:56 +0200
From:      Borja Marcos <borjam@sarenet.es>
To:        Willem Jan Withagen <wjw@digiware.nl>
Cc:        fs@freebsd.org
Subject:   Re: ZFS freeze/livelock
Message-ID:  <98AF4752-7881-4C50-8A59-243F1AD55318@sarenet.es>
In-Reply-To: <4CB1DD0F.6000209@digiware.nl>
References:  <4CB1DD0F.6000209@digiware.nl>

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On Oct 10, 2010, at 5:34 PM, Willem Jan Withagen wrote:

> Hi,
>=20
> Just had my FreeBSD freeze on me with what I would think is sort of an =
livelock....
>=20
> While I was receiving zfs snapshots on my data pool.
>=20
> Top and systat just kept running,
> but anything getting near a shell (and perhaps disk-io) ended up in:
>=20
> root@zfs.digiware.nl# gpart create -s gpt da6
> load: 0.00  cmd: csh 12393 [zfsvfs->z_teardown_inactive_lock] 26.12r =
0.00u 0.00s 0% 2480k
> load: 0.10  cmd: csh 12393 [zfsvfs->z_teardown_inactive_lock] 96.01r =
0.00u 0.00s 0% 2480k
>=20
> Trying to execute to execute shutdown -r now had no effect what so =
ever.
> Neither did the three-finger salute.
> (Well at least not in 60 sec I was willing to wait.)
>=20
> Only way out of this situation was hard-reset. And I do have to admit =
I like ZFS for the speed it recovers after unexpected reboot.
>=20
> To bad there was no alt-ctrl-backspace escape to debugger compiled in. =
I'll do that with the next kernel, just in case.


There is an (as far as I know) unsolved deadlock situation when =
receiving a snapshot while you read the target dataset.

I found it  in a redundant server configuration. I replicate some =
datasets periodically doing an incremental send-receive. It works =
perfectly but it can deadlock if I have a process reading the =
destination dataset on the secondary server. And those things can happen =
if you have, for example, one of the nightly periodic tasks running.=20

Were you doing a siimilar thing? Or are you sure there was no reading =
activity on the destination dataset?





Borja.




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