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Date:      Fri, 25 May 2012 15:25:09 +1000
From:      Jan Mikkelsen <janm@transactionware.com>
To:        Pete French <petefrench@ingresso.co.uk>
Cc:        freebsd-stable@freebsd.org
Subject:   Re: VirtualBox, AIO and zvol's - a cautionary tale
Message-ID:  <F1C457B5-0CFF-4286-95E0-C9871CA8021A@transactionware.com>
In-Reply-To: <E1SXBLw-000AZw-Ee@dilbert.ingresso.co.uk>
References:  <E1SXBLw-000AZw-Ee@dilbert.ingresso.co.uk>

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Hi,

On 23/05/2012, at 11:11 PM, Pete French wrote:

> Am posting this to stable not really as a question, but more in case =
anyone
> else hits the same problem. Last patch tuesday one of my virtual =
Windows
> machines  running under VirtualBox started crashing. By which I mean
> that VirtualBox would quit. This had been running tsably for a long
> tine, so it puzzled me.
>=20
> First thought was it was sme patch from patch-tuesday. But rolling =
back
> to an earlier version of the disc showed it wasn't - the crashes were
> occurring before the patch had been applied.
>=20
> I'll skip the hours of puzzlement which followed - it turrned out that
> the indirect cause was that a few weeks ago I had installed Samba
> onto the same server. In doing so I had enabled AIO, as this improves
> Samba performance.
>=20
> What I didn't realise is that if VirtualBox finds AIO loaded it =
proceeds
> to use it.  So by doing that I had switched on AIO inside my virtual
> machines as well. The disc I use for my virtual machines are all zvols =
(it
> performs better, and it seems that VirtualBox has a problem using AIO
> to access zvols.
>=20
> But this didn;t show up for weeks because in the normal scheme of =
things
> my virtual machines dont acccess the local dirve very much. It was =
only
> when they started downloading patches that the crash happened.
>=20
> Solution is simple - disable AIO. All then goes back to being nice
> and stable again. But it did take a while to find.

I have seen similar behaviour, but I did not disable AIO to solve it. =
Instead, in the VirtualBox VM, I made sure that the storage controller =
was created with the "--hostiocache on" option. Without that, the =
virtual machines were unreliable on ZFS with the same behaviour you saw.

Do you have the hostiocache enabled or disabled in your VM? Does it make =
a difference?

Regards,

Jan.




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