VirtualBox, AIO and zvol's - a cautionary tale

[Jan Mikkelsen]

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.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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.