VirtualBox, AIO and zvol's - a cautionary tale

[Pete French]

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.

Hope someone else finds the info usefull!

-pete.