ZFS zvol support improved - VirtualBox gets unstable with it

[Adam Vande More]

On Wed, Aug 13, 2014 at 7:16 AM, Volodymyr Kostyrko <c.kworr at gmail.com>
wrote:

> Hi all.
>
> There was a long time ago when I started to use pure zvol's as VirtualBox
> disks. This way virtual machines were working faster due to default smaller
> cluster size and I also can snapshot them and mount right from the OS. Too
> bad this also required setting 'use host i/o cache' at storage controller
> properties. Without that most guest OS would just break or hang.


I've been using ZVOL's as Vbox disks for maybe @ 3 years including some
exported as ISCSI.  The break/hang you describe is totally unfamiliar to me
even though my guests include various Linux distros, different
versions/types of BSD, and several flavors of Windows.  These included a
mixture of i386/amd64 guests.  I do not set "use host i/o cache" on any
systems(not all have ZVOL snapshots but most do).


>
> Recently 10-STABLE was shown some improvements:
>
> 1. zfs set volmode=dev <zvol>
>
> Now you can mark zvol as a dev node preventing system from touching it.
> Also this makes working with zvol simpler.
>
> 2. r269429 (MFC r269123): Implement separate I/O dispatch method for ZVOLs
> in "dev" mode.
>
> After this one if you have zvol's marked as volmode=dev you can turn off
> useHostIOCache for your virtual machines. And they would also work much
> faster! But alas after that VirtualBox becomes unstable and your machines
> can also fail on heavy IO.
>

1 & 2 look very interesting.  If that makes it into 10.1, I'll be giving it
a whirl.


-- 
Adam