[CentOS-virt] kvm guest from zfs dataset

George Dunlap

dunlapg at umich.edu
Mon Jan 19 13:00:02 UTC 2015

On Fri, Jan 16, 2015 at 12:47 AM, aurfalien <aurfalien at gmail.com> wrote:
> Hi all,
> I thought I’d post this in case any one has issues similar to mine.
> First, my initial email to the list which I didn’t send;
> I’m trying to run a KVM based guest OS off of a mirrored ZFS dataset.
> It won’t run with errors "invalid argument..", but will run when on the root
> volume, or when that ZFS dataset has been removed in favor of an EXT4
> volumes.
> Thanks in advance,
> PS I do see this thread which is not promising;
> http://www.redhat.com/archives/redhat-list/2014-July/msg00017.html
> Lastly, the solution;
> Set your cache policy on the KVM guest as write back.
> This allows it to run on a ZFS volume.
> My apology if this has been covered.  I did spend some time on this so
> hopefully you can save time by setting the cache policy.

The virt-manager / KVM angle is covered here:


It looks like virt-manager asks for "cached=none" by default, which
causes qemu to pass the "O_DIRECT" flag when opening the file, which
ZFS doesn't support.

Be advised that if you use your work-around, you may be risking
problems with disk corruption unless your OS understands how to use
flush commands; see cached=writeback here:



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