Create VM w/ cache=none on tmpfs

[Miguel Duarte de Mora Barroso]

On Fri, Mar 27, 2020 at 1:12 PM Daniel P. Berrangé <berrange at redhat.com> wrote:
>
> On Fri, Mar 27, 2020 at 12:31:07PM +0100, Miguel Duarte de Mora Barroso wrote:
> > Hi,
> >
> > I've seen that in the past, libvirt couldn't start VMs when the disk
> > image was stored on a file system that doesn't support direct I/O
> > having the 'cache=none' configuration [0].
> >
> > On the KubeVirt project, we have some storage tests on a particular
> > provider which does just that - try to create / start a VM whose disk
> > is on tmpfs and whose definition features 'cache=none'.
> >
> > The behavior we're seeing is that libvirt throws this warning:
> > ```
> > Unexpected Warning event received:
> > testvmig4zsxc2f8swkxv22xkhx2vrb4ppqbfdfgfgqh5gq8plqzrv5,853ff3d9-70d4-43c5-b9ff-4d5815ea557d:
> > server error. command SyncVMI failed: "LibvirtError(Code=1, Domain=10,
> > Message='internal error: process exited while connecting to monitor:
> > 2020-03-25T10:09:21.656238Z qemu-kvm: -drive
> > file=/var/run/kubevirt-ephemeral-disks/disk-data/disk0/disk.qcow2,format=qcow2,if=none,id=drive-ua-disk0,cache=none:
> > file system may not support O_DIRECT\n2020-03-25T10:09:21.656391Z
> > qemu-kvm: -drive
> > file=/var/run/kubevirt-ephemeral-disks/disk-data/disk0/disk.qcow2,format=qcow2,if=none,id=drive-ua-disk0,cache=none:
> > Could not open backing file: Could not open
> > '/var/run/kubevirt-private/vmi-disks/disk0/disk.img': Invalid
> > argument')"
> > ```
> >
> > But actually proceeds, and is able to start the VM - but seems it
> > coerces the cache value to writeThrough.
>
> Are you sure it is really continuing to start the VM - those log messages
> strongly suggest that is not the case.

Actually no, you're right; it fails, and does not proceed; kubevirt
itself re-queues the request, this time without specifying that
attribute (I have yet to figure out why / where).

Two different requests are clearly seen in the libvirt logs in the pod
that encapsulates libvirt.

Thanks for the reply.

>
>    "internal error: process exited while connecting to monitor:"
>
> means libvirt lost its connection to the QMP monitor, which means QEMU
> has shutdown.
>
> Similarly the error message about being unable to open the disk is
> something that QEMU treats as a fatal error AFAIK.
>
> > Is this the expected behavior ? e.g. cache = none can't be used when
> > the disk images are on a tmpfs file system ? I know it was, not sure
> > about now (libvirt-5.6.0-7) ...
>
> Yes, cache=none is incompatible with tmpfs, so you'll need to pick
> a different cache setting.


>
> Regards,
> Daniel
> --
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