Create VM w/ cache=none on tmpfs

Fri Mar 27 12:12:31 UTC 2020

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.

   "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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