[libvirt] [PATCH v1] qemu-migration: Disallow migration of read only disk

Jason J. Herne jjherne at linux.vnet.ibm.com
Wed Aug 17 21:10:53 UTC 2016

On 08/11/2016 08:57 AM, Corey S McQuay wrote:
> On 08/10/2016 09:16 AM, Koniszewski, Pawel wrote:
>>> -----Original Message-----
>>> From: libvir-list-bounces at redhat.com [mailto:libvir-list-
>>> bounces at redhat.com] On Behalf Of Corey S. McQuay
>>> Sent: Friday, August 5, 2016 8:34 PM
>>> To: jjherne at linux.vnet.ibm.com; libvir-list at redhat.com
>>> Cc: Corey S. McQuay <csmcquay at linux.vnet.ibm.com>
>>> Subject: [libvirt] [PATCH v1] qemu-migration: Disallow migration of
>>> read only
>>> disk
>>> From: "Corey S. McQuay" <csmcquay at linux.vnet.ibm.com>
>>> Currently Libvirt allows attempts to migrate read only disks. Qemu
>>> cannot
>>> handle this as read only disks cannot be written to on the
>>> destination system.
>>> The end result is a cryptic error message and a failed migration.
>>> This patch causes migration to fail earlier and provides a meaningful
>>> error
>>> message stating that migrating read only disks is not supported.
>> What will happen if read-only disk is copied to destination prior to
>> migration start? Currently such scenario works, will it still work
>> with this code?
> Based on our testing, pre-copying a read only disk image to the
> destination system has no effect on the outcome of attempting to migrate
> a non-shared read only disk. I'm not sure what scenario you are
> referring to but here is what we tried:
> Relevant guest xml:
>      <disk type='file' device='cdrom'>
>        <driver name='qemu' type='raw' cache='writethrough'/>
>        <source file='/disk-images/guest.iso'/>
>        <backingStore/>
>        <target dev='sdz' bus='scsi'/>
>        <readonly/>
>        <alias name='scsi0-0-0-0'/>
>        <address type='drive' controller='0' bus='0' target='0' unit='0'/>
>      </disk>
> The disk image exists at /disk-images/guest.iso on the source. Before
> migration we copied the image to the same path on the destination
> system. Then we attempted migration:
>      virsh migrate --live --copy-storage-all --migrate-disks sdz
> --verbose kvm1 qemu+ssh://dstHost/system tcp://dstHost
> The error message we get is:
> error: internal error: info migration reply was missing return status
> Running journalctl shows additional information:
> Aug 10 16:02:16 collin-kvm libvirtd[41616]: operation failed: migration
> of disk sdz failed.
> I'm pretty sure this patch does not stop the user from doing anything
> that works today. But if your scenario is different from ours in some
> way please let us know and we'll do some more testing.


Thanks for taking a look. Does Corey's reply address your concerns?

-- Jason J. Herne (jjherne at linux.vnet.ibm.com)

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