Re: [Qemu-devel] [PATCH 1/1] blk: do not select PFLASH device for internal snapshot

["Denis V. Lunev" <den@openvz.org>]

On 01/12/2016 05:16 PM, Kevin Wolf wrote:
> Am 12.01.2016 um 07:03 hat Denis V. Lunev geschrieben:
>> There is a long-long story. OVMF VMs can not be snapsotted using
>> 'virsh snapshot' as they have "pflash" device which is configured as
>> "raw" image. There was a discussion in the past about that.
>>
>> Good description has been provided on topic by Laszlo Ersek, see below:
>>
>> "It is true that a pflash drive is "just a drive" *internally* to QEMU.
>> It is also true that it more or less takes the same -drive options as
>> any other *disk* drive. But those facts are just implementation details.
>>
>> The relevant trait of pflash storage files is that they are not *disk
>> images*, on the libvirt domain XML level. They are not created in
>> storage pools, you cannot specify their caching attributes, you don't
>> specify their guest-visible frontend in separation (like virtio-blk /
>>      virtio-scsi / pflash). Those details are hidden (on purpose).
>>
>> Consequently, pflash storage files are expected to be *small* in size
>> (in practice: identically sized to the varstore template they are
>> instantiated from). They are created under /var/lib/libvirt/qemu/nvram.
>> Although you can edit their path in the domain XML, they are not
>> considered disks."
>>
>> Thus we should avoid selection of "pflash" drives for VM state saving.
>>
>> For now "pflash" is read-write raw image as it configured by libvirt.
>> Thus there are no such images in the field and we could safely disable
>> ability to save state to those images inside QEMU.
> This is obviously broken. If you write to the pflash, then it needs to
> be snapshotted in order to keep a consistent state.
>
> If you want to avoid snapshotting the image, make it read-only and it
> will be skipped even today.
>
> Kevin
you interpret the patch a bit wrong.

It will be snapshoted once I'll raw image with qcow2 image, but this image
will not be selected for state saving, i.e. it will remain compact.

Den