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Re: [Qemu-block] [PATCH 1/1] blk: do not select PFLASH device for intern

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH 1/1] blk: do not select PFLASH device for internal snapshot
Date: Tue, 12 Jan 2016 15:16:07 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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