Re: [Qemu-devel] [PATCH v5 0/5] Geometry and blocksize detection for backing devices.

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Dec 18, 2014 at 04:59:50PM +0100, Christian Borntraeger wrote:
> Are you ok with the patches? If yes, can you take care of these patches in the block tree?

This series looks close, I've left comments on the patches.

The series is fine for command-line QEMU users where probing makes the
command-line more convenient, so we can merge it.  But the approach is
fundamentally wrong for stacks where libvirt is in use.

Libvirt is unaware of the guest geometry and block sizes that are probed
in QEMU by this patch series.  This breaks non-shared storage migration
and also means libvirt-based tools that manipulate drives on a guest may
inadvertently change the guest-visible geometry and cause disk problems.

For example, what happens when you copy the disk image off a host DASD
and onto NFS?  QEMU no longer probes the geometry and the disk geometry
has changed.

The right place to tackle guest-visible geometry is in libvirt, not in
QEMU, because it is guest state the needs to be captured in domain XML
so that migration and tooling can preserve it when manipulating guests.

Stefan

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