[libvirt] Quorum block driver libvirt support proposal
Daniel P. Berrange
berrange at redhat.com
Fri May 16 13:54:43 UTC 2014
On Fri, May 16, 2014 at 12:33:04PM +0200, Benoît Canet wrote:
>
> Hello list,
>
> I want to implement libvirt Quorum support.
> (https://github.com/qemu/qemu/commit/c88a1de51ab2f26a9a37ffc317249736de8c015c)
> Quorum is a QEMU RAID like block storage driver.
> Data are written on n replicas and when a read is done a comparison between the
> replica read is done. If more than threshold reads are identical the read succeed
> else it's and error.
>
> For example a Quorum with n = 3 and threshold = 2 would be made of three QCOW2
> backing chains used as identicals replicas. threshold = 2 means that at least
> 2 replica must be identical when doing a read.
>
> I want to make use of the new backingStore xml element to implement quorum.
>
> Proposed Quorum libvirt format:
> -------------------------------
>
> <disk type='quorum' device='disk'>
> <driver name='qemu' type='quorum'/>
> <threshold value=2/>
> <backingStore type='file'>
> <format type='qcow2'/>
> <source file='/var/lib/libvirt/images/file1.qcow2'/>
> </backingStore>
> <backingStore type='file'>
> <format type='qcow2'/>
> <source file='/var/lib/libvirt/images/file2.qcow2'/>
> </backingStore>
> <backingStore type='file'>
> <format type='qcow2'/>
> <source file='/var/lib/libvirt/images/file3.qcow2'/>
> </backingStore>
> <target dev='vda' bus='virtio'/>
> </disk>
It feels rather odd to have <backingStore> elements but no top level
disk images. Really these are all top level images
Regards,
Daniel
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