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Re: [libvirt] [PATCH 0/5]Add startupPolicy attribute support for hard disks



On Tue, Mar 19, 2013 at 03:56:40PM +0800, Guannan Ren wrote:
> On 03/18/2013 10:42 PM, Daniel P. Berrange wrote:
> >On Mon, Mar 18, 2013 at 05:10:17PM +0800, Guannan Ren wrote:
> >>The set of patches is trying to add 'startupPolicy' attribute support
> >>to the source element of hard disks. Policy levels are using the
> >>mandatory, requisite, optional levels as originally documented.
> >>
> >>For the 'optional' policy, there is a little difference from CDROM and
> >>Floppy which only drop its source path, for disks, if missing, the
> >>checking function will drop their definitions, because qemu doesn't
> >>allow missing source path for hard disk.
> >What is the motivation for this feature ? I personally find even the
> >existing CDROM code for this to be of rather dubious value, so would
> >like to see a clear use case for why we need to expand this hack.
> >
> >Daniel
> 
>    There is a real case from a libvirt customer request, which needs
>    to make disk of block type use "optional" policy.
> 
>    "The DR concept for our virtualization infrastructure is based upon
>     host-based mirroring of independent SAN LUNs.
>     Because RH does not support CLVM spanning multiple data centers
>     we have to pass the LUNs to the KVM and mirror inside of the KVM.
>     If there is an outage taking down a storage box or an entire DC,
>     we will be unable to migrate, relocate or start any KVM,
>     despite the KVM being able to run just on one side of the mirror."
> 
>     "Declare all devices as optional. If there aren't any mirror
> parts left,
>      the KVM will fail, but that is an acceptable behaviour that will be
>      recognized by our monitoring software. There is no need to have
>      libvirt guard against failure of storage devices in such a setup. "

Ok, so they are doing multi-pathing inside the guest, and so don't care
if some of the LUNs on the host are not available when started. This
makes more sense now as a feature.

Regards,
Daniel
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