Re: [Xen-devel] Any plan to support disk driver domain?

["G.R." <firemeteor@xxxxxxxxxxxxxxxxxxxxx>]

On Thu, Jul 25, 2013 at 11:35 PM, G.R. <firemeteor@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>> I believe I should be able to serve HVM domU in this way (using dom0
>>> as a proxy).
>>> But this appears to introduce some overhead (dom0 proxy).
>>> I wonder if it will be hard to provide HVM support for disk domain?
>>
>> This is not possible due to the fact than when using Qemu the Qemu
>> process in Dom0 needs access to the disk you are attaching to the guest.
>> For PVHVM guests this is not a big deal, because the emulated device
>> will only be used for the bootloader, and then the OS switches to PV,
>> which doesn't use Dom0 as a proxy.
>>
>> The only improvement here would be to auto attach the disk to Dom0 in
>> order to launch Qemu, which now has to be done manually. The same
>> happens with PV domains that use pygrub.
>
> Thanks for your comment, Roger.
> But I'm not sure how to specify the domain config in this case.
> Previously the dual-interface is handled by the toolchain, generated
> from the same 'disk' config entry.
> In this case, the qemu will expect config in the form of
> vdev=hda,target=/dev/xvda, while the pv-driver should use the config
> in the form of vdev=xvda,domain=<driver>,target=/path/in/driver/domain
> How does the toolchain know that these two are actually the same disk?
>
> Also, will this be another "attach twice" case? Any potential
> consistency issue? (Since this bypasses NFS that deals with
> consistency)

Hi Roger,
Could you comment on my questions above? I would be nice if you could
provide an example config to make the dual-interface work.
For the consistency issue, I also care about the interoperability with
NFS -- Can I share the same disk image through NFS while having it
mounted through blk-backend?

Thanks,
Timothy

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