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

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

>
>> In your example, the driver is setup using zvol. I wonder if there are
>> any constraint prohibiting using a file based backend?
>
> I have not tried it, but FreeBSD blkback should be able to handle raw
> files also.
>
>> Finally, I saw this limitation in the wiki:
>>>> It is not possible to use driver domains with pygrub or HVM guests yet, so 
>>>> it will only work with PV guests that have the kernel in Dom0.
>> While I can imagine why pygrub does not work, I don't understand the
>> reason HVM is affected here. Could you explain a little bit?
>> And what about a HVM with PV driver? (e.g. those windows guests)
>
> If you use HVM, Qemu needs to access the block/file used as disk, so if
> the disk is on another domain Qemu has no way to access it (unless you
> plug the disk to the Dom0 and then pass the created block device
> /dev/xvd* to Qemu).
>

I just upgrade to xen 4.3 and here is my quick report.

With some preliminary test, I can confirm that freebsd 8.3 is able to
serve as disk backend, exporting file as a disk.
I was able to block-attach it and mount it in dom0.
However, there are some issues with the xl block-list / block-detach command.
The attached disk cannot be listed. And block-detach will report fail
when I try to get it removed.
However, it appears to have removed the data in xenstored in spite of
the remove error report.
And I was able to re-attach the same file again later.

There is also an issue about block-attach -- it does not prevent me
attaching the same file twice accidentally.
This appears to crash the whole system later when I tried some further
operations.

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?
Will it be able to provide dual-interface like hda/xvda so the
overhead is eliminated after PV driver is loaded?

It will be great if you can share your plan / schedule about further
enhancement of this feature.

Thanks,
Timothy

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