Re: Any plan to support disk driver domain?

["Roger Pau Monné" <roger.pau@citrix.com>]

On 21/06/13 05:26, G.R. wrote:
> On Tue, May 28, 2013 at 6:16 PM, Roger Pau Monné <roger.pau@citrix.com> wrote:
>> On 04/04/13 12:26, G.R. wrote:
>>> Hi developers,
>>> I'm currently running a PVHVM freeNAS domU to serve storage (SATA
>>> controller passed-through) to all other domUs. However, some issues are
>>> observed in the network / NFS performance for the NAS domU.
>>> I noticed that there is a blkback driver in freebsd which is documented
>>> as capable to export disk to other domains, which looks very promising.
>>> However, the wiki says that disk driver domain is not supported (at
>>> least for now).
>>>
>>> I wonder if there is any plan to support such disk driver domain? It'll
>>> be a great feature for my use case.
>>
>> I've added a tutorial that explains how to use storage driver domains
>> with Xen 4.3, it contains a FreeBSD section that explains how to use ZFS
>> ZVOLS as disk backends for other domains, see:
>>
>> http://wiki.xen.org/wiki/Storage_driver_domains
>>
> 
> Hi Roger,
> 
> I just checked out your wiki page and it seems to be a fairly simple setup.
> It looks like a miracle that the freebsd version does not require
> those helper utils.
> 
> As I understand from what Daniel stated in his mail, the linux driver
> domain would
> rely on some hotplug scripts delivered with xen to hook up the connection.
> I was imagining some similar sciprts will need to be hooked into
> FreeBSD devd in similar manner.
> How does it work without these?

Linux use this hotplug scripts to write the physical-device node, which
FreeBSD doesn't need.

> I'm not familiar with the FreeBSD kernel.
> Also, does it has any assumption on the FreeBSD version? I'm on 8.3.1,
> which is a kind of old but still PVHVM capable.

If it contains blkback you should be fine.

> 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).