Re: [Xen-devel] Xen guest disk online resize, xenstore/blkback/blkfront questions

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Jan 06, 2010 at 08:44:25PM +0000, Ian Campbell wrote:
> On Wed, 2010-01-06 at 20:26 +0000, Pasi Kärkkäinen wrote:
> [...] 
> > Is this more or less correct? Please correct if I've missed something or
> > understood something wrong.
> 
> All sounds vaguely familiar, although it's been a while since I last had
> to follow those twisty paths around...
> 
> > So, what I need to do next:
> > 
> > - LVM online-resize the guest disk LV in dom0.
> > 
> > - write something to xenstore block device backend structures, 
> >   to get the blkback driver notified about the 'block-resize'.
> > 
> >   Should I add a new xenbus_watch for some, say, 'resize' field, so I could 
> > get
> >   callback to blkback device_resize() easily when xenstore is updated? 
> > 
> > - blkback driver then needs to update/fetch the new size of the vbd, 
> >   and update the xenstore /local/domain/0/backend/vbd/X/sectors field.
> > 
> >   Any problems getting the new size/sectors on-the-fly in the kernel? 
> 
> You might need to close and reopen the block device? 
> 
> Idle speculation: perhaps instead of an explicit "resize" field in
> xenstore you could just have the backend continue watching the
> 'physical-device' node even after everything is connected the first time
> and if it is rewritten (including to the same value) reopening the
> physical device and setting things up again (picking up a size change as
> a side effect).
> 

Good idea. I'll experiment with this.

> There's no particular reason why the physical device couldn't change
> over this operation either, you could maybe imagine changing to a
> different device mapper node (e.g. perhaps some sort of wierd snapshot
> mechanism?) or perhaps implementing some sort of PV-CDROM media change
> in the same way.
> 
> > - blkback then needs to write something to xenstore block device frontend
> >   /local/domain/X/device/vbd/Y/ to notify the blkfront driver in the guest.
> > 
> >   Same thing here, should blkfront have a watch for some 'resize' field
> >   or so?
> 
> blkfront can probably just watch /local/domain/0/backend/vbd/X/sectors
> 

Sounds good. 
Thanks for the reply!

-- Pasi


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel