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xen-devel
Re: [Xen-devel] Xen guest disk online resize, xenstore/blkback/blkfront
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).
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
Ian.
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