[Xen-users] Expanding a virtual block device

[Vivien Bernet-Rollande <vivien.bernet-rollande@xxxxxxxxxxxxxxxxx>]

Hi list.

I'm running Xen 4.0.1 on Debian.

I'm trying to live resize a block device :

 - I have a arbitrary block device in my dom0 (/dev/mapper/vm-vol42).
 - I start a PV domain with that device as a disk (disk = 
['phy:/dev/mapper/vm-vol42,xvda,w']).
 - I resize vm-vol42 in the dom0 (adding size only)
 - the dom0 sees the device's new size.
 - the domU still has a disk of the old size.
 - I want the domU to see the new disk with it's actual size, so it can 
resize partitions and filesystems on said disk.
From my research I have gathered :
 - on XenServer or XCP, there is a "xe vdi-resize" command. But since 
I'm on debian, no "xe" for me, and I'm not even sure it works online.
 - Browsing in the XenApi code showed an interesting method, 
VBD_media_change. However, it seems to allow only for changing read-only 
CD images. However, Xxm does not look like it uses it.
 - I cannot pause the domU, remove the device, add it back and unpause 
the domain (the block-dettach operation hangs on a paused domain). I can 
do it quite fast without unpausing, but it's not fully transparent, and 
can't be done on the root device.
 - I cannot issue a scsi rescan in dom0 since the xvd devices don't go 
through the scsi subsystem (and I couldn't find anything relevant in 
/sys/devices/vbda nor /sys/classes/block/xvda (apart from a read only 
"size" file).
I know I could just use LVM in the domU, create and attach a new block 
device, and grow my vg and lv in the domU. However this means managing 
more volumes, and I'd rather manage a 50GB volume than 5 10GB volumes.
So, the end question : how can I notify Xen that the device's size has 
changed, so the VBD's size in the domU will also increase ?
--
Vivien Bernet-Rollande
Systems&  Networking Engineer
Alter Way Hosting


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