[Xen-users] Issues with Adding RBD Block Devices

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

I'm having some problems in Xen trying to create a new block script for rbd 
devices (Ceph RBD) and using those devices in domUs.  I've attached the 
block-rbd script that I've created and placed in /etc/xen/scripts.  Basically, 
this should allow me to specify the following in my config file:
disk = [ 'rbd:image-name,hda,w' ]

...where image-name is the name of an rbd image.  The script uses Ceph's "rbd" 
command to map the image to a device (/dev/rbdN where N is a number) and then 
feeds that to the xenstore commands necessary for accessing the device.  
Everything seems to work fine - script runs, devices are mapped, and xenstore 
gets updated - but I cannot see the disk in my HVM domU.  The only clue as to 
what might be wrong is the following entry in my 
/var/log/xen/qemu-dm-<domU>.log file:

qemu: type (image format) 'rbd' unknown for vbd 
'/local/domain/0/backend/vbd/29/768/mode' or image 'image-name'

There are a couple of puzzling things about this.  First, the domU is getting 
passed a block device in the form of /dev/rbd1, which it should be able to 
access, so why does it care from the type/image format is?  The block device is 
no different than a phy: device with a physical HDD or an LVM volume, and, 
while the setting for "type" is rbd, I don't know why that would impact it.  
Second, why is it referencing the "mode" node of the xenstore tree for that 
backend device.  That really doesn't make any sense - the vbd should be one 
level above that, and the mode is definitely set to "w" for read-write 
(verified with with xenstore-ls).

Has anyone else used Ceph RBD to back Xen-based VMs?  Or has anyone else 
successfully added another type of block device to Xen with a script?  Any 
hints on where to go from here?

Thanks - Nick



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