Re: [Qemu-block] Disconecting /dev/nbdX leaves stale partitions and device

[Eric Blake]

On 07/27/2018 10:07 AM, Richard W.M. Jones wrote:

> > Does it work also for running vms?
>
> I've not actually tried, but it's built into the qemu raw-format block
> driver (see qemu.git/block/raw-format.c) so it should work.  Note AIUI
> it'll only work for raw format drives, not qcow2 which is probably a
> critical drawback for oVirt.

Not necessarily. You can always layer BDS in qemu, so instead of the normal:

qcow2 -> file

or

raw -> file

you can instead request:

qcow2 -> raw -> file

from the command line and/or QMP, in order to interpret qcow2 format 
from a subset of the overall file with the raw layer doing the offset 
translation. Libvirt doesn't have enough hooks to declare such layering, 
though, so that may be a limitation.

> > Example use case:
> >
> > 1. Attach LUN or activate LV
> > 2. wipe start of the block device, or write some header
> > 3. Start guest with the block device with an offset, so guest see the disk
> > starting at offset.
> >
> > What about patthrough? can we attach a host device to a guest using an
> > offset?
>
> Passthrough likely not.  If qemu isn't processing each block of data
> then the qemu block driver wouldn't be able to change offsets.

If you are trying to do passthrough of a host device, can you convince 
the kernel to expose a subset of the host device (aka a partition) as 
needed?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org