libvirt users - Mar 2022 - Resize backing store

Apologies in advance if this be a stupid question, as everything I've 
read seems to suggest it's impossible.

Slowly running out of space on a 200GB root partition.

Is it possible to create a snapshot - leaving the QCOW2 image 
'quiescent' - resize it using 'qemu-img resize' - then pivot the
snapshot back? (Then shutdown, attach the storage to another bootable 
OS, and run fdisk and resize2fs.)

Everything I've read says to shutdown the VM; but besides the real 
possibility of the snapshot blockcommit being corrupted when pivoted 
into the newly resized backing store, I don't see why it shouldn't work.
And save some significant downtime.

Please can someone advise if this is actually possible, or if I'm 
wasting my time?

Thanks!

-- 
Jeff Brown
Future Foundation
Cell 074 101 5170
VoIP 087 550 1210
Fax: 086 532 3508

On Tue, Mar 08, 2022 at 06:17:32 +0200, Jeff Brown
wrote:
> Apologies in advance if this be a stupid question, as everything I've
read
> seems to suggest it's impossible.
> 
> Slowly running out of space on a 200GB root partition.
> 
> Is it possible to create a snapshot - leaving the QCOW2 image
'quiescent' -
> resize it using 'qemu-img resize' - then pivot the snapshot back?
(Then
> shutdown, attach the storage to another bootable OS, and run fdisk and
> resize2fs.)
> 
> Everything I've read says to shutdown the VM; but besides the real
> possibility of the snapshot blockcommit being corrupted when pivoted into
> the newly resized backing store, I don't see why it shouldn't work.
And save
> some significant downtime.
> 
> Please can someone advise if this is actually possible, or if I'm
wasting my
> time?
We actually also have 'virsh blockresize' which allows to modify the
size of the disk while the VM is running.

There are some caveats, e.g. certain disk bus and operating system
combinations will not notice that the size increased, but in most common
cases it actually works while the VM is running. The workaround if that
happens is to just reboot the guest.

The common approach is to:

virsh blockresize $VM $DISK $NEWSIZE (look into the manual for how size is
treated)

in the vm then:

1) increase the size of the partition
2) resize LVM (physical volume) if used
3) resize filesystem using resize2fs or similar