Get the block device from vm
root@overcloud-compute-01:~# virsh domblklist instance-00000001
Target Source
------------------------------------------------
vda compute/f0984ae9-f9c9-439b-80b9-f1c6201c914c_disk
Shutdown the instance from OSP
Map the disk using rbd command
root@overcloud-compute-01:~# rbd map compute/f0984ae9-f9c9-439b-80b9-f1c6201c914c_disk --name client.admin
Mount the disk
root@overcloud-compute-01:~# mount /dev/rbd/compute/f0984ae9-f9c9-439b-80b9-f1c6201c914c_disk-part1 /mnt
Do the changes you want.
Unmount the disk
root@overcloud-compute-01:~# umount /mnt
List rbdmaps
root@overcloud-compute-01:~# rbd showmapped
id pool image snap device
1 compute f0984ae9-f9c9-439b-80b9-f1c6201c914c_disk - /dev/rbd1
Unmap the disk
root@overcloud-compute-01:~# rbd unmap /dev/rbd1