Comment 0 for bug 2023078

Nova incorrectly sets the libvirt XML after an online volume migration when the source is a backend that doesn’t support discard (Cinder doesn't return `discard: true` in the connection dictionary) to one that does.

It seem like Nova doesn't rebuild the disk XML, so it's missing the discard=unmap that should have for the new volume.

This bug results in the trimming/unmapping commands not working on the new volume until the next time Nova connects the volume.

For example an instance reboot will not be enough, but a shelve and unshelve will do the trick and fstrim will work again.