3 compute nodes: A, B and C.

A exports its /var/lib/nova/instances over NFS v4.

B and C mount that at /var/lib/nova/instances.

(This is the kind of setup that I understand to be required for live migration to be possible.)

If I create a VM on A, wait for it to be ACTIVE, then `nova reboot --hard testvm1`, it's fine, i.e. it reboots and returns to ACTIVE state.

If I create a VM on B, wait for it to be ACTIVE, then `nova reboot --hard testvm2`, it goes permanently into ERROR state.  The Nova compute log has

Stderr: "qemu-img: Could not open '/var/lib/nova/instances/02bf258b-41bc-451a-9d3c-af6741c6060f/disk': Could not open '/var/lib/nova/instances/02bf258b-41bc-451a-9d3c-af6741c6060f/disk': Permission denied\n": nova.exception.InvalidDiskInfo: Disk info file is invalid: qemu-img failed to execute on /var/lib/nova/instances/02bf258b-41bc-451a-9d3c-af6741c6060f/disk : Unexpected error while running command.

and the ownership on that file is in fact now:

-rw-r----- 1 root root 29491200 Apr  7 18:27 disk

Before the reboot operation it was:

-rw-r----- 1 libvirt-qemu kvm  29491200 Apr  7 18:25 disk

There isn't a similar ownership change when rebooting a VM on node A, so presumably this is something to do with NFS being involved.  But I believe I've followed standard NFS setup advice, and my user and group IDs are the same on all compute nodes:

$ getent group nova
nova:x:1003:libvirt-qemu
$ getent passwd nova
nova:x:1002:1003::/home/nova:/bin/bash
$ getent passwd libvirt-qemu
libvirt-qemu:x:64055:109:Libvirt Qemu,,,:/var/lib/libvirt:/usr/sbin/nologin
$ getent group kvm
kvm:x:109:nova

Any clues on how to debug or fix this next?

Many thanks - Nell