On Tue, 30 Jul 2019 11:35:45 +0100 Daniel P. Berrangé <berrange@redhat.com> wrote:
On Mon, Jul 29, 2019 at 02:04:14PM +0200, Stephan von Krawczynski wrote:
Hello,
is there some immanent code in libvirt that forces UID/GID of the libvirt standard user to be the same on two boxes migrating qemu vms against each other? The migration itself uses root obviously (password is requested). But if a vm xml does not contain any definition regarding UID/GID what else could prevent this from working?
I believe I ran into such a problem trying to migrate and ending up in an error, a vm still working on original host but its fs (netfs pool (nfs/raw)) being switched to read-only...
When migrating a VM whose image is hosted on NFS, you have 2 QEMU processes which both need to be able to open the same image file at the same time. QEMU runs as an unprivileged user normally, and so the disk images get chowned to this unprivileged user by libvirt when QEMU is started. If the QEMU on the target host is given a UID/GID that's different from the QEMU on the source host, then the target QEMU will likely have problems opening the image.
Basically when using shared FS storage, the rule is to have all your hosts configured in the same way from libvirt's POV.
Regards, Daniel
Hello Daniel, thank you for the short explanation. The key words are "both need to be able to open the same image file at the same time". I would not have expected that. I thought qemu 1 will close and exit, and then qemu 2 will open the image, which means he can change the uid/gid right before - just as in normal operation. Is this the reason why my failing try leaves me with a read-only fs on the guest? Which I would see as a bug, not? Turning it read-only is possibly the only way to not corrupt the fs image if two qemus have it open simultaneously. -- Regards, Stephan