On Thu, Jul 05, 2012 at 03:00:26PM +0100, Daniel P. Berrange wrote: > Now, when using 'nova volume-attach': > > # nova volume-attach 05eb16df-03b8-451b-85c1-b838a8757736 > a5ad1d37-aed0-4bf6-8c6e-c28543cd38ac /dev/sdf > > nova will import an iSCSI LUN from the nova volume service, on the compute > node. The kernel will assign it the next free SCSI drive letter, in my > case '/dev/sdc'. > > The libvirt nova driver will then do a mknod, using the volume name > passed to 'nova volume-attach'. > eg it will do > > mknod /var/lib/nova/instances/instance-0000000e/rootfs/dev/sdf
Opps, I'm slightly wrong here. What it actually does is mount --bind /dev/sdc /var/lib/nova/instances/instance-0000000e/rootfs/dev/sdf so you get a 'sdf' device, but with the major/minor number of the 'sdc' device. I can't say I particularly like this approach. Ultimately I think we need the kernel support to make this work correctly. In any case, even using mount --bind, doesn't deal with the fact that the guest' /dev is not visible from the host > this is where it has all gone horribly wrong... > > * The iSCSI LUN is completely randomly allocated, and unrelated to the > block device name the user will give to 'nova volume-attach'. So there > is no association between the /dev/sdf in the container and the > /dev/sdc in the host, and you can't expect the caller of 'volume-attach' > to be able to predict what the next assigned LUN will be on the host. > > * The /var/lib/nova/instances/instance-0000000e/rootfs/dev/ directory > where nova did the mknod is a completely different filesystem to > the one seen by the container. The /dev in the container is a tmpfs > that is never visible to the host, so a mknod in the host won't > appear to the container. > > AFAIK, there is no way to resolve either of these problems given the > current level kernel support for LXC, which is why libvirt has never > implemented block volume attach itself. > > Thus I'm wondering how this LXC volume-attach code in Nova has ever > worked, or was tested ? My testing of Nova shows no sign of it working > today. Unless someone can demonstrate a flaw in my logic, I'm inclined > to simply revert this whole commit from Nova. > Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| _______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : [email protected] Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp
