[fedora-virt] uniquely identifying drives in virt configurations

[Richard W.M. Jones]

On Wed, Feb 03, 2010 at 07:54:05PM -0500, Bill McGonigle wrote:
> I'm wondering what folks are doing for uniquely identifying plain disks 
> in their virtual machine configs.
> 
> For instance, I'm trying out the new Xen version and stuck a couple SATA 
> drives in a box.  I'd like to use them as physical devices, but not 
> break my vm config if I re-cable the drives (or move them to another 
> machine).
> 
> I could create a degraded RAID mirror, and then reference them by mdX 
> and let md worry about the UUID's, but that's ugly.  If I were using 
> luks on the drives they'd get tagged and could be referenced uniquely. 
> Same with iscsi, those are available uniquely under /sys.
> 
> Maybe something with device mapper?  But I'm actually interested in 
> benchmarking ZFS through Xen with this, so I'd hate to add another layer 
> just for this purpose, since it's only needed at setup time.  I could 
> script some ugly hacks but I'd rather not.
> 
> I was hoping to find a /sys/block/scsi/model/serialnumber type of thing 
> symlinked back to the ../../devices/pci... , but I didn't.  So, I hope 
> I'm missing something obvious and folks here have this solved.

I wonder if you'd be better off uniquely identifying the filesystems
instead of the devices.  Filesystems have a unique ID:

# tune2fs -l /dev/sda1 | grep UUID
Filesystem UUID:          868b1447-0ec5-41bf-a2e5-6a77a4c9b66d

and you can use this to mount the filesystem, eg in /etc/fstab:

UUID=868b1447-0ec5-41bf-a2e5-6a77a4c9b66d /boot ext3 defaults  1 2

This is well tested too -- all Fedora installs use UUIDs like this.

This will survive physical rearrangement of the disks containing the
filesystem, and even work if you decide to turn the filesystem into a
virtual disk.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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