uniquely identifying drives in virt configurations

[Bill McGonigle]

Hi, all,

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.

Thanks,
-Bill

-- 
Bill McGonigle, Owner
BFC Computing, LLC
http://bfccomputing.com/
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[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
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://et.redhat.com/~rjones/virt-df/

[Daniel P. Berrange]

On Wed, Feb 03, 2010 at 07:54:05PM -0500, Bill McGonigle wrote:
>  Hi, all,
>  
>  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 suspect you are looking for /dev/disk/by-id whcih gives you a stable
links based on model/serial number details

$ ls -l /dev/disk/by-id/
total 0
lrwxrwxrwx. 1 root root  9 2010-02-03 18:04 ata-HTS721010G9SA00_MPCZ12Y0GNGWSE ->
../../sda
lrwxrwxrwx. 1 root root 10 2010-02-03 18:04 ata-HTS721010G9SA00_MPCZ12Y0GNGWSE-part1 ->
../../sda1
lrwxrwxrwx. 1 root root 10 2010-02-03 18:04 ata-HTS721010G9SA00_MPCZ12Y0GNGWSE-part2 ->
../../sda2
lrwxrwxrwx. 1 root root  9 2010-02-03 18:04 scsi-SATA_HTS721010G9SA00_MPCZ12Y0GNGWSE ->
../../sda
lrwxrwxrwx. 1 root root 10 2010-02-03 18:04 scsi-SATA_HTS721010G9SA00_MPCZ12Y0GNGWSE-part1
-> ../../sda1
lrwxrwxrwx. 1 root root 10 2010-02-03 18:04 scsi-SATA_HTS721010G9SA00_MPCZ12Y0GNGWSE-part2
-> ../../sda2


Daniel
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[Bill McGonigle]

On 02/04/2010 05:23 AM, Daniel P. Berrange wrote:
>  I suspect you are looking for /dev/disk/by-id which gives you a
> stable
>  links based on model/serial number details 
Yes, thank you, I was only looking around in /sys.  /dev/disk/by-id is 
perfect!

Thanks, Daniel (and the sender of the private reply I received).

-Bill

-- 
Bill McGonigle, Owner
BFC Computing, LLC
http://bfccomputing.com/
Telephone: [phone]
Email, IM, VOIP: bill(a)bfccomputing.com
VCard: http://bfccomputing.com/vcard/bill.vcf
Social networks: bill_mcgonigle/bill.mcgonigle