starting Fedora Server SIG

Les Mikesell lesmikesell at gmail.com
Thu Nov 20 14:05:58 UTC 2008


Matej Cepl wrote:
> On 2008-11-20, 07:00 GMT, Les Mikesell wrote:
>> Maybe - but it isn't a real solution.  You still have to deal 
>> with identifying the device before and during the labeling 
>> process.  If you can do that, you didn't really need the label.
> 
> Sorry, maybe I don't understand, but what's so difficult on the 
> following?
> 
> tune2fs -l /dev/sda1 |awk '/UUID/ {print $3}'
> 

The disk may be unformatted at this point and not have a UUID.  Or if it 
has filesystems, they may be types that don't support labels or UUID's 
(vfat, etc.).   But the point is that if you already knew it was 
/dev/sda1, why do you need to care about anything else?

There are at least two different use cases here.  One is that you have a 
single machine where you move a known small set of drives around to 
different positions (or they are usb/firewire, etc. where you don't have 
a concept of position) and you always want the same partition mounted in 
the same place regardless of where it ends up in the physical order.  I 
do have a machine like that and understand why the concept of labels or 
uuids is useful - but in that case I made all of the partitions into 
raid1 md devices (some mirrored, some with the 2nd member specified as 
'missing' which accomplishes the same thing and lets me plug in a disk 
with a matching partition and make a backup by adding it to the raid.

The other use case is where you have a large number of servers with 
swappable drives and regularly swap them around.  Scsi/sca backplanes 
are usually predictable as to drive ordering so if you physically insert 
a disk you automatically know the /dev/sd? name for it.  This may not be 
true for newer sas/sata controllers with raid logic that hides the 
physical drives behind 'volumes' even if you configure one drive per 
volume, though.  But in any case you need a way to discover and identify 
devices before and during the partitioning and filesystem creation 
steps, and labels can't help you then.

-- 
   Les Mikesell
    lesmikesell at gmail.com




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