system down - can't find luns (correction)

[Ed Wilts]

On Mon, May 23, 2005 at 11:02:36AM -0500, Rigler, Stephen C. wrote:
> On Mon, 2005-05-23 at 10:53 -0500, Dana Holland wrote:
> > Rigler, Stephen C. wrote:
> > > First, try doing "mount /dev/sdb1 /usr/local/bbls".
> > > If that works, then unmount and try doing "mount /usr/local/bbls".
> > > 
> > > If the second try doesn't work, then make sure your filesystem has the
> > > label specified in the fstab ("e2label /dev/sdb1").  If it doesn't have
> > > the label, then you can relabel it by doing
> > > "e2label /dev/sdb1 /dev/sdb1".  Repeat for /dev/sdc1.
> > 
> > mount /dev/sdb1 /usr/local/bbls works - mount /usr/local/bbls doesn't work
> > 
> > I guess I'm being dense, but I'm not following what you and Ed are 
> > telling me about the label in /etc/fstab - here is what I have:
> > 
> > LABEL=/dev/sdb1      /usr/local/bbls         ext2    defaults        1 2
> > LABEL=/dev/sdc1      /usr/local/bbcm         ext2    defaults        1 2
> > 
> > What is it that's incorrect?
> 
> The entries in your fstab are causing the system to mount filesystems by
> label rather than by the device file.  The label makes things convenient
> because, if things devices change their mappings, you can still mount
> the filesystem because you know what it's label is.  The labels can be a
> pain for the same reason (like if you want to change system disks).

For SCSI devices, it's usually considered mandatory to label the drives
- that's normally true for non-SAN drives but probably not as critical
for SAN storage.

What happens by default is that the first SCSI drive will be /dev/sda1.
The second drive will be /dev/sdb1.  This may work fine for years, but
imagine what happens if /dev/sda1 fails on boot.  Suddenly all of your
drives got renumbered and what used to be /dev/sdb1 is now /dev/sda1.  

The purpose of the labels is to ensure that this doesn't happen - the
label can be anything and does not have to match the device name.  In
your case, a more useful label would be something like bbls.  So you
could have:
# e2label /dev/sda1 bbls
and in your fstab have:
LABEL=bbls /usr/local/bbls ...

When the device is ready to be mounted, there will be a scan for the
labels on all the disks your system can see, and when it finds one with
bbls, it will be mounted on /usr/local/bbls - you never need to know
that it's /dev/sdb1 at all.

> It should be pretty straight forward.  I'm betting that whomever set up
> the mounts probably forgot to add the labels. 

Given that the data is apparently intact, that's a safe bet.

Incidentally, unless your system is really old, now might be a good time
to convert the disk from ext2 to ext3 so that you've got a journalling
file system that recovers for errors a lot more cleanly.

        .../Ed

-- 
Ed Wilts, RHCE
Mounds View, MN, USA
mailto:ewilts at ewilts.org
Member #1, Red Hat Community Ambassador Program