Lost my partition table - can I recover?

[bscott at ntisys.com]

  I saw that you already decided to go ahead with the restore from tape
(often the best call in that kind of situation), but I figured this might be
useful to know anyway, so:

On Thu, 5 Aug 2004, at 9:38am, sgarman at iname.com wrote:
> My situation: I have an HP server with two hot-swap SCSI drive bays. It's
> got a RAID controller in it ...

  I know some of HP's RAID controllers are just re-badged AMI (now LSI)
MegaRAID controllers.  So are some of Dell's.  Nice cards, really.

  The MegaRAID line presents logical drives (LDs) to the OS.  It builds LDs
out of physical disks (PDs).  The OS doesn't know about the physical disks;  
it just sees one big logical unit.  The controller does whatever is needed
(striping/mirroring/parity/etc) to turn your PDs into LDs.

  The controller keeps configuration information in NVRAM and on disk.  
When you deleted the LD, it wrote that fact to NVRAM and disk.  That means
no more LD for the OS to see.

  In theory, if you re-create the LD with the *exact same parameters* that
it had before, the contents of the LD will still be there.  Things get a bit
tricky, depending on the RAID level you use, the firmware rev you have, and
the type of RAID array (e.g., a RAID-1 mirror member looks just like a
regular disk in most respects, but a RAID-5 array will be totally
scrambled).  But it can work.  It's not guaranteed, of course, but I have
done it before.

> I am certain that all that's happened is the RAID controller re-wrote a
> new partition table with no partitions.

  Not exactly a partition table.  The RAID configuration information lives
"outside" the LD the OS normally sees.  I think it is kept at the end of
each PD, although I'm not sure on that.  The OS then puts a partition table
inside the LD.  If the LD-recreate trick described above works, then your 
partition table will re-appear, along with everything else that used to be in 
the LD.

> Can I recover from this without having to do a reinstall? This is, ahem, a
> time-critical problem. :( :( :(

  866-NTI-LINUX, $95/hour is our regular rate.

-- 
Ben Scott <bscott at ntisys.com>
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