I have gone a few rounds with this issue myself, and here is what I 
learned:
- Catalog disk needs read performance, not write.
- RAID-5 is faster than RAID-10 for sequential reads.
- If you put your catalog on SAN disk, don't use the same HBA/port that 
you use for tape.
- Most servers' internal RAID controllers are brain dead

My catalog is about 70GB and I can back it up from internal disk 
to LTO2 in about 50 minutes.  Given what I have seen in the past, I can't 
complain about this number.

My master is an IBM x345 with six U320 SCSI drives.  Once upon a time 
(before me), it was configured with all six drives in a single RAID-5 and 
when the catalog ran the OS would suffer because it was all the same 
virtual disk.  I had an opportunity to rebuild the box and I thought I'd 
be smart and change it up a bit.  I made a 2-disk mirror for OS, and a 
4-disk RAID-10 for the NBU catalog.  This did not work well.  It turns out 
the internal LSI RAID controller is brain dead (as are most low-end raid 
controllers) and was doing a round robin read on what actually appeared 
to be a RAID-0+1.  So, my performance was actually worse than it was on a 
single spindle.

As a temporary measure, I moved the catalog to SAN space.  It screamed. 
SAN was the best performance I have seen out of the catalog (see two HBAs 
above), but I wasn't too comfortable putting my catalog on SATA disk, 
SAN or not, so this was just a temporary home while I converted the 
4-disk "RAID-10" to RAID-5.  Once this was complete, I had acceptible 
speeds again (above the minimum 10MB/s for my HP LTO2 drives).

-- 
Joe Royer / SysAdmin / Digital Motorworks / 512-692-1028


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