Frank,
You definitely DO NOT want to do RAID 5 on a database server. That
is probably the worst setup you could have, I've seen it have lower
performance than just a single hard disk.
I've seen that on RAID0 and RAID10 as well.
This is more about the quality and modernity of the RAID controller than
anything else at this point, although there are some theoretical
advantages of RAID10 from a random seek standpoint even if the adapter
CPU is infinitely fast at checksumming. We're using RAID5 in practice
for OLAP / Data Warehousing systems very successfully using the newest
RAID cards from 3Ware (9550SX).
Note that host-based SCSI raid cards from LSI, Adaptec, Intel, Dell, HP
and others have proven to have worse performance than a single disk
drive in many cases, whether for RAID0 or RAID5. In most circumstances
I've seen, people don't even notice until they write a message to a
mailing list about "my query runs slowly on xxx dbms". In many cases,
after they run a simple sequential transfer rate test using dd, they see
that their RAID controller is the culprit.
Recently, I helped a company named DeepData to improve their dbms
performance, which was a combination of moving them to software RAID50
on Linux and getting them onto Bizgres. The disk subsystem sped up on
the same hardware (minus the HW RAID card) by over a factor of 10. The
downside is that SW RAID is a pain in the neck for management - you have
to shut down the Linux host when a disk fails to replace it.
- Luke
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