Re: Advice configuring ServeRAID 8k for performance

Definitely switch to RAID-10 .... it's not merely that it's a fair bit
faster on normal operations (less seek contention), it's **WAY** faster than
any parity based RAID (RAID-2 through RAID-6) in degraded mode when you lose
a disk and have to rebuild it. This is something many people don't test for,
and then get bitten badly when they lose a drive under production loads.

Use higher capacity drives if necessary to make your data fit in the number
of spindles your controller supports ... the difference in cost is modest
compared to an overall setup, especially with SATA. Make sure you still
leave at least one hot spare!

In normal operation, RAID-5 has to read and write 2 drives for every write
... not sure about RAID-6 but I suspect it needs to read the entire stripe
to recalculate the Hamming parity, and it definitely has to write to 3
drives for each write, which means seeking all 3 of those drives to that
position. In degraded mode (a disk rebuilding) with either of those levels,
ALL the drives have to seek to that point for every write, and for any reads
of the failed drive, so seek contention is horrendous.

RAID-5 and RAID-6 are designed for optimum capacity, protection, and low
write performance, which is fine for a general file server.

Parity RAID simply isn't suitable for database use .... anyone who claims
otherwise either (a) doesn't understand the failure modes of RAID, or (b) is
running in a situation where performance simply doesn't matter.

Cheers
Dave

On Thu, Aug 5, 2010 at 1:28 PM, Kenneth Cox <kenstir(at)gmail(dot)com> wrote:

> My questions are simple, but in my reading I have not been able to find
> definitive answers:
>
> 1) Should I switch to RAID 10 for performance? I see things like "RAID 5
> is bad for a DB" and "RAID 5 is slow with <= 6 drives" but I see little on
> RAID 6. RAID 6 was the original choice for more usable space with good
> redundancy. My current performance is 85MB/s write, 151 MB/s reads (using
> dd of 2xRAM per
> http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm<http://www.westnet.com/%7Egsmith/content/postgresql/pg-disktesting.htm>
> ).
>