Re: BBU still needed with SSD?

On 07/18/2011 11:56 PM, Andy wrote:
> I'm talking about after I get 2 Intel 320s, should I spend the extra
> money on a RAID BBU? Adding RAID BBU in this case wouldn't improve
> reliability, but does it improve performance? If so, how much
> improvement can it bring?

It won't improve performance enough that I would bother. The main
benefit of adding a RAID with BBU to traditional disks is that you can
commit much, much faster to the card RAM than the disks can spin. You
can go from 100 commits/second to 10,000 commits/second that way (in
theory--actually getting >2000 at the database level is harder).

Since the Intel 320 drives can easily hit 2000 to 4000 commits/second on
their own, using the cache that's built-in to the drive, the advantage
of adding a RAID card on top of that is pretty minimal. Adding a RAID
cache will help some, because that layer will be faster than the SSD at
absorbing writes, and putting another cache layer into a system always
helps with improving burst performance. But you'd probably be better
off using the same money to add more RAM, or more/bigger SSD drives.
The fundamental thing that RAID BBU units do--speed up commits--you will
only see minimal benefit from with these SSDs.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD