Re: SSD + RAID

On 11/13/09 7:29 AM, "Merlin Moncure" <mmoncure(at)gmail(dot)com> wrote:

> On Fri, Nov 13, 2009 at 9:48 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
> wrote:
>> I think RAID6 is gonna reduce the throughput due to overhead to
>> something far less than what a software RAID-10 would achieve.
>
> I was wondering about this. I think raid 5/6 might be a better fit
> for SSD than traditional drives arrays. Here's my thinking:
>
> *) flash SSD reads are cheaper than writes. With 6 or more drives,
> less total data has to be written in Raid 5 than Raid 10. The main
> component of raid 5 performance penalty is that for each written
> block, it has to be read first than written...incurring rotational
> latency, etc. SSD does not have this problem.
>

For random writes, RAID 5 writes as much as RAID 10 (parity + data), and
more if the raid block size is larger than 8k. With RAID 6 it writes 50%
more than RAID 10.

For streaming writes RAID 5 / 6 has an advantage however.

For SLC drives, there is really not much of a write performance penalty.
>