From: | Scott Carey <scott(at)richrelevance(dot)com> |
---|---|
To: | Merlin Moncure <mmoncure(at)gmail(dot)com>, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> |
Cc: | Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: SSD + RAID |
Date: | 2009-11-13 17:22:17 |
Message-ID: | C722D5C9.169FC%scott@richrelevance.com |
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Lists: | pgsql-performance |
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.
>
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