On Fri, Nov 13, 2009 at 12:22 PM, Scott Carey <scott@xxxxxxxxxxxxxxxxx> > On 11/13/09 7:29 AM, "Merlin Moncure" <mmoncure@xxxxxxxxx> wrote: > >> On Fri, Nov 13, 2009 at 9:48 AM, Scott Marlowe <scott.marlowe@xxxxxxxxx> >> 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. how does raid 5 write more if the block size is > 8k? raid 10 is also striped, so has the same problem, right? IOW, if the block size is 8k and you need to write 16k sequentially the raid 5 might write out 24k (two blocks + parity). raid 10 always writes out 2x your data in terms of blocks (raid 5 does only in the worst case). For a SINGLE block, it's always 2x your data for both raid 5 and raid 10, so what i said above was not quite correct. raid 6 is not going to outperform raid 10 ever IMO. It's just a slightly safer raid 5. I was just wondering out loud if raid 5 might give similar performance to raid 10 on flash based disks since there is no rotational latency. even if it did, I probably still wouldn't use it... merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
