On Thu, 14 Jan 2010, Scott Marlowe wrote:
I've just received this new server:
1 x XEON 5520 Quad Core w/ HT
8 GB RAM 1066 MHz
16 x SATA II Seagate Barracuda 7200.12
3ware 9650SE w/ 256MB BBU
2 discs in RAID 1 for OS + pg_xlog partitioned with ext2.
12 discs in RAID 10 for postgres data, sole partition with ext3.
2 spares
I think your first choice is right. I use the same basic setup with
147G 15k5 SAS seagate drives and the pg_xlog / OS partition is almost
never close to the same level of utilization, according to iostat, as
the main 12 disk RAID-10 array is. We may have to buy a 16 disk array
to keep up with load, and it would be all main data storage, and our
pg_xlog main drive pair would be just fine.
The benefits of splitting off a couple of discs for WAL are dubious given
the BBU cache, given that the cache will convert the frequent fsyncs to
sequential writes anyway. My advice would be to test the difference. If
the bottleneck is random writes on the 12-disc array, then it may actually
help more to improve that to a 14-disc array instead.
I'd also question whether you need two hot spares, with RAID-10. Obviously
that's a judgement call only you can make, but you could consider whether
it is sufficient to just have a spare disc sitting on a shelf next to the
server rather than using up a slot in the server. Depends on how quickly
you can get to the server on failure, and how important the data is.
Matthew
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