Hi John,
On Thu, May 3, 2012 at 12:54 AM, John Lister
<john.lister@kickstone.co.uk> wrote:
On 03/05/2012 03:10, Jan Nielsen wrote:
300GB RAID10 2x15k drive for OS on local storage
*/dev/sda1 RA* 4096
*/dev/sda1 FS* ext4
*/dev/sda1 MO*
600GB RAID 10 8x15k drive for $PGDATA on SAN
*IO Scheduler sda* noop anticipatory deadline [cfq]
*/dev/sdb1 RA* 4096
*/dev/sdb1 FS* xfs
*/dev/sdb1 MO* allocsize=256m,attr2,logbufs=8,logbsize=256k,noatime
300GB RAID 10 2x15k drive for $PGDATA/pg_xlog on SAN
*IO Scheduler sdb* noop anticipatory deadline [cfq]
*/dev/sde1 RA* 4096
*/dev/sde1 FS* xfs
*/dev/sde1 MO* allocsize=256m,attr2,logbufs=8,logbsize=256k,noatime
*
I was wondering if it would be better to put the xlog on the same disk as the OS? Apart from the occasional log writes I'd have thought most OS data is loaded into cache at the beginning, so you effectively have an unused disk. This gives you another spindle (mirrored) for your data.
Or have I missed something fundamental?
I followed Gregory Smith's arguments from PostgreSQL 9.0 High Performance, wherein he notes that WAL is sequential with constant cache flushes whereas OS is a mix of sequential and random with rare cache flushes. This (might) lead one to conclude that separating these would be good for at least the WAL and likely both. Regardless, separating these very different use-patterns seems like a "Good Thing" if tuning is ever needed for either.
Cheers,
Jan
