On 20/02/13 12:24, Josh Berkus wrote:
NM, I tested lowering dirty_background_ratio, and it didn't help, because checkpoints are kicking in before pdflush ever gets there. So the issue seems to be that if you have this combination of factors: 1. large RAM 2. many/fast CPUs 3. a database which fits in RAM but is larger than the RAID controller's WB cache 4. pg_xlog on the same volume as pgdata ... then you'll see checkpoint "stalls" and spread checkpoint will actually make them worse by making the stalls longer. Moving pg_xlog to a separate partition makes this better. Making bgwriter more aggressive helps a bit more on top of that.
We have pg_xlog on a pair of PCIe SSD. Also we running the deadline io scheduler.
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