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On 05/03/2012 02:28 AM, Robert Klemme wrote:
Maybe this also has some additional input: http://www.fccps.cz/download/adv/frr/hdd/hdd.html
Be careful with that link. His recommendations for dirty_ratio and dirty_background_ratio would be *very bad* in a database setting. Note this from the actual article:
"I am aware that my tuning values are probably quite insane in some respects, may cause occasional longer periods of high read latency, may cause other problems. Still I guess the exercise was worth it - the tests did show some interesting results."
That's putting it lightly. With some of those settings in a very large memory server, you could see *minutes* of synchronous IO waits if dirty_ratio gets saturated. I like to follow this:
http://www.westnet.com/~gsmith/content/linux-pdflush.htmAs a note, there are actually new tunables for some of this: dirty_bytes, and dirty_background_bytes. With them, you can match them better to the actual size of your controller write cache so you can avoid page flush storms causing IO stalls. It's unfortunate, but database servers are not the target platform for most of the kernel devs, and really have a much different profile from everyday systems. We need to address latency more than throughput, though both are important.
I think Greg mentioned something that setting these too low can cause VACUUM to lag, but I'm willing to take that tradeoff. We've had IO stalls in the past when our background ratio was too high, and it wasn't pretty. Ironically, we never had a problem until we tripled our system memory, and suddenly our drive controllers were frequently getting choked to death.
Mr. Nielsen's setup actually looks pretty darn good. It's my personal opinion he might run into some IO waits if he plans to use this for heavy OLTP, thanks to having only 8 spindles in his RAID1+0, but he may eventually grow into a SAN. That's fine. It's a good starting point.
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