Re: Postgres, fsync and RAID controller with 100M of internal cache & dedicated battery

At 11:28 PM 8/22/2007, Dmitry Koterov wrote:
>Hello.
>
>We are trying to use HP CISS contoller (Smart Array E200i) with
>internal cache memory (100M for write caching, built-in power
>battery) together with Postgres. Typically under a heavy load
>Postgres runs checkpoint fsync very slow:
>
>checkpoint buffers dirty=16.8 MB (3.3%) write=24.3 ms sync=6243.3 ms
>
>(If we turn off fsync, the speed increases greatly, fsync=0.) And
>unfortunately it affects all the database productivity during the checkpoint.
>Here is the timing (in milliseconds) of a test transaction called
>multiple times concurrently (6 threads) with fsync turned ON:

It's likely your controller is probably not doing the write caching
thingy or the write caching is still slow (I've seen raid controllers
that are slower than software raid).

Have you actually configured your controller to do the write caching?
Won't be surprised if it's in a conservative setting which means
"write-through" rather than "write-back", even if there's a battery.

BTW, what happens if someone replaced a faulty battery backed
controller card on a "live" system with one from a "don't care test
system" (identical hardware tho) that was powered down abruptly
because people didn't care? Would the new card proceed to trash the
"live" system?

Probably not that important, but what are your mount options for the
partition? Is the partition mounted noatime (or similar)?

Regards,
Link.