On 10/05/10 08:54, Yan Cheng CHEOK wrote:
I realize that, when I have 1 threads perform frequent read, and another 3
threads to perform frequent write. I will get the following error log
*sometimes*
2010-05-07 17:16:10 MYT LOG checkpoints are occurring too frequently (22
seconds apart)
2010-05-07 17:16:10 MYT HINT Consider increasing the configuration parameter
"checkpoint_segments".
2010-05-07 17:31:37 MYT LOG checkpoints are occurring too frequently (29
seconds apart)
2010-05-07 17:31:37 MYT HINT Consider increasing the configuration parameter
"checkpoint_segments".
This is a performance warning.
My observation is, even the above logging happen, "write" operation is success
when I exam the content of database.
What happen is that, the read operation fail.
Er, what?
read thread is spawned after 3 write threads, and read thread will only be considered as
"success", only when its read result is same as what is being written by the 3
threads. The read threads will perform retry for every 200ms, lasted for 4 seconds.
OOOoooh, the readers aren't failing at all, they're just not returning
the results you expect.
At a guess, your code has a race condition, where readers are not
synchronizing themselves properly with writers. Remember that readers
cannot see what writers have written until the writer commits that work.
I was thinking, what is the consequent of increasing "checkpoint_segments",
say, from 3 to 6. Any bad thing might happen?
No, nothing bad. It can increase the size of the WAL stored on disk, but
that's about it.
--
Craig Ringer
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