Ho Fat Tsang wrote:
Hi Richard,
I've tested again according your suggestion. I noticed that for each
time the pgsql slow down, there is a short period a process called
"pdflush"
eating up lot of I/O. I've goolgled and know it is a process for writing
dirty pages back to the disk by the Linux kernel. I will have further
investigation on this process with my limited knowledge on Linux kernel.
Well, pdflush is responsible for flushing dirty pages to disk on behalf
of all processes.
If it's doing it every 3 minutes while checkpoints are happening every
30 seconds then I don't see how it's PG that's responsible.
There are three possibilities:
1. PG isn't actually checkpointing every 30 seconds.
2. There is a burst of query activity every 3 minutes that causes a lot
of writing.
3. Some other process is responsible.
Correct me if i am wrong. It seems postgresql 8.0 does not bundle
auto-vacuum by default. So all vacuum and analyse are done manually ? So
what i have tested related to vaccuum is running auto-vacuum (a executeable
located in /bin) parallel under normal production load but it seems won't
help.
Can't remember whether 8.0 had autovacuum bundled and turned off or not
bundled at all. If it's not running it can't be causing this problem though.
--
Richard Huxton
Archonet Ltd
