Re: VERY slow queries at random

could be that the checkpoints are done too seldom.
what is your wal checkpoint config?

Kristo
On 07.06.2007, at 0:27, Scott Marlowe wrote:

> Gunther Mayer wrote:
>> Hi there,
>>
>> We run a small ISP with a FreeBSD/freeradius/postgresql 8.2.4 backend
>> and 200+ users. Authentication happens via UAM/hotspot and I see a
>> lot
>> of authorisation and accounting packets that are handled via PL/PGSQL
>> functions directly in the database.
>>
>> Everything seems to work 100% except that a few times a day I see
>>
>> Jun  6 10:41:31 caligula postgres[57347]: [4-1] radiususer: LOG:
>> duration: 19929.291 ms  statement: SELECT fn_accounting_start(...)
>>
>> in my logs. I'm logging slow queries with
>> log_min_duration_statement =
>> 500 in my postgresql.conf. Sometimes another query runs equally
>> slow or
>> even slower (I've seen 139 seconds!!!) a few minutes before or
>> after as
>> well, but then everything is back to normal.
>>
>> Even though I haven't yet indexed my data I know that the system is
>> performant because my largest table (the accounting one) only has
>> 5000+
>> rows, the entire database is only a few MB's and I have plenty of
>> memory
>> (2GB), shared_buffers = 100MB and max_fsm_pages = 179200.  Also from
>> briefly enabling
>>
>> log_parser_stats = on
>> log_planner_stats = on
>> log_executor_stats = on
>>
>> I saw that most queries are 100% satisfied from cache so the disk
>> doesn't even get hit. Finally, the problem seems unrelated to load
>> because it happens at 4am just as likely as at peak traffic time.
>>
>> What the heck could cause such erratic behaviour? I suspect some
>> type of
>> resource problem but what and how could I dig deeper?
>
> Maybe your hard drive is set to spin down after a certain period of
> idle, and since most all your data is coming from memory, then it
> might be that on the rare occasion when it needs to hit the drive
> it's not spun up anymore.
>
> Maybe some other process is cranking up (cron jobs???) that are
> chewing up all your I/O bandwidth?
>
> Hard to say.  Anything in the system logs that would give you a
> hint?  Try correlating them by the time of the slow pgsql queries.
>
>
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