On 08/13/2013 01:51 PM, Barth Weishoff wrote:
Hello
I'm having an interesting issue with PGSQL. It seems that I'm
experiencing timeouts at various times. The servers are not busy and
have plenty of resources. The databases are ~50GB in size, the
systems typically have 8-12GB physical RAM, and the connections are
low (less than 15 at any given time).
The issue I'm seeing is that randomly I'm getting these pauses, or
stalls, while trying to simply connect to the database server(s) from
connected clients using the psql command line client. I cannot tell
if the server is even getting the request for service as they don't
seem to show up in the logs at the time the event is occurring, so I'm
thinking it's maybe a client-side issue.
Is there a good general starting place for debugging these types of
issues ?
General recommendations:
1. A copy of "PostgreSQL High Performance" by Greg Smith should be on
your shelf.
2. Increase your logging and be sure to log connections so you can
correlate what you observe with your PostgreSQL logs. I'm currently
logging all queries and connections with minimal overhead (though I'm
routing the logs to a separate server as they accumulate about 4GB/week).
3. Use a log-analyzer. Pgbadger for PostgreSQL and, perhaps, a general
system activity graphing program. Sysusage is easy to install if you
don't have something in place already for system activity graphing.
4. Be specific when reporting your issue. There are issues that only
occur with certain versions of PostgreSQL or on selected
operating-systems (even on specific kernel versions).
When you have seemingly random slowdowns and have eliminated the usual
suspects like unusually large queries or request spikes you should look
at write cache sizing. The default settings on many Linux distributions
are not optimal for a dedicated database server and can lead to periodic
storms of writes. Since the OS kernel parameters are ratios of available
RAM, large amounts of RAM can actually exacerbate this problem since it
allows the OS to accumulate larger amounts of data that require writing
to disk before hitting aggressive disk write trigger threshold.
Cheers,
Steve
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