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Under postgres 7.3 logging is incredibly slow!

I have applied the following settings:

syslog = 2
syslog_facility = 'LOCAL0'
syslog_ident = 'postgres'

log_connections = true
log_duration = true
log_pid = true
log_statement = true
log_timestamp = true

This severely impacted the performance of our production system, a search
page which took 1-3 seconds now takes over 30, is this normal?

I need to get some performance indicators from our production db, however I
cant turn on logging with such performance degradation.

Theo





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Theo Galanakis wrote:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
> log_connections = true
> log_duration = true
> log_pid = true
> log_statement = true
> log_timestamp = true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, however I
> cant turn on logging with such performance degradation.

Linux syslog has this bad behavior of fsync'ing all log writes. See the
syslog manual page for a way to turn off the fsync.

--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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...and on Mon, Dec 20, 2004 at 03:17:11PM +1100, Theo Galanakis used the keyboard:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
> log_connections = true
> log_duration = true
> log_pid = true
> log_statement = true
> log_timestamp = true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, howeverI
> cant turn on logging with such performance degradation.
>

Hi Theo,

One thing you should be sure about is that whichever logfile you have
configured for the local0 facility is being written to asynchronously.
Synchronous logging is REALLY expensive.

If you're using the standard syslogd, you can achieve that by prefixing
the filename in syslogd.conf with a dash. For example,

local0.* /var/log/postgresql.log

would become

local0.* -/var/log/postgresql.log

One other option would be to turn off syslog logging completely and let
postmaster take care of the log on its own, which may or may not be
possible for you, depending on the policy in effect (remote logging, etc.).

Hope this helped,
--
Grega Bremec
gregab at p0f dot net

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On Mon, Dec 20, 2004 at 03:17:11PM +1100, Theo Galanakis wrote:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
> log_connections = true
> log_duration = true
> log_pid = true
> log_statement = true
> log_timestamp = true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, however I
> cant turn on logging with such performance degradation.


I've experienced this problem many times due to hanging dns
lookups. /etc/resolv.conf may point to a nonexistent
nameserver. Comment it out and restart syslogd. Or use a syslog
implementation that allows you to disable dns lookups. Or just give
the nameserver a kick.

-Mike Adler

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