Re: log buffer space vs. redo log space requests

Howard J. Rogers wrote:
>
> "Scott Watson" <rscottwatson_at_hotmail.com> wrote in message
> news:mGiR6.18515$UH5.955932_at_wagner.videotron.net...
> > Thanks, for the input guys! Here is some more information...
> >
> > My log buffer is at 2Megs when I was observing these waits.
> >
> > There are 4 groups of online redo logs of 20 megs each. When the app was
> > running under a regular load I noticed that the logs were switching at
> > around 45 seconds. So I have since bumped by log files up to 200Megs each
> > and waiting to see what this will give. I was thinking that the log
> > switches were causing the LGWR to fall behind.
> >
> > If this fails I will try increasing my log buffer again.
>
> I wouldn't do that. I'd be increasing the size of the redo logs to
> something more like 2Gb. You want the rate of log switching to come down
> (usually to something like once per hour. My own preference is for once per
> day. Obviously the rate of switching affects the time taken to perform
> instance recovery, though).
>
> > What do you think about decreasing the checkpoint timeout/ interval so
 there
> > is less work to do once a log switch does come around.
> >
>
> The more mini-checkpoints you introduce, the lower your overall performance.
> But better a steady low performance than one that roller-coasters up and
> down as wait events are encountered. Use them as a last resort, in my book.
>
> Regards
> HJR
>
> > Thanks,
> > scott.
> >
> > "Mark J. Bobak" <mark.NOSPAM_at_bobak.net> wrote in message
> > news:20010530.203754.1244316437.24699_at_bobak.net...
> > > In article <9f3b7i0cci_at_drn.newsguy.com>, "Thomas Kyte"
> > > <tkyte_at_us.oracle.com> wrote:
> > >
> > > > In article <ip7R6.748$m76.424872_at_news20.bellglobal.com>, "Scott
 says...
> > > >>
> > > >>I am a little confused as to the values I am getting from these two
> > > >>statistics.
> > > >>
> > > >>log buffer space in V$SYSTEM_EVENT shows a value for waits which leads
> > > >>me to believe that the log buffer is not large enough however the redo
> > > >>log space requests from V$SYSSTAT shows a value of zero.
> > > >>
> > > >>Therefore one stat tells me there is not enough space while the other
> > > >>tells me everything is ok. I think I am misinterpreting these
> > > >>statistics. Can someone set me straight.
> > > >>
> > > >>Oracle 8.1.6 running on solaris.
> > > >>
> > > >>thanks,
> > > >>Scott.
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > > log buffer space is a wait for space in the redo log buffer (in the
> > > > SGA). you'll get this when your log buffer is too small or LGWR is not
> > > > able to empty it fast enough (slow IO)
> > > >
> > > > redo log space is a wait for space in the physical online redo log
 file
> > > > (on disk). you might get this on a "checkpoint not complete".
> > > >
> > > > The are somewhat un-related.
> > > >
> > > > Thomas Kyte (tkyte_at_us.oracle.com) Oracle Service Industries Howtos and
> > > > such: http://asktom.oracle.com/ http://asktom.oracle.com/~tkyte/
 Oracle
> > > > Magazine: http://www.oracle.com/oramag Opinions are mine and do not
> > > > necessarily reflect those of Oracle Corp
> > > >
> > >
> > > In addition to what Tom said, if you have log buffer space waits,
> > > this could be due to excessive log switches, which cause a backlog
> > > in the log buffer.
> > >
> > > How big are your log files?
> > > How big is your log buffer?
> > > How often are you seeing log switches?
> > >
> > > Also, Steve Adams has some good info on log buffer and log file
> > > tuning, check out http://www.ixora.com.au/
> > >
> > > Hope that helps,
> > >
> > > -Mark
> >
> >

I have to admit I'm a big fan of mega size logs, and then I cron a manual switch each (insert your preferred time unit here). In particular, you can use this to "guarantee" the out-of-date-ness of a standby

hth
connor

-- 
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Connor McDonald
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"Some days you're the pigeon, some days you're the statue"