RE: Way to much log switching!!!

  • From: Jeremiah Wilton <jwilton@xxxxxxxxxxxxx>
  • To: "'oracle-l@xxxxxxxxxxxxx'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Wed, 30 Jun 2004 13:10:27 -0700 (PDT)

On Wed, 30 Jun 2004, Powell, Mark D wrote:

> I for one like my online logs sized such that in 24 hours the db generates
> 24 to 48 logs.  This is a reasonable number of logs to work with if you ever
> have to perform manual roll forward during recovery.  This might be the case
> where you do not have enough room to keep all the uncompressed logs since
> the last backup on disk in one location.  Then there is that little feature
> where checkpoints are triggered on a log switch and all dirty blocks are
> written to disk.  I see no reason to force buffer flushing this frequently.

In modern Oracle, checkpoints happen constantly as part of the fast
start mechanism.  In 10g this is the default.  Even in the days of
log_checkpoint_interval, the only stuff written to disk was what was
on the LRUW list, not the whole buffer cache!

So I don't buy that frequent checkpoints adversely affect the cache or
performance.  Checkpoints only use a small portion of the DBW0's write
batch anyway.

As for recovery, why is it more difficult to recover with 100 32M
logfiles than with 10 320M logfiles?  I use wildcards to copy stuff
and automatic log apply to roll forward.  I could even argue that in
the case of a restore, the first 32M logfile is going to be restored
sooner than the first 320M log, allowing me to commence roll forward
sooner, and shorten overall time to recover.

Jeremiah Wilton

> -----Original Message-----
> From: oracle-l-bounce@xxxxxxxxxxxxx
> [mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Jeremiah Wilton
> On Wed, 30 Jun 2004, Mark Moynahan wrote:
> > Currently, we have an ODS system with 3 redo log files sized at 200M each.
> > Looking at v$log_history, the logs are switching every 1 to 2 minutes. If
> a
> > 200M log switches at 2 minutes then a 400M log should switch in 4 minutes
> > and 1G log should switch in 10 minutes. I've recommended to size the redo
> to
> > 1G along with determining what is causing all the redo with Logminer. Is
> 20
> > minutes a good rule of thumb to go by when seeing how often the redo logs
> > switch?
> If checkpoints and archiving are keeping up, then I can't think of a
> good reason logs shouldn't switch every 2 minutes.
> You don't need logminer to find the redo culprit, just this SQL:
> select module, osuser, sql_hash_value, value / (sysdate - logon_time) redo
> from v$session s, v$sesstat ss, v$statname sn
> where s.sid = ss.sid
> and ss.statistic# = sn.statistic#
> and name = 'redo size'
> order by redo;
> --
> Jeremiah Wilton
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