Re: log buffer space vs. redo log space requests

From: Mark J. Bobak <mark.NOSPAM_at_bobak.net>
Date: Thu, 31 May 2001 00:37:53 GMT
Message-ID: <20010530.203754.1244316437.24699@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 Received on Wed May 30 2001 - 19:37:53 CDT