RE: Recommended number of redo log switches per hour

<<The three log switches per hour recommendation from Oracle support is delusional. >>

As a baseline, I look for log switches to occur from somewhere between 10 minutes and 15 minutes on my higher transaction systems that must perform. Thus, I have log files that are quite big.. Then I monitor and tune accordingly.

The exposure to data loss is limited in 8i+ with the incremental checkpoints (and my segregated online redo log files), so I don't worry about it. The requirement in these systems is response, number one. Granted, if you have a small database, but high transactional volume, then the idea of having log files that are potentially larger than your whole database, might be counter intuitive, but it *is* a performance issue (and if you have a database with that kind of footprint, I'd think it was even more important from a performance reason because you have a great deal of volatility going on!)

<<And any standby would be miserably behind at all times. >> 9i Data Guard offers several different solutions that make this no longer an issue. Of course, replication would also offer a solution to this problem (but not one of my favorite ones).

<<Wall clock time is a bad metric for this question>> Well, generally one metric is bad for any question... :-)) However, my general experience says that rapid log switching in a high transactional environment generally leads to performance impacts. If you require high concurrency, sub-second response, then disk space needs to not be a constraint. Look at your waits and see if they justify larger redo log sizes. I think generally you will find that in high volume systems, they will.

<<If checkpoints are usually able to finish before the next log switch, then the logfile sizes are fine.>>
BUT... a log switch does much more than influence checkpoints. There is latching that occurs, it causes the ARCH process to fire up and start moving the online redo log to the archive log directory, etc....

RF

Robert G. Freeman - Oracle8i OCP
Oracle DBA Technical Lead
CSX Midtier Database Administration

The Cigarette Smoking Man: Anyone who can appease a man's conscience can take his freedom away from him.

-----Original Message-----
Sent: Thursday, March 14, 2002 11:19 AM
To: Multiple recipients of list ORACLE-L

On Thu, 14 Mar 2002, Khedr, Waleed wrote:

> In my opinion not less than 5-10 minutes in very busy times.

The three log switches per hour reccomendation from Oracle support is delusional. I would need 5 to 10 Gb logfiles to accomodate that reccomendation. And any standby would be miserably behind at all times. And recovery time (assuming log switch checkpoints) on instance failure would be unacceptable.

Wall clock time is a bad metric for this question, and support should not have told Patrice this.

If checkpoints are usually able to finish before the next log switch, then the logfile sizes are fine.

Robert F., Here's one for ya.

--
Jeremiah Wilton
http://www.speakeasy.net/~jwilton



> -----Original Message-----


> Sent: Thursday, March 14, 2002 8:19 AM

> To: Multiple recipients of list ORACLE-L

> 

> An Oracle Support technician recommended that I try to aim for three log

> switches per hour, during busy times.

> 

> According to the throughput I got yesterday between 3 and 4 o'clock PM, my

> redo log files would each have to be 1G in size.

> 

> That seems much too large to me, and I am not even sure it is possible to

> create redo log files that size.  Under Oracle 7.3. the largest I could

> create on Tru64 UNIX 4.0g was 40M redo log files.

> 

> Is this another "rule of thumb" that should go down the drain?


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