RE: redo log timing

  For a heavy transactional system, I'd (like) to try to keep it around 20-30 minutes per switch, but most of the databases I'm dealing with right now are hybrids. Basically, lots of reporting all day long with occassional updates to data (unless they decide to re-calculate the figures during the day then they get lots of updates--yeah, I know it's not a good thing, but they had that part designed and implemented before I took over here). Then they do batch processing of new data from clients at night (but usually these clients put off their data entry until the end of the week, and at some places the end of the month). Usually this will give me up to 2-3 days between switches and then during heavy hit times I've tried to plan for it being in the 15-30 minute range (right now it's about one every 1.5 hours during high activity, but there will be a lot more clients by the end of the year). Like Rodd pointed out, it does make a difference how much time you can afford to be without data (I have redundant files so I can reload daily data if worse comes to worse and only have to worry about minor client updates during the day...although I am working on a strategy to check for low activity and then switch the logs on a regular basis to help minimize any recovery burden). As for upper and lower limits, I've seen databases switching every 5-6 minutes and not have any problems, but I've seen some where the redo logs were created too small and when the users hit the system heavily, the database would cycle through the redo logs all the way to the one being currently archived. There are a few variables here, but I generally follow the advice I got a long time back for this, "Just run through all the different scenarios and go with the one that makes you feel least nauseous when you think about it."

Jeffery Stevenson
Chief Databeast Tamer
Medical Present Value, Inc.
Austin, TX

-----Original Message-----
Sent: Tuesday, March 20, 2001 12:32 PM
To: Multiple recipients of list ORACLE-L

Ian,
This is one of those where you can get the old "it depends" answer. What type of database is this? What level of transaction activity do you have? How big are your redo logs if you switch every 15 min, 20 min, 30 min?
How critical is recovery to the current time to you?

If you don't want the overhead of lots of log switches and you can stand to have a possible loss of 30 minutes of data on a recovery then 2/hr would work. (Possiblity of data loss based on losing all logs and mirror logs in a

recovery scenario).

Others may have differing opinions on this, but I normally keep mine running

in the 15-20 minute range. The overhead difference between 2 switches/hour and 3 or 4 per/hour is generally not noticeable. It makes for smaller archives which makes the gzip of the log run faster and take up less cpu time. (I run a cron job that compresses the archived logs to save disk space).

On Tuesday 20 March 2001 11:36, you wrote:
>
> Now for an Oracle question. It is often touted that redo logs should
> switch every 15 minutes to 30 minutes. Are these the lower and upper
> limits, should the average time to switch logs be within that range, or
> should the median be between that range?
>
> Ian MacGregor
> Stanford Linear Accelerator
> ian_at_slac.stanford.edu
>
>

-- 
Rodd Holman
Oracle DBA
(605) 988-1373
rodney.holman_at_lodgenet.com
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