Re: Redo Log Sizes

In article <38F13F58.DFD44FE_at_erols.com>,   jgitomer_at_erols.com wrote:
> stuarti wrote:
> >
> > I am trying to decide how to size my redo logs
> > on average data is inserted into tables 10 - 20 K chunks
> > should the initial extent of the rbs be about 20K ???
> >
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> How often does your system do a log switch? The ideal is no
> more than 3 times an hour during peak periods.
> --
> Jerry Gitomer
> Once I learned how to spell DBA, I became one
>

I think it needs to be pointed out that rbs, roll back segments, and redo logs are separate entities and are sized differently.

The redo log should be sized based on the amount of time it takes to switch logs and its accompaning checkpoint while rbs segment extents should be sized based on the average transaction size and number of concurrent transactions.

As a general rule for rbs segments initial = next and minextents should be at least 10 and probably set to 20. If you want to be able to support at least 10M of change data in one larger transaction then for 20 extents you would need an initial extent size of 512K and every rbs segment would cost you 10M in space. If you up the extent size it is easy to calculate the size of each rbs segment. As a general type rule for the minimum size of an extent I would use 1.5 the average transaction size times twice the average number of tractions I expect to be assigned the rollback segment under peak load. You should consider batch transaction needs in you calculations.

For redo logs pick the ideal number of logs you think should be generated in one day and keep ajusting your redo log size till you hit that number. I would recomment you aim for 1 or 2 logs per hour which would be 24 to 48 logs per day in an archive log environment. This is a reasonable number of logs to keep track of and should allow you to not experience multiple log switchs within very short time periods like more than one switch in a 1 to 3 minute period. You do not want a new log switch to occur before the checkpoint has had time to complete.

The above is just my opinion and some leadway must be applied to fit these rules to your application. Good luck.

--
Mark D. Powell  -- The only advice that counts is the advice that
 you follow so follow your own advice --


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