Re: Question about sizing log files

On Wed, 02 Apr 2003 15:18:50 -0800, Chuck <ccarson_at_echeeba.com> wrote:

>
>
>Hans Forbrich wrote:
>> Chuck wrote:
>>
>>
>>>We currently have our redo logs set to 20mb and we get an average of 4
>>>log switches per minute. Just wondering how high people tend to set redo
>>>log sizes on very busy OLTP database. We are currently running 8.1.7.4
>>>on Solaris 8 64-bit. We are currently trying to migrate to 9.2.0.1.
>>>
>>
>>
>> This becomes a 'philosophy' question ... basically how much paper backup do you
>> want to keep in case the systems fails. It's really a balance between cost of
>> data protection and cost of CPU cycles to accompdate the load for high data
>> protection.
>>
>> Generally I try to configure the redo logs to switch for 10-15 minutes at 'normal
>> busy load'. In some cases this has required log files of 150 or 200 MB. You
>> indicate 80Mb/minute or roughly 800Mb in the 10 minutes I'd target; that's
>> slightly high for my taste but not totally unreasonable. Flip side, depending on
>> the speed of disks, archiving the 800Mb logs might cause a noticable stutter in
>> which case I'd probably head back down to 150-200 again.
>>
>> Are you getting that 80Mb/min rate continuously? Or is that a peak load? (If
>> continuous & it's an order entry system, I want the stock-ticker!)
>>
>>
>>
>
>*lol* Yea I too wish it was an order entry system. Actually I am at a
>Bio-tech company and the data is bio-tech related. We have a lot of
>automated processes, primarily a 500-node linux cluster that hammers out
>a ton of data.
>
>My main reason for increasing the interval betweem log switched is to
>make 'life easier' and reduce the quantitity of archive logs.
>
>-Chuck
>
>
>
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Hi Chuck,

Increasing redo log file size does not reduce archive log quantity, it just packs the log into fewer, larger files. Should not be a reason for increasing redo log file size, tar + compress can do the same job.

"Life easier" : Hmmm....increasing redo log file size will reduce the overhed of log switch. That's right. And 4 log switches pr. minute definitely sounds like too much. So for that reason, increase the redo log file size.

But remember that log file size interacts with other parameters and increasing that may have decisive impacts on your system. If your checkpoint interval is very high, the log switch may be the only event that causes checkpoints on your system. If so, enlarging redo log files to, say, 128M, and leaving all other parameters the same may increase your MTTR dramatically in case of instance failure. So other parameters affecting MTTR should be adjusted as well when adjusting redo log file size. Among these are (in 8i) FAST_START_IO_TARGET, LOG_CHECKPOINT_INTERVAL and LOG_CHECKPOINT_TIMEOUT.

• Kenneth Koenraadt