RE: MTTR and Optimal Logfile Size

From: Jonathan Lewis <>
Date: Fri, 16 Jan 2015 10:42:49 +0000
Message-ID: <CE70217733273F49A8A162EE074F64D928268946_at_EXMBX01.thus.corp>

It's interesting that Oracle still has that "ca. 20 minutes" as a best practice. It probably came about from the days when a log file switch triggered an immediate checkpoint to clear the log file, which meant you got a spike in database writes on the switch. Over the years and versions Oracle has become increasingly "lazy" about clearing log files, so you could find that although the checkpoints are nominally triggered on each switch you could see a long lag (measured in terms of number of outstanding log files available) before dbwr exhibited any urgency in writing to clear an old log switch checkpoint.

As far as Jeff is concerned the 8GB optimal could be Oracle working an outdated principle backwards. If he's not interested in recovering in less than an hour Oracle may be estimating file size on the basis of needing to roll forward an entire log file for instance recovery, and has decided that it could recover 8GB in that time. With the largest possible log file for the time there is a possibility that writes due to checkpoints will be kept to a minimum.

Jonathan Lewis

From: [] on behalf of Brent Day [] Sent: 16 January 2015 09:37
Subject: Re: MTTR and Optimal Logfile Size


How many logfile switches per hour occur at the heaviest switching hour? Here is a query that can help you find that information:

select trunc(first_time,'hh24'), count(*) from v$archived_log
where first_time >= sysdate - 30
group by trunc(first_time,'hh24')
order by 2;

Ideally you want a logfile switch about every 20 minutes (Oracle's documented best practice) and have your redo log files multiplexed. With current numbers provided this would be closer to 50GB.


On Thu, Jan 15, 2015 at 11:06 AM, Jeff C <<>> wrote: I set my fast_start_mttr_target to 3600 to get a baseline but I noticed that it suggest the optimal logfile size to 8561mb which seems very high. Currently my logfile size is 50mb and I have been getting a some more frequent log switch than in the past so I was going to increase it. And I never had mttr set so I set it to see what it suggest. But why would it suggest an 8gb log file? If I have 3 groups that is 24 gigs of a logs for a 30gb database.


Received on Fri Jan 16 2015 - 11:42:49 CET

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