Re: MTTR and Optimal Logfile Size

From: Mladen Gogala <>
Date: Fri, 16 Jan 2015 11:35:20 -0500
Message-ID: <>

Hi Jonathan,
The speed of recovery may also depend on the way he's doing backup and the speed of IO devices. If the backup type is storage snapshot and if the archive logs are on solid state devices, 8GB log files sound perfectly reasonable and the recovery will be very quick.

Also, recovery from backup is always the last line of defense. The primary line of defense is RAC, standby or a recovered copy in the flashback area. Recovery from backup is only done if everything else fails, in which case it is probably not expected for the recovery to last less than 1 hour. The only case of full recovery from backup, with the tapes fetched from the Iron Mountain, in my last 5 years of work, was caused by the lady named Sandy, which has visited the New York City area in the year 2012. To paraphrase Marlon Brando from the "Godfather", RAC was sleeping with the fishes.

On 01/16/2015 05:42 AM, Jonathan Lewis wrote:
> 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 outdated principle backwards.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.
> Regards
> Jonathan Lewis
> _at_jloracle
> ------------------------------------------------------------------------
> *From:* []
> on behalf of Brent Day []
> *Sent:* 16 January 2015 09:37
> *To:*
> *Cc:*
> *Subject:* Re: MTTR and Optimal Logfile Size
> Jeff,
> 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.
> Brent
> 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.
> Thanks,
> Jeff

Mladen Gogala
Oracle DBA

Received on Fri Jan 16 2015 - 17:35:20 CET

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