RE: MTTR and Optimal Logfile Size
- From: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
- To: "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 16 Jan 2015 16:56:20 +0000
Mladen,
I was offering a possible answer to the original question, viz: "why would
Oracle choose such a large value for the log file size?": Paraphrasing myself,
my suggestion was simply: "if Oracle thinks you're not in a hurry to recover
quickly maybe it's going suggest the largest log file that it thinks it can
recover in the requested time".
Any considerations of hardware and speed of devices may, or may not, be
relevant to the way that Oracle produces this suggestion - just as the level of
caching is not relevant to the optimizer when it's calculating the cost of a
particular execution path.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
@jloracle
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on behalf
of Mladen Gogala [dmarc-noreply@xxxxxxxxxxxxx]
Sent: 16 January 2015 16:35
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: MTTR and Optimal Logfile Size
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
http://jonathanlewis.wordpress.com
@jloracle
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>
[oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>] on behalf
of Brent Day [coloradodba@xxxxxxxxx<mailto:coloradodba@xxxxxxxxx>]
Sent: 16 January 2015 09:37
To: backseatdba@xxxxxxxxx<mailto:backseatdba@xxxxxxxxx>
Cc: oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>
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
<backseatdba@xxxxxxxxx<mailto:backseatdba@xxxxxxxxx>> 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
http://mgogala.freehostia.com
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