Re: Log Switches in RAC

Here is the short answer to your 'WHY' part. During media recovery (in RAC) we need to get the changes from ALL threads and merge them before applying to the database. If any of the thread is not archived for longer duration and if the critical changes are recorded in that thread, you may not be able recover the database as the physiological logging requires all the changes from ALL the thread to make the recovery complete

Also that thread is inactive for longer time and you have other threads active, the inactive thread may become a single point of failure during the recovery (if it is not archived for longer duration). To avoid this the other threads force the logswitch to the sleeing thread to keep the SCN ordering intact during recovery. I have discussed the stuff in details in my RAC book. But the key part to your WHY question is to keep the recover ability.

-Gopal

Best Regards,
K Gopalakrishnan

Oracle Database 10g Real Application Clusters Handbook http://www.amazon.com/gp/product/007146509X/

Oracle Wait Interface: A Practical Guide to Performance Diagnostics & Tuning http://www.amazon.com/exec/obidos/tg/detail/-/007222729X/

On Tue, Nov 10, 2009 at 12:33 PM, Rajesh Rao <Rajesh.Rao_at_jpmchase.com>wrote:

> Sorry, I mean, other than the alter system commands, Why and when would a
> log switch on one instance of a RAC trigger cause log switches to happen
> across one or more of the other instances?
>
>
>
> *From:* Surachart Opun [mailto:surachart_at_gmail.com]
> *Sent:* Tuesday, November 10, 2009 1:24 PM
> *To:* Rajesh Rao
> *Cc:* oracle-l_at_freelists.org
> *Subject:* Re: Log Switches in RAC
>
>
>
> If you are meaning switch log by manual...
>
>
>
> "alter system switch logfile"
>
> This command switches log on that instance.
>
>
>
> "alter system archive log current"
>
>
>
> This command switches log all instance in RAC
>
>
>
> Example:
>
> SQL> alter system switch logfile;
>
>
>
> System altered.
>
>
>
> SQL> alter system archive log current;
>
>
>
> System altered.
>
>
>
>
>
> Good Luck
>
> Surachart Opun
> http://surachartopun.com
>
> On Wed, Nov 11, 2009 at 1:11 AM, Rajesh Rao <Rajesh.Rao_at_jpmchase.com>
> wrote:
>
>
>
> Why and when would a log switch on one instance of a RAC trigger cause log
> switches to happen across one or more of the other instances?
>
>
>
> Thanks
>
> Raj
>
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