Re: Fast roll-back

From: Mladen Gogala <>
Date: Sat, 24 Jan 2009 11:55:40 +0000 (UTC)
Message-ID: <glevjs$iuo$>

On Sat, 24 Jan 2009 02:38:36 +0100, The Boss wrote:

> If the following comment on oracle-l is true, both Mladen and yourself
> could be right:
> "When you kill a process using alter system kill session then pmon will
> do the rollback/recovery, if instead you kill the OS process smon will
> do the recovery which might look more attractive sometimes since
> parallel can be used. Be aware of 100% CPU usage by smon and his
> parallel processes though."

The manual, which I used to consider the ultimate resource which is always right says this:
"System monitor (SMON)         

The system monitor performs recovery when a failed instance starts up again. In an Oracle Real Application Clusters database, the SMON process of one instance can perform instance recovery for other instances that have failed. SMON also cleans up temporary segments that are no longer in use and recovers dead transactions skipped during system failure and instance recovery because of file-read or offline errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back online.

Process monitor (PMON)         

The process monitor performs process recovery when a user process fails. PMON is responsible for cleaning up the cache and freeing resources that the process was using. PMON also checks on the dispatcher processes (described later in this table) and server processes and restarts them if they have failed."

This is from the 11g DBA guide:

I did, however, read the article from Fairlie Rego and I did look in V$SESSTAT and V$STATNAME for all statistics with the name like '%roll%' Result looks like this:

IMU CR rollbacks  	0
cleanouts and rollbacks - consistent read gets 	33
rollback changes - undo records applied 	2
rollbacks only - consistent read gets 	32
transaction rollbacks 	1
transaction tables consistent read rollbacks 	0
user rollbacks 	0

So, SMON has definitely done a transaction rollback and some other rollbacks. The same stats for PMON are all zeroes.

Received on Sat Jan 24 2009 - 05:55:40 CST

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