RE: Measuring the impact of Redo Log change

From: Roberts, David (GSD - UK) <>
Date: Tue, 9 Sep 2008 09:47:03 +0100
Message-ID: <>

While the following SQL isn't going to be able to tell you anything about the performance of the application you are running, it may give an indication of increased stability, and depending on the size of your control file may give you data from both before and after the change for you to compare.  

One of the problems I have encountered is when redo logs are all on a single disk device, there will be contention between the archive log process and the log writer, which will be seen in an increasing amount of time taken to archive logs (I'm assuming you're in archive log mode) when the system is under duress.  

SELECT recid,

       To_Char( next_time, 'YYYY-MON-DD HH24:MI:SS' ),

       (completion_time - next_time) * ( 24 * 60 * 60 ) "Seconds to archive"

  FROM v$archived_log

 ORDER BY recid  

If the peek or average time to archive logs has dropped, it would be possible to argue that you have increased the theoretical throughput!  

Obviously if you had the opportunity, placing redo log groups on alternating physical devices would obviously be a better way to reduce this form of contention!    


David Roberts

Logica - Releasing your potential  

Logica UK Limited
Registered in England and Wales (registered number 947968) Registered office: Stephenson House, 75 Hampstead Road, London NW1 2PL, United Kingdom

-----Original Message-----
[] On Behalf Of Deepak Sharma Sent: 09 September 2008 05:33
Subject: Measuring the impact of Redo Log change  

We recently (today) moved the redo logs from RAID5 to RAID1+0.

The obvious reason of doing this is to make things better in terms of performance, as RAID1+0 is better suited for read/writes. Someone pls correct me if that's not true.

What are different ways to measure the impact of this change?

The platform is AIX, DB is and DB size is 55TB, and 2TB redo is generated each day. There are plans underway to reduce the redo generation using direct path etc. (so let's not get diverted by that).

We can try measuring the wait event stats using AWR, session stats etc., but what exactly to look for? 'log file switch completion', 'log file sync', are some things that come to mind - what else?


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Received on Tue Sep 09 2008 - 03:47:03 CDT

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