RE: Mirroring redo log groups or not ?

From: Timur Akhmadeev <>
Date: Wed, 8 Apr 2009 11:56:34 +0400
Message-ID: <>

Good point to start: Tuning 'log file sync' wait events < events/>  


Timur Akhmadeev

[] On Behalf Of Crisler, Jon Sent: Tuesday, April 07, 2009 8:17 PM
Subject: Mirroring redo log groups or not ?  

I am dealing with a possible performance issue with "redo log sync wait". This is for 10g on Linux 64. In troubleshooting this problem, one of the things I did was to cut down the redo logs so that there is only 1 member per group rather than 2 (i.e. redo log mirroring). Each member goes to a different LUN, and the LUN's are Raid-5.  

Here is the layout-  

LUN 1 is 1 TB approx, Raid-5 used for database files

LUN 2 is 500 gb Raid 5 used for RMAN backups to disk,

LUN 3 is 500 gb Raid 5 used for archive log destination  

All are SAN connected via 2 - 4gb Qlogic HBA's with multipath.  

Now, currently online redo logs are going to Lun 2 for the first member and Lun 3 for the second. So there is going to be a little bit of contention when archiving, but whichever LUN I point to is going to encountered contention. It would be very difficult to add more LUN's due to SAN restrictions, but I have the option of converting either LUN 2 or 3 to Raid 1 / 0. If I converted to Raid 1/0 I could point one or both members to this LUN. There appears to be some performance difference with our hardware between Raid 1 and Raid 5, but not a great deal in actual testing.  

So, if the LUN that holds the online redo logs is already protected (raid 1, 5 etc) do I really need to mirror the online redo logs ? Should I convert the LUN to Raid 1 / 0 ? if I convert to Raid 1/ 0 should I keep redo log mirroring and point to the same LUN ?

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Received on Wed Apr 08 2009 - 02:56:34 CDT

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