Mirroring redo log groups or not ?

  • From: "Crisler, Jon" <Jon.Crisler@xxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 7 Apr 2009 12:16:42 -0400

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


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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