RE: duplexing redos and archives

I think Jonathan Lewis describes this in his book as well. When Oracle needs to do recovery, it may read from the corrupted disk and fail recovery.

Suhen

Joe,

        #2 could result from the processes/hardware path outside of Oracle's control. For example, the redo logs are written to disks with separate caches. Disk #1 cache soft corrupts the data being stored, but it is not caught by the system. Disk#2 cache writes it correctly. The key is that Oracle 'thinks' the log has been written correctly when it was not. I'm not a hardware/systems expert, but the scenario of one file being corrupted where another is not sounds possible, but unlikely. Then again...why take the chance.                 

-----Original Message-----
Sent: Wednesday, August 07, 2002 6:53 AM To: Multiple recipients of list ORACLE-L

Dan,

If scenario #2 happened wouldn't both sets of logs be corrupted since oracle is writing the same thing to both files?

BTW, from the tone of everyone responding I think I'll push for continuing to duplex. Thanks.

Joe

"Fink, Dan" wrote:
>
> This may be 'old school' thinking, but I prefer to duplex my redo logs.
Why?
> Because there are some things that mirroring cannot protect you against.
> 1. The deletion of a redo log through human error (been there, recovered
> from it...)
> 2. The logical corruption of a redo log (never experienced it myself, but
it
> is theortically possible)
> 3. Oracle's fault tolerance regarding the ability to function as long as 1
> redo log in a group is accessible
>
> Considering the critical nature of the redo logs (lose one and your
recovery
> is done...and perhaps your job) and their small size in relation to
overall
> databse size, duplexing in a group makes sense.
>
> This is one of those issues where valid arguements exist on both sides. In
> some ways, the logical arguements are on the side of not duplexing redo
logs
> that are mirrored. However, when you consider that the loss or corruption
of
> a single redo log can cause the database to be unrecoverable is enough to
> justify the cost of duplexing.
>
> Just my humble (old school) opinion
>
> Dan Fink
>
> -----Original Message-----
> Sent: Tuesday, August 06, 2002 12:13 PM
> To: Multiple recipients of list ORACLE-L
>
> Currently we are duplexing our redo logs and archive logs through
> oracle. We do this at the oracle level because we are using mirroring
> (raid0?) and if we lose a redo log or archive file the database will not
> be affected.
>
> We will be moving to raid5 arrays in the future and are wondering
> whether we still need to do the duplexing. Since the arrays are made up
> of data disks plus a parity disk plus a hot swap do we need to worry
> about losing a redo log? Our sys admins are telling us we don't and I
> tend to agree with them but we want to get other opinions on the
> subject.
>
> How are shops handling this who are using raid5?
>
> Thanks.
> Joe
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