Re: Redo + Archive + Data Layout...

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Tue, 21 Oct 2003 20:04:34 +1000
Message-Id: <3f950534$0$10411$afc38c87@news.optusnet.com.au>

Daniel Morgan wrote:

>>

> You are correct. I forgot that Hari seems to have a compulsive need to
> re-ask questions as though they
> were never answered. But I disagree about mirror what has been
> multiplexed. I can't think of a possible
> failure that would be better solved by having 4 control files than 2.
> The issue is eliminating a single point
> of failure. If you had two corrupt control files ... your hardware
> mirroring would just give you mirrored
> corruption.
>

Hardware mirroring protects against hardware failure; software multiplexing protects against software corruptions and user errors.

If I rm control*, hardware mirroring will not save me, because the rm will be replicated on the mirror. Or if CKPT is updating the controlfile, throws a wobbly, and introduces corruption, that corruption will be mirrored.

Only by making CKPT write to two separate and independent multiplexed versions of your controlfiles can you protect against that sort of error.

Of course, having two multiplexed copies means (maybe) you don't need to bother with the hardware mirroring in the first place. And if money and resources were tight, I'd go for multiplexing over mirroring every time.

But there's one thing hardware mirroring can do for you that Oracle multiplexing doesn't do: splitting mirrors for the taking of backups.

It's also true that in a world of 9 possible archive destinations, 2 control files is not sufficient protection. So why not Oracle-multiplex 3, 4 or more times? Because doing so causes CKPT to write to more files (and LGWR, and other processes), and you've therefore increased Oracle's workload. Having 2 files managed by Oracle, and then mirrored by the hardware, gives you the larger number of files you may require for resilience and peace of mind, without increasing the workload for Oracle unacceptably.

So, if I had the choice, I'd multiplex and then mirror. Best of both worlds.

Regards
HJR

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Received on Tue Oct 21 2003 - 05:04:34 CDT