Re: Configuring Oracle on RAID 5

"Sybrand Bakker" <postbus_at_sybrandb.demon.nl> wrote in message news:taqj15oiul2f67_at_beta-news.demon.nl...
>
> ""Anurag Minocha"" <aminocha_at_herold.com> wrote in message
> news:EBBA6D3A81229C42981E3AEA79BF5A4E111233_at_jshemail.herold.com...
> Hi,
> I was wondering that since RAID5 creates one logical volume of disk across
> multiple drives there is no need to have multiple copies of control files
> and also multiplexing redo logs is not required.
>
> I might be wrong ... I am not sure.
>
> what care needs to be taken in creating and placing datafiles, redo log
> files and control files on a RAID 5 machine.
> where can i find more information about Oracle on RAID 5 machines.
> Thanks
> Anurag
>
>
>
> As has been repeatedly discussed here, using RAID 5 for redo log files and
> control files is *NOT* recommended, because of the CRC penalty on writes.
> Not multiplexing control files and redo log files is *NOT* to be
> recommended in *ANY* configuration
> because
> a) Oracle is not aware of any hardware mirroring
> b) if the controller managing a mirrored disk fails, you still have
 nothing
> at all.
> Oracle also needs at least 2 control files, otherwise it simply refuses to
> start.

Really??? You must be working with some weird version or operating system, because that's simply not true on 8.0.5, 8.1.5, 8.1.6 or 8.1.7 on NT, W2K or Solaris.

(The DBA course, chapter 6, requires Students to take a single-Controlfile system and mirror it. Chapter 4 requires them to create a database, and to keep things simple, I never get them to mirror the Controlfile at that point -so a single-Controlfile database is always created. Currently, there are 18 people on the course, and all successfully created their databases today. Unmirrored. Some might question the presence of 18 people on a training course. I would be amongst them!).

That aside, you are absolutely correct: even if you've got RAID 103-and-a-half, you would be plain daft not to have Oracle attempt to write to at least 2 copies of both redo logs and Controlfiles. Too many examples of corruption written to a single Controlfile being instantly, and beautifully, mirrored by the hardware to all mirrored copies have been encountered for the single-file-hardware-mirror approach to be recommended. At least with multiplexing inside Oracle, you have a fighting chance that a duff write by Oracle to one copy of the file won't be repeated when it writes to the second (and preferably third) copy.

You are also correct, of course, in warning about the write penalties associated with RAID-5.

Regards
HJR
>
> Further information on Oracle and RAID 5 can be found on
> http://www.ipass.net/~dsisk/oont.htm
> Also the excellent book by James Morle, Scaling Oracle 8i discusses RAID-5
> in depth. My above comments are an abstract of both references.
>
> Regards,
>
> Sybrand Bakker, Oracle DBA
>
> PS PLEASE DON'T POST IN HTML
>
>
>
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Received on Tue Mar 13 2001 - 02:40:28 CST