Re: Is Raid 5 really that bad for Oracle?

"Joel Garry" <joel-garry_at_home.com> wrote in message news:91884734.0408021508.4bb00e13_at_posting.google.com...
> niall.litchfield_at_dial.pipex.com (Niall Litchfield) wrote in message
news:<b6beca79.0408020226.19e847f9_at_posting.google.com>...
> > "joe bayer" <joebayerii(no-spam)@hotmail.com> wrote in message
news:<qDPOc.1753$%J6.1677_at_trndny07>...
> > > I am quoting from Jonathan Lewis's book, Practical Oracle 8i, page 206
> > >
> > > Raid 5 has an undeservedly bad reputation as far as Oracle database
systems
> > > are concerned. ....
> > > However, for most small systems, it is almost necessary and perfectly
> > > acceptable; and for many large systems it is totally adequate.
> >
> > I'm not sure that I buy that it is almost necessary - it would be
> > common in 'small' systems.
> >
> > I'd suggest that whether RAID5 is technically appropriate or not
> > depends entirely on the application (that looks to be behind some of
> > Jonathan's words too). No-one disputes (with the odd storage vendor
> > sales droid exception) that RAID5 has a rather serious write penalty
> > and so is slower than RAID10 (for the same capacity).
>
> I know a hardware-oriented IS manager who disputes that very thing.
> Given a W2K 2+GHz dual Xeon, the argument is that the dual controllers
> to the raid are going to be faster than the motherboard can hand
> things off to a mirrored C drive. I get the "trust me" response if I
> try to put anything anywhere but the raid-5. And there really isn't
> enough room to do otherwise when you start adding in backups and arcs
> and future data.

Your IS manager is still missing the point, then. Generally, the write penalty for RAID-5 is going to be irrelevant, because no-one waits on DBWn to do its thing in any case (different matter for online redo logs, of course). The real bummer with RAID-5 is performance during a single hard disk failure, as each I/O operation gets translated into individual I/Os for every disk in the stripe set. Oh, and the fact that it's not "I" anymore, but "compartively bloody 'E'". Cary's paper says it all, really.

The write penalty during normal operation, in other words, is the least of RAID-5's problems. The main bummer is the READ penalty under failure conditions.

And I say again, if someone doesn't have "enough room" to use anything other than RAID-5, that sounds like an iffily cheap Intel box running Windows... and my recommendation would therefore be Access or SQL Server at a pinch. With sensibly-grandiose hardware, RAID-5 should still and always be the last option to be considered -as I suspect you agree.

Regards
HJR Received on Mon Aug 02 2004 - 18:59:40 CDT