Re: Performance with RAID5 / mirroring ?

LandRoverManREMOVETHIS_at_tiscalimail.nl (Hans van Dam) wrote in message news:<3e391280.511401468_at_pollux.casema.net>...
> On 29 Jan 2003 06:36:22 -0800, Mark.Powell_at_eds.com (Mark D Powell)
> wrote:
>
> >LandRoverManREMOVETHIS_at_tiscalimail.nl (Hans van Dam) wrote in message news:<3e37be05.424238406_at_pollux.casema.net>...
> >> On Wed, 29 Jan 2003 09:43:54 +0100, "RoBog" <robog_at_tin.it> wrote:
> >>
> >> >We have Oracle 8.17 (win NT) on 4 hd (data + index + redo + system)
> >> >
> >> >If we use RAID5 what's the impact on performance?
> >> >
> >> >If we use mirroring (4+4 hd) what's the impact on performance?
> >>
> >> RAID 0 = performance (striping)
> >> RAID 1 = safety (mirrorring)
> >> RAID 5 = compromise between performance and safety
> >> RAID 0+1 = performance and safety without compromise
> >>
> >> Read all (?) about it at http://www.mylex.com/solutions/aboutrdr.html
> >>
> >> Groeten,
> >>
> >> Hans van Dam.
> >
> >Given a choice I would use OS level stripping with mirroring, but it
> >data loss prevention is a very high priority I would not hesitate to
> >mirror RAID-5. We run EMC and their version of RAID-5 is called
> >RAID-S and we perform OS mirroring of the RAID-S disk system to a
> >second RAID-S device. Performance is (amazingly) good.
>
> That doesnt seem like a logical configuration to me. If you need
> mirroring and striping, the best choice is RAID 0+1 because a with
> mirrored RAID S you overdo things, because of the "redundant
> redundancy".
>
> Furthermore OS mirroring is always slower and less reliable than
> hardware mirroring, even if you are satisfied it is not optimal and
> certainly not maximal.
>
> So if you're happy with it, fine, but you can do better with less
> disks (= money).
>
> Regards,
>
> Hans van Dam.

I wasn't asked about the disk setup before the system administrators and vendor set it up. EMC guaranteed that the disk system performance would exceed the system being replaced and it does so who am I to complain. Management is aware of the additional expense, but we have experienced the loss of more than one disk in a raid stripe and that is all RAID-5 protects you from. So the choice was really tripple mirror or mirror RAID-5. Mirroring RAID-5 won.

As far as OS mirroring vs hardware well we have seen several hardware mirror systems fail to protect the data over the last decade. When the hardware itself starts to go bad it corrupts the data and mirrors it. If you set up your OS mirroring correctly this is unlikely and at least one of the mirrors is valid. But more importantly hardware mirrors usually cannot handle the distance separating our disk storage devices or require too much money to do so, leaving OS mirroring as the only affordable option.

It is all about trade-offs and the experts choose this setup so I would with what I have, and it does seem to work well. Is it the best choice, well that will depend on the site, its requirements, and its budget. But I did say my first choice was stripe and mirror.

• Mark D Powell --