Re: RAID Newbie question...

The idea of separate disk files for indexes, tables, rollback segments and so on is simply to try and keep the simultaneous i/o's that inevitably occurs during normal DML separate from each other. With a RAID array, yes -it looks like one disk, but of course is comprised of multiple physical disks -so, in fact, the RAID array is separating those conflicting i/o's out for you already.

Hence, no -you won't need multiple RAID arrays. Effectively, you bung everything onto the one array, and let the RAID hardware deal with the i/o problem.

That said, RAID 5 is a real big no-no when it comes to Oracle (or any RDBMS for that matter)... because whilst the striping bit of RAID 5 is good (separating out those i/os across lots of disks), the calculation of parity bit of RAID 5 makes writes (relatively speaking) extremely slow. Certainly, the last place you want to use RAID 5 is for the redo log sub-system, because the frequent, intensive and sequential nature of the writes makes it inappropriate, and if your redo subsystem is running slow, the entire database will run slow in sympathy. There's an argument that RAID 5'ing your data files is not nearly so bad, because of the scattered nature and relative infrequency of the writes.

Personally, I wouldn't touch RAID 5 with a barge pole, but pure striping without the parity (I can never remember my RAID numbers, but I think that's RAID 0!) is excellent for performance -though, of course, there's no redundancy. You will hear people talk about RAID1+0, but the mirroring element there seems to me to be of dubious benefit, since a cock-up on one half of the mirror is invariably reflected onto the other half in the twinkling of an i/o. The Redo Log/Archive Log mechanism provides sufficient redundancy for many situations, without the need for hardware mirroring. I guess it all depends on your budget.

Regards
HJR <gdas_at_my-deja.com> wrote in message news:93efdm$5h4$1_at_nnrp1.deja.com...
> Hi,
> I'm hoping someone can help. I've been doing alot of reading of RAID
> stripping levels and everything seems clear (but no hands-on experience
> yet).
>
> I'm still somewhat confused as to how Oracle and RAID 5 work together.
> I've read several posts on this newsgroup and even read the white
> papers at orapub.com, but I'm confused on one thing still.
>
> If you are running a raid 5 array, it is my understanding that all the
> disks in the array appear as one single logical disk. Is that true?
>
> If so, how does a DBA allocate his/her database on this array?
>
> I've got it fixed in my mind that I need to put data on their own
> physical disks, indexes on their own and temp/system/rollback on their
> own.
>
> If I have 4 physical disks, this is easy to do. But if I have a RAID 5
> array where the entire array appears as one logical disk, what do I
> do?
>
> Does this mean that I need 4 separate arrays? I believe you need at
> least 3 disks for raid 5...so this means 12 disks? Is that correct?
>
> I've read alot about when to and when not use different stripping
> levels. I'm asking this question from a purely academic perspective...
> Let's say for some reason that you have made the choice to implement
> oracle on a raid 5 array. How is it done?
>
> Thanks for any help.
> Gavin
>
>
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Received on Tue Jan 09 2001 - 04:50:15 CST