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Re: Is RAID useful for Oracle database files?

From: Art S. Kagel <kagel_at_bloomberg.net>
Date: Wed, 13 Jan 1999 12:55:16 -0500
Message-ID: <369CDE04.6AC9@bloomberg.net>


Connor McDonald wrote:
>
> Monique van Daal wrote:
> >
> > We know it is possible to use RAID 5 on SUn Solaris for our Oracle
> > databases. But does anybody know if it is also USEFUL to apply RAID 5
> > with Oracle database?
> >
> > Thanks
>
> With RAID you can have 2 of a possible 3 benefits...Take your pick
>
> RAID 0 1 0+1 5
> Redundancy No Yes Yes Yes
> Performance Yes Maybe Yes No
> Inexpensive Yes No No Yes

I have to add my crusade comments to the above. RAID5 provides redundancy, yes, but ONLY protects against catastrophic failure of a single drive. RAID5 provides NO PROTECTION against partial media failure. The not uncommon situation where the disk media becomes flaky (is that a technical term? ;-) because the parity is NOT checked at read time only updated at write time without prior verification so if you read a bad sector from one of the drives and write it back with changes the parity for that stripe will be updated with garbage.

RAID 1 and RAID 0+1/1+0 as well as RAID 3 and RAID 4 offer partial media failure protection as well as better write performance than RAID 5. RAID 1, RAID 0+1 & RAID 1+0 write to both sides of the mirror independently so partial media failure cannot corrupt the redundance. RAID 3 & RAID 4 always check parity on read so partial media failure is detected, and if possible corrected or if not reported, as soon as the affected sector is read back. BTW many RAID 3 sstems implement parity in hardware so the old performance problems with early RAID 3 does not apply any longer. (No I do not work for a RAID or drive manufacturer.)

Also note that there is a difference between RAID 0+1 and RAID 1+0. There is no convention and so there is always confusion but the order of the RAID levels in combinations usually implies the order in which the levels are applied. So RAID 0+1 is normally a mirrored stripe set (ie make two stripe sets of N drives each and mirror one to the other) while RAID 1+0 is striped mirrored pairs (ie mirror each of N drives to another drive and stripe the pairs). RAID 0+1 and RAID 1+0 will, at least in theory, perform equally well and redundancy is the same. However, in case of a drive failure, RAID 0+1 must rebuild the entire stripe set from its mirror which keeps all N spindles rather busy reducing the RAID set's performance significantly during recovery. In contrast RAID 1+0 will only have to recover a single drive so that only one spindle is busy with recovery and the RAID set's performance will only be reduced by less than 1/N of normal. Also RAID 0+1 is totally lost if ANY two drives from opposite sides of the mirror are lost at the same time while RAID 1+0 will recover from losing as many as N drives as long as both drives of the same mirrored pair are not lost.

Art S. Kagel Received on Wed Jan 13 1999 - 11:55:16 CST

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