Q

RAID 3 or 5 for digital video storage

One reader asks, should I use RAID 3 or RAID 5 for Oracle databases? Read the answer from our expert.

I have a need for RAID; it is being used for real-time multiple digital video storage expansion. Should I use RAID 3 or RAID 5? RAID 3 is supposed to have better write performance compared to RAID 5 (what is the real difference?), but what about single-point failure? If I lose the parity bit disk (RAID 3), can I recover the data? While with RAID 5, there is no single-point failure that cannot be recovered, is that correct?

Personally, I like to stay away from RAID 5 for Oracle databases. As you are already aware, RAID 5 has a write penalty associated with it. This write penalty is typically twice as slow when compared to write operations on non-RAID devices. And databases are write-intensive creatures. So a bottleneck of this magnitude is probably not conducive to smooth database operations.

For write and read performance with databases, I prefer RAID 0+1, but the cost may be prohibitive. It costs twice as much since you need twice as much disk to mirror everything.

If you cannot do RAID 0+1, then look at RAID 3. It is better than RAID 5 for database performance, but not as good as RAID 0+1.

Losing the parity disk in a RAID 3 configuration is not a single point of failure. RAID 3 systems will continue to run if you lose this disk, but you won't have redudancy until that disk is rebuilt. This is why people tend to devote at least one drive in their RAID configuration to a "spare" drive. Should any drive become unavailable (data drive or parity drive), the spare drive is used. If it is a data drive, all data is rebuilt onto this spare using the parity bits. If it is a parity drive, the parity bits are recomputed from the data drives. You won't experience downtime during this operation. The RAID system will handle the rebuld for you.

 

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This was first published in September 2002

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