topic Re: RAID 3/5? in Disk Enclosures

RAID 3/5?

Brian_195 — Fri, 30 May 2003 10:55:23 GMT

What does "RAID 3/5" mean? My HSG80 uses this terminology, but I can't find an explanation.

My understanding is that RAID 3 is striping with a dedicated parity disk, while RAID 5 is striping with distributed parity. What am I missing?

Thanks.

Re: RAID 3/5?

Leif Halvarsson_2 — Fri, 30 May 2003 12:20:45 GMT

Hi,
You are correct about RAID 3 and RAID 5. You can read about "RAID Storagesets" in the manual for HSG80 (see page 2-23).

ftp://ftp.compaq.com/pub/products/storageworks/techdoc/acs/AA-RFBEJ-TE.pdf

As I can find RAID 3/5 uses distributed parity (and should be a RAID 5). I can't see the difference.

Re: RAID 3/5?

Peter Mattei — Mon, 02 Jun 2003 07:15:22 GMT

Brian

Both Raid levels stripe over multiple disks. The difference is in the placement of the parity data. R3 has a dedicated parity disk while R5 distibutes parity over all disks.
R3 is optimal for sequential data while R5 is better for random access.
Read more in the attached doc.

Cheers
Peter

Re: RAID 3/5?

Vincent Farrugia — Mon, 02 Jun 2003 07:25:56 GMT

Hello,

RAID 3/5 is ONE RAID level. RAID 3 has an excellent data rate but poor request rate, on the other hand, RAID 5 has excellent read performance but moderate write performance.

If the controller is able to handle both parity generation schemes of RAID 3 and RAID 5, and uses them in case of advantage, the combination of them gives better write throughput overall compared to the pure RAID 3 or RAID 5 implementations. It is just the best out of both RAID levels.

HTH,
Vince