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Re: Do you still ned RAID 5 if you follow OFA?

From: Liz Reen <Liz_at_reading.news>
Date: Tue, 08 Dec 1998 18:21:10 GMT
Message-ID: <MPG.10d738f9f38b1e65989699@news.cwix.com>


In article <01be21ef$e2c4c800$a12c6394_at_J00679271.ddc.eds.com>, Mark.Powell_at_eds.com says...
> In answer to a post about the pros and cons of using OFA compliant naming
> conventions on a raid 5 disk farm, Tom Pall stated >> The write penalty for
> RAID 5 is 1/n where n is the number of disks used to create the array. <<
>
> Please explain. I always thought the write penalty for Raid 5 was that
> every time you write you have to do two I/O's. One to write the data and
> one to write the parity and both I/O's must complete before the I/O is
> done. So how do you get 1/n?

Under stripping all the disks in a stripeset are divided into chunks, usually a small number of bits. Assume that there are 5 drives in the set 4 data and 1 checksum. Each chunk has a matching chunk on the other drives. These the 4 data chunks are check summed and theh checksum is written out to the 5th chunk. When you read you go directly to the chunk needed. When you write the checksum must be recalculated. This is done either by rereading the other chunks and recalculating the checksum or by a set of calculations. What happens depends on the hardware.

Raid 5 has a definite write penalty. Below ~ 75% reads it performs worse than a normal disk. Yes, you get security. However, if you lose a disk your performance goes out the window as you must now do calculations to figure out what the missing chunk was. Back when 1 gig disks cost $90k, I ran Raid 5.

Now that disks are cheap, I prefer Raid0+1, stripped mirror sets. There is no write penalty. Reads speed is doubled as there are two disk available for reads. With Raid 5, you lose 1 disk you're ok. Lose 2 disks you are dead in the water. With Raid 0+1, you can lose up to 4 disks (the example of 4 data disks) and still continue. Granted you cannot lose both members of a mirror set.

So at worst case 0+1 is as secure as 5. It does have the potential of being better.

Liz
--
My real address is L R E E N a t P E R R I E R G R O U P d o t C O M Received on Tue Dec 08 1998 - 12:21:10 CST

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