Comments on: RAID Stripe width http://storageadvisors.adaptec.com/2007/09/07/raid-stripe-width/ Storage Solutions for Real World IT Professionals Sat, 05 Jul 2008 07:56:57 +0000 http://wordpress.org/?v=1.5.2 Comment on RAID Stripe width by: maobo http://storageadvisors.adaptec.com/2007/09/07/raid-stripe-width/#comment-80977 Tue, 16 Oct 2007 00:17:30 +0000 http://storageadvisors.adaptec.com/2007/09/07/raid-stripe-width/#comment-80977 I want to ask a question about the read/write cache size ratio in the real storage controller. As we know the write cache is usually a battery-backed RAM in storage controller. But for battery life limit, its size are restrict to a small size. What the really ratio in the common storage controller? Thank you very much. I want to ask a question about the read/write cache size ratio in the real storage controller. As we know the write cache is usually a battery-backed RAM in storage controller. But for battery life limit, its size are restrict to a small size. What the really ratio in the common storage controller? Thank you very much.

]]> Comment on RAID Stripe width by: Alex http://storageadvisors.adaptec.com/2007/09/07/raid-stripe-width/#comment-76611 Tue, 25 Sep 2007 08:24:19 +0000 http://storageadvisors.adaptec.com/2007/09/07/raid-stripe-width/#comment-76611 Thanks for the info. I have seen a post on the web that says RAID-5 write performance degrades with more disks, because: "writing to a RAID-5 random location requires 2 writes (to the block and it's parity) plus N-2 reads (so you have something to compute the parity with). As the number of drives in the RAID-5 set gets larger, the write performance gets worse. " I assume the N-2 comment is wrong (where n is the stripe size)? A RAID-5 write will always take 4 operations regardless of stripe size (in a non-degraded state, of course) One further point: Will an 8-disk RAID 5 or 2 x 4-disk RAID 5 arrays perform similarly for writes, as long as the writes are distributed evenly between the two arrays? cheers alex Thanks for the info.

I have seen a post on the web that says RAID-5 write performance degrades with more disks, because:

“writing to a RAID-5 random location requires 2 writes (to the block and it’s parity) plus N-2 reads (so you have something to compute the parity with). As the number of drives in the RAID-5 set gets larger, the write performance gets worse. ”

I assume the N-2 comment is wrong (where n is the stripe size)? A RAID-5 write will always take 4 operations regardless of stripe size (in a non-degraded state, of course)

One further point: Will an 8-disk RAID 5 or 2 x 4-disk RAID 5 arrays perform similarly for writes, as long as the writes are distributed evenly between the two arrays?

cheers

alex

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