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Technology Stocks : Disk Drive Sector Discussion Forum -- Ignore unavailable to you. Want to Upgrade?

To: Dwight E. Karlsen who wrote (1477)	11/10/1997 1:01:00 PM
From: David A. Lethe	Respond to of 9256

Close, Dwight ... but let's set the record straight. People don't buy RAID-5 for safety. They buy it to get a lower cost/MB on storage. Most know about the penalty in performance. If you have a stripe of 5 disks, in RAID5 configuration, then the overhead is 20%. i.e., 5 x 4 GB drives in RAID5 yield 16 GB protected/usable. Actually, Mirroring (RAID1) provides slightly better protection and availability. The overhead is 100%. Each drive must have a mirror copy of its data on another disk. The penalty is 100%. In order to get 16 GB protected/useable in RAID1, you need 8 x 4 GB disks. RAID-1 will almost always be moderately or significantly faster than RAID-5. The user, however, needs to factor in cost of controller, mirroring software, and other limitations into the equation. I have seen RAID-1 implementations cost LESS than RAID-5 implementations, and vise-versa. Other RAID technologies, such as EMCs (propriatary) RAID-S and RAID-3 have different characteristics and are better discussed outside of this forum. Back to this thread, IBM and Seagate are implementing XOR technology at the drive level. This will enable the disk to make the necessary calculations at the drive level for computing the necessary parity information for RAID-5. This is big for the industry and will make performance calculations and generalizations even more confusing. David