Re: New RAID-5 algorithm on V4R5 ? -- MIDRANGE-L

This has nothing to do with V4R4 vs V4R5.
No matter how many drives are in the RAID set, the device parity striping information is (and always has been) spread over either the first 4 drives, or the first 8 drives, in the set. This is why you have a 4 drive minimum in a RAID set. If you add a 5th, 6th or 7th drive to the set they will show at full capacity (not 25% less) because the device parity striping (and also the additional disk workload to maintain this) is still spread over the first 4 disk units. When you add an 8th disk (or 8th, 9th, 10th) you CAN spread the device parity over the first 8 disks - BUT you must end and restart device parity protection for that RAID set in order for the system to do this. When you start parity protection the system determines whether to spread the parity information over 4 or 8 disks based on how many are in the set.
Obviously with the device parity information spread over 8 disks you have less of a performance bottleneck than if you just have the first 4 disks doing all the extra work to maintain the parity information, That's why if you have 4 disks in a set, and are going to add more, you shouldn't add just 3 - spend the extra little to add the 8th disk and spread the workload over all 8 disks (which will each lose 12.5% of their usable capacity for device parity striping).

Neil Palmer DPS Data Processing Services Canada Ltd.
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Philipp Rusch <Philipp.Rusch@rusch-edv.de>
Sent by: owner-midrange-l@midrange.com

2000/12/03 13:47
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Subject: New RAID-5 algorithm on V4R5 ?

Hi all, I noticed while being at a transition job from one AS/400 to another, that there is a difference in how V4R4 and V4R5 handles disk sets on a 2740 controller. I used to calculate the resulting capacity when "raiding" a set of disks on a 274x controller as a loss of about 20% of total capacity. When working on a 620-2179 with V4R4 and 6713 disks (8.58 GB)we got a resulting capacity of about 6400 MB each, the same disks gave me round about 7512 MB on a system 170-2385 with V4R5 and both systems were using a 2740 RAID controller. Looks to me as if we have a better algorithm as before, because only about 10% is used up for running RAID-5 on that set. Anyone to confirm this ? Regards from germany, Philipp Rusch ----------------------------------------------------------------- | EDV Beratung Rusch EDP Consulting Rusch | | Philipp Rusch Mailto: Philipp.Rusch@rusch-edv.de | | Am Errlich 9 WWW : http://www.rusch-edv.de/ | | D-61191 Rosbach, | | Germany | | Phone: (+49) 6003 3972 Mobile : (+49) 172 89 86 230 | | Fax : (+49) 6003 3795 | | | -----------------------------------------------------------------