Re[2]: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS

Dear Jurijs,

>>- Level 1: would have to process (R + W) I/O requests per second
>>- Level 5: would have to process (R + 4W) I/O requests per second

JVal> Can I kindly ask you to clarify few questions?
JVal> 1. Is 4W figure (in formula above) constant in context of RAID 5 array and 
JVal> not depend on spindles count? I suspect that it can be constant in any 
JVal> RAID5 implementation. In case of 6 spindles block will be distributed as:

Unless I'm missing something than according to raid specs it doesn't mater how many disks are in raid5 array, you just need one additional disk for checksums, so in case of 6 spindle array you can create raid5 that will operate according to your schema (it actually will be two raid5 arrays) or you can create one raid5 array that will use 5 disks for data and one disk for checksums.

Please, correct me if I'm wrong.

JVal> 2. If we need to change one of 3 data blocks belonging to one RAID5 set, 
JVal> block. Do I understand correctly? So for writing one block into RAID5 we
JVal> need 2W+2R. Or I am wrong?

Actually it's 6 step process something similar to 2W+2C+2R where 2C is for cpu service. For more information read excelent article by Cary Millsap (http://www.miracleas.dk/BAARF/0.Millsap1996.08.21-VLDB.pdf) at page 11.

-- 
Best regards,
 Edgar                  

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