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IBM i SAS drive parity sets and usable GBs - TD105880



fixed

Per the V7R1 InfoCenter the max size of a Raid set is 18 drives. However
when using a FC5913 Controller that Max is 32 per the TechDoc listed below.

Back in the days of SCSI ( vs SAS ) drives if you told the system to build
the raids sets based on Performance and you had and there were 8 drives
available to build raid it would have built 2 sets of 4. Per the charts in
the link below, the 5913 will build Performance Based raid sets of 24
drives depending on the total number of drives and if Hot Spare is used.

Does someone know a good doc on how Raid works in a SAS environment? back
in the SCSI days if I remember correctly ( ignore striping here ) and for a
4 drive set

The Grey is turning Greyer so this could be all wrong.
A Block of data is written to Disk #1
Controller Reads Drives #2 and #3
Calcs new Checksum and writes that to drive#4

Now with a 8 drive set
A Block of data is written to Disk #1
Controller Reads Drives #2 thru #7
Calcs new Checksum and writes that to drive#8
So there would be at least 4 more reads for that block of data and therefor
you get better performance with smaller raid sets.

How does SAS Raid5 work differently that 24 drive raid set is Perfomance?

TD105880
http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/TD105880





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