Short overview on different configuration possibilities for FC arrays

Our DDN FC array currently houses 600 disks (400 3 TB drives and 200 2 TB drives). DDN only allows certain configurations to be exported (and a maximum of 272 of these including the spare pool). These possibilities, the usable size and the "wasted" space is summarized in the next table

RAID type	number of disks	usable space [2/3TB drives]	"wasted" space[2/3 TB drives]	comment
RAID1	2	2 / 3 TB	2 / 3 TB	"wasted" space useful for improved read performance
RAID5	5	8 / 12 TB	2 / 3 TB	may fail prematurely on double disk outage
RAID6	6	8 / 12 TB	4 / 6 TB
RAID5	9	16 / 24 TB	2 / 3 TB	may fail prematurely on double disk outage, 24 TB problematic for sammkfs
RAID6	10	16 / 24 TB	4 / 6 TB	24 TB problematic for sammkfs

Based on these numbers, one could construct these many pools dedicated to a single file system (without RAID5 as this may not be robust enough for recovery)

RAID type	number of disks/size	number of pools	usable space [TB]	"wasted" space[TB]
RAID1	2 / 2TB	100	200	200
	2 / 3 TB	200	600	600
RAID6	6 / 2TB	33	264	132
	6 / 3 TB	66	792	396
RAID6	10 / 2TB	20	320	80

-- CarstenAulbert - 23 May 2014