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
Topic revision: r1 - 23 May 2014, CarstenAulbert
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