IBM DS8000 manual Raw or physical DDM layer, Array site layer, Array layer

Raw DDM layer	DDM 16-pack Y
DDM 16-pack X
Array Site X	Array Site Y
array site layer
Array X RAID5 or RAID10	Array Y
array layer
	RAID5 or RAID10

Rank1 rank layer

Rank2

CKD or FB Virtualization Layers Hierarchy

(Extent Pool A)

Each extent = 1

GB

extent pool layer 6

Volume layer

Volumes/LUNs

Host

Figure 10-7 View of the raw DDM to LUN relationship

Raw or physical DDM layer

At the very top of Figure 10-7you can see the raw DDMs. There are 16 DDMs in a disk drive set. DDM X represents one 16-pack and DDM Y represents another 16-pack. Upon placing the 16-packs into the DS8000, each 16-pack is grouped into array sites, shown as the second layer.

Array site layer

At the array site level, predetermined groups of eight DDMs of the same speed and capacity are arranged. An arrays across loop strategy is used in the predetermined groupings so that an array does not consist of the same, eight physical raw disks in the disk drive set.

Array layer

This level is where the format is placed on the array. Sparing rules are enforced depending on which RAID format is chosen. If you choose RAID-5, then one spare is created and a RAID-5 format is striped across the remaining 7 drives. You must calculate the equivalent of one disk that is used for parity out of the array. Although parity is not placed on one physical disk, but striped across all the remaining disks, that parity equals one disk’s worth of capacity. See Figure 10-8 on page 201. In this figure the RAID format is a 6 + P + S. If you add up the parity chunks it equals one disk’s worth of capacity. If you chose RAID-10 then you would have two spares with no parity and a 3 X 3 + 2(spares) configuration. This would continue for each RAID array until the sparing rules are met.

200DS8000 Series: Concepts and Architecture