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154 DS6000 Series: Concepts and Architecture
DDM
A Disk Drive Module (DDM) is a customer-replaceable unit that consists of a single disk drive
and its associated packaging.
Array sites
An array site is a predetermined grouping of four individual DDMs of the same speed and
capacity.
Arrays
Arrays consist of DDMs from one to two array sites, used to construct one RAID array. An
array is given either a RAID-5 or RAID-10 format.
Ranks
One array forms one CKD or Fixed Block (FB) rank. When the rank is configured, either CKD
or FB characteristics are taken on at this point. Presently only one array can reside in a rank,
but in the future one or more arrays will be able to reside in one rank.
Extent pools
An extent pool consists of one or several ranks. Ranks in the same extent pool must be of the
same data format (CKD or FB). Each extent pool is associated with server 0 or server 1.
Although it is possible to create extent pools with ranks of different drive capacities, speeds,
and RAID types, we recommend creating them to consist of the same RAID type, speed, and
capacity. Also, we recommend that only half of the total of number of ranks be configured to
reside in one pool (server 0) and the other half in the other pool (server 1).
Extent pools contain one or more ranks divided into fixed-size extents as follows:
CKD extents are equal to a 3390 Mod1
FB extents are 1 GB
The storage in an extent pool (the extents from each rank in the extent pool) is used to create
logical volumes. We recommend creating one extent pool out of only one rank to start with,
unless the size required for a Fixed Block logical volume (LUN) is larger than the combined
free extents residing in one single rank.
Note: The ranks have no pre-determined relation to an LSS.