IBM DS8000 manual Extent pools, Forming an FB rank with 1 GB extents

a Model 1, and a Model 1 has 1113 cylinders which is about 0.94 GB. The extent size of a CKD rank therefore was chosen to be one 3390 Model 1 or 1113 cylinders.

One extent is the minimum physical allocation unit when a LUN or CKD volume is created, as we discuss later. It is still possible to define a CKD volume with a capacity that is an integral multiple of one cylinder or a fixed block LUN with a capacity that is an integral multiple of 128 logical blocks (64K bytes). However, if the defined capacity is not an integral multiple of the capacity of one extent, the unused capacity in the last extent is wasted. For instance, you could define a 1 cylinder CKD volume, but 1113 cylinders (1 extent) is allocated and 1112 cylinders would be wasted.

Figure 5-5shows an example of an array that is formatted for FB data with 1 GB extents (the squares in the rank just indicate that the extent is composed of several blocks from different DDMs).

Data

Data

Data

Data

Data

Data

Parity

Spare

RAID Array

D1	D7	D13	...
D2	D8	D14	...
D3	D9	D15	...
D4	D10	D16	...
D5	D11	P	...
D6	P	D17	...
P	D12	D18	...

Creation of

a Rank

			. . . .
			. . . .	FB Rank
			. . . .
1GB	1GB	1GB	1GB . . . .	of 1GB
			. . . .	extents
			. . . .
			. . . .
			. . . .

Figure 5-5 Forming an FB rank with 1 GB extents

5.3.4 Extent pools

An extent pool is a logical construct to aggregate the extents from a set of ranks to form a domain for extent allocation to a logical volume. Typically the set of ranks in the extent pool would have the same RAID type and the same disk RPM characteristics so that the extents in the extent pool have homogeneous characteristics. There is no predefined affinity of ranks or arrays to a storage server. The affinity of the rank (and it's associated array) to a given server is determined at the point it is assigned to an extent pool.

One or more ranks with the same extent type can be assigned to an extent pool. One rank can be assigned to only one extent pool. There can be as many extent pools as there are ranks.

Chapter 5. Virtualization concepts 89