IBM DS8000 manual Logical volumes, Fixed Block LUNs, CKD volumes

5.3.5 Logical volumes

A logical volume is composed of a set of extents from one extent pool.

On a DS8000 up to 65280 (we use the abbreviation 64K in this discussion, even though it is actually 65536 - 256, which is not quite 64K in binary) volumes can be created (64K CKD, or 64K FB volumes, or a mix of both types, but the sum cannot exceed 64K).

Fixed Block LUNs

A logical volume composed of fixed block extents is called a LUN. A fixed block LUN is composed of one or more 1 GB (230) extents from one FB extent pool. A LUN cannot span multiple extent pools, but a LUN can have extents from different ranks within the same extent pool. You can construct LUNs up to a size of 2 TB (240).

LUNs can be allocated in binary GB (230 bytes), decimal GB (109 byes), or 512 or 520 byte blocks. However, the physical capacity that is allocated for a LUN is always a multiple of 1 GB, so it is a good idea to have LUN sizes that are a multiple of a gigabyte. If you define a LUN with a LUN size that is not a multiple of 1 GB, for example, 25.5 GB, the LUN size is 25.5 GB, but 26 GB are physically allocated and 0.5 GB of the physical storage is unusable.

CKD volumes

A zSeries CKD volume is composed of one or more extents from one CKD extent pool. CKD extents are of the size of 3390 Model 1, which has 1113 cylinders. However, when you define a zSeries CKD volume, you do not specify the number of 3390 Model 1 extents but the number of cylinders you want for the volume.

You can define CKD volumes with up to 65520 cylinders, which is about 55.6 GB.

If the number of cylinders specified is not an integral multiple of 1113 cylinders, then some space in the last allocated extent is wasted. For example, if you define 1114 or 3340 cylinders, 1112 cylinders are wasted. For maximum storage efficiency, you should consider allocating volumes that are exact multiples of 1113 cylinders. In fact, integral multiples of 3339 cylinders should be consider for future compatibility.

If you want to use the maximum number of cylinders (65520), you should consider that this is not a multiple of 1113. You could go with 65520 cylinders and waste 147 cylinders for each volume (the difference to the next multiple of 1113) or you might be better off with a volume size of 64554 cylinders which is a multiple of 1113 (factor of 58), or even better, with 63441 cylinders which is a multiple of 3339, a model 3 size.

A CKD volume cannot span multiple extent pools, but a volume can have extents from different ranks in the same extent pool.

Chapter 5. Virtualization concepts 91