IBM System storage DS6000 Series, Redbooks

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DS6000 Series: Concepts and Architecture
4.2.5 Logical volumes
A logical volume is composed of a set of extents from one extent pool.
On a DS6000 up to 8192 (8K) volumes can be created (8K CKD, or 8K FB volumes, or a mix
of both types (4K CKD plus 4K FB).
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 (2
30
) 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 (2
40
) in any integral multiple of 64K bytes.
The capacity allocated to a LUN is always a multiple of the 1 GB extent, so any LUN size that
is not a multiple of 1 GB wastes some space in the last extent allocated to the LUN. LUNs can
be allocated in binary GB (2
30
bytes), decimal GB (10
9
bytes), or 512 or 520 byte blocks.
However, when you define a LUN that is not a multiple of 1 GB, the capacity up to the next
multiple of 1 GB 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, your 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).
A CKD volume cannot span multiple extent pools, but a volume can have extents from
different ranks in the same extent pool.