Lun Calculation - IBM DS8880 Series Introduction And Planning Manual

If you migrate an existing non-striped volume to the same extent pool with a
rotate extents allocation method, then the volume is "reorganized." If you add more
ranks to an existing extent pool, then the "reorganizing" existing striped volumes
spreads them across both existing and new ranks.
You can configure and manage storage pool striping using the DS Storage
Manager, and DS CLI, and DS Open API. The default of the extent allocation
method (EAM) option that is allocated to a logical volume is now rotate extents.
The rotate extents option is designed to provide the best performance by striping
volume extents across ranks in extent pool.
Managed EAM: Once a volume is managed by Easy Tier, the EAM of the volume
is changed to managed EAM, which can result in placement of the extents
differing from the rotate volume and rotate extent rules. The EAM only changes
when a volume is manually migrated to a non-managed pool.
Rotate volumes allocation method
Extents can be allocated sequentially. In this case, all extents are taken from the
same rank until there are enough extents for the requested volume size or the rank
is full, in which case the allocation continues with the next rank in the extent pool.
If more than one volume is created in one operation, the allocation for each
volume starts in another rank. When allocating several volumes, rotate through the
ranks. You might want to consider this allocation method when you prefer to
manage performance manually. The workload of one volume is going to one rank.
This method makes the identification of performance bottlenecks easier; however,
by putting all the volumes data onto just one rank, you might introduce a
bottleneck, depending on your actual workload.

LUN calculation

The storage system uses a volume capacity algorithm (calculation) to provide a
logical unit number (LUN).
In the storage system, physical storage capacities are expressed in powers of 10.
Logical or effective storage capacities (logical volumes, ranks, extent pools) and
processor memory capacities are expressed in powers of 2. Both of these
conventions are used for logical volume effective storage capacities.
On open volumes with 512 byte blocks (including T10-protected volumes), you can
specify an exact block count to create a LUN. You can specify a standard LUN size
30
(which is expressed as an exact number of binary GiBs (2 )) or you can specify an
9
ESS volume size (which is expressed in decimal GiBs (10 ) accurate to 0.1 GB). The
unit of storage allocation for fixed block open systems volumes is one extent. The
extent sizes for open volumes is either exactly 1 GiB, or 16 MiB. Any logical
volume that is not an exact multiple of 1 GiB does not use all the capacity in the
last extent that is allocated to the logical volume. Supported block counts are from
1 to 4 194 304 blocks (2 binary TiB) in increments of one block. Supported sizes are
from 1 to 16 TiB in increments of 1 GiB. The supported ESS LUN sizes are limited
to the exact sizes that are specified from 0.1 to 982.2 GB (decimal) in increments of
0.1 GB and are rounded up to the next larger 32 K byte boundary. The ESS LUN
sizes do not result in standard LUN sizes. Therefore, they can waste capacity.
However, the unused capacity is less than one full extent. ESS LUN sizes are
typically used when volumes must be copied between the storage system and ESS.
53
Chapter 2. Hardware features