Allocation, Deletion, And Modification Of Volumes; Lun Calculation - IBM DS8900F Introduction And Planning Manual

Allocation, deletion, and modification of volumes

Extent allocation methods (namely, rotate volumes and pool striping) determine the means by which
actions are completed on storage system volumes.
All extents of the ranks assigned to an extent pool are independently available for allocation to logical
volumes. The extents for a LUN or volume are logically ordered, but they do not have to come from one
rank and the extents do not have to be contiguous on a rank. This construction method of using fixed
extents to form a logical volume in the storage system allows flexibility in the management of the logical
volumes. You can delete volumes, resize volumes, and reuse the extents of those volumes to create other
volumes, different sizes. One logical volume can be deleted without affecting the other logical volumes
defined on the same extent pool.
Because the extents are cleaned (overwritten with zeros) after you delete a volume, it can take some time
until these extents are available for reallocation for volume specific metadata. The reformatting of the
extents is a background process.
There are two extent allocation methods used by the storage system: rotate volumes and storage pool
striping (rotate extents).
Storage pool striping: extent rotation
The default storage allocation method is storage pool striping. The extents of a volume can be striped
across several ranks. The storage system keeps a sequence of ranks. The first rank in the list is randomly
picked at each power on of the storage subsystem. The storage system tracks the rank in which the last
allocation started. The allocation of a first extent for the next volume starts from the next rank in that
sequence. The next extent for that volume is taken from the next rank in sequence, and so on. The system
rotates the extents across the ranks.
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 provisioned capacity algorithm (calculation) to provide a logical unit number
(LUN).
In the storage system, usable 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.
32 IBM DS8900F: DS8900F Introduction and Planning Guide