IBM DS8880 Series, Introduction And Planning Manual

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All extents of the ranks that are 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 that are
defined on the same extent pool.
Because the extents are cleaned after you delete a volume, it can take some time
until these extents are available for reallocation. The reformatting of the extents is a
background process.
There are three allocation methods that are used by the storage system: rotate
capacity (also referred to as storage pool striping), rotate volumes, and managed.
Rotate capacity allocation method
The default allocation method is rotate capacity, which is also referred to as storage
pool striping. The rotate capacity allocation method is designed to provide the best
performance by striping volume extents across arrays in a pool. The storage system
keeps a sequence of arrays. The first array in the list is randomly picked at each
power-on of the storage subsystem. The storage system tracks the array in which
the last allocation started. The allocation of a first extent for the next volume starts
from the next array 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
arrays.
If you migrate a volume with a different allocation method to a pool that has the
rotate capacity allocation method, then the volume is reallocated. If you add arrays
to a pool, the rotate capacity allocation method reallocates the volumes by
spreading them across both existing and new arrays.
You can configure and manage this allocation method by using the DS8000 Storage
Management GUI, DS CLI, and DS Open API.
Rotate volumes allocation method
Volume extents can be allocated sequentially. In this case, all extents are taken from
the same array until there are enough extents for the requested volume size or the
array is full, in which case the allocation continues with the next array in the pool.
If more than one volume is created in one operation, the allocation for each
volume starts in another array. You might want to consider this allocation method
when you prefer to manage performance manually. The workload of one volume is
allocated to one array. This method makes the identification of performance
bottlenecks easier; however, by putting all the volume data onto just one array, you
might introduce a bottleneck, depending on your actual workload.
Managed allocation method
When a volume is managed by Easy Tier, the allocation method of the volume is
referred to as managed. Easy Tier allocates the capacity in ways that might differ
from both the rotate capacity and rotate volume allocation methods.
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DS8880 Introduction and Planning Guide