Custom-sized provisioning
Custom-sized (or variable-sized) provisioning has more flexibility than fixed-sized provisioning and is the traditional storage-based volume management strategy typically used to organize storage space.
To create custom-sized volumes on a storage system, an administrator creates volumes of the desired size from individual array groups. These volumes are then individually mapped to one or more host ports as a logical unit.
Following are three scenarios where custom-sized provisioning is an advantage:
In fixed-sized provisioning, when several frequently accessed files are located on the same volume and one file is being accessed, users cannot access the other files because of logical device contention. If the custom-sized feature is used to divide the volume into several smaller volumes and I/O workload is balanced (each file is allocated to different volumes), then access contention is reduced and access performance is improved.
In fixed-sized provisioning, not all of the capacity may be used. Unused capacity on the volume will remain inaccessible to other users. If the custom-sized feature is used, smaller volumes can be created that do not waste capacity.
Applications that require the capacity of many fixed-sized volumes can instead be given fewer large volumes to relieve device addressing constraints.
The following illustrates custom-sized provisioning in an open-systems environment using standard volumes of independent array groups:
To change the size of a volume already in use, you first create a new volume larger (if possible) than the old one, then move the contents of the old volume to the new one. The new volume would be remapped on the server to take the mount point of the old one, which is retired.
A disadvantage is that this manual intervention can become costly and tedious and this provisioning strategy is appropriate only in certain scenarios.