When you calculate the size and number of volumes for storage pools that are defined with a FILE or DISK device class, consider the type of storage, how much storage is available, and other variables.
The optimal size for storage pool volumes depends on these elements:
If you do not have the information to estimate a size for FILE device class volumes, start with volumes that are 50 GB.
The FILE device class allows only one session or process to write to a volume at a time. Therefore, you must have at least as many volumes as you have sessions and processes that you expect to write to the pool at the same time.
If you are using collocation, a minimum number of volumes is required. For example, if you are running collocation by node, use at least one volume per node. When you use collocation with preallocated volumes, consider the potential unused storage within those volumes. Collocation with preallocated volumes requires more space. If you use collocation by group, you can reduce the amount of unused storage by designing groups to minimize this waste.
You can use preallocated volumes to avoid file system fragmentation, but you must estimate how much space is required for the storage pool and allocate enough volumes to handle that load. This estimate assumes that the amount of data you are backing up does not fluctuate. For example, processing that is done at the end of the month might use more storage pool space than regular daily processing. If you use scratch volumes, fluctuation is not a factor because Tivoli® Storage Manager allocates what is needed for each day’s processing. If you are using preallocated volumes at a recovery site, some additional time is required for the recovery process because volumes must be preallocated and formatted.