Rules for striping volume selection

For both guaranteed and nonguaranteed space allocations, the following key rules apply for striping volume selection:

Storage groups containing mixed device types are not considered.
Target volumes
- An SDR of 0 results in a target stripe count of 1 for both guaranteed and nonguaranteed space requests, and conventional volume selection occurs. A nonzero SDR that is 1 when divided by the device value results in a target stripe count of 1, but striping volume selection is used instead of conventional volume selection.
- For nonguaranteed space, the number of target volumes is computed by dividing the SDR that is specified in the storage class by a value of 3 for 3380 devices, by a value of 4 for 3390 devices, and rounding up the result, if required. For example, an SDR of 18 results in a target stripe count of 6 on a 3380 device and a target stripe count of 5 (after rounding up) on a 3390 device.
- For guaranteed space, if the SDR value is 1 or greater, it is ignored; in which case, the target number of stripes is the greater of either the volume count that is specified or the number of specified volume serial numbers. All specified volumes must be in the same SMS storage group for a guaranteed space request. SMS assumes that the amount of space that the user wants is the target number of stripes times the specified primary space.
All temporary data sets with a volume count greater than one are allocated as nonstriped.
The volume must be able to satisfy the primary space requested by the number of stripes.
The maximum number of stripes (volumes) for VSAM data sets is 16. The maximum number of stripes for physical sequential (PS) data sets is 59.
The maximum number of extents is five for each space allocation.
The maximum number of extents per stripe is 123. For VSAM data sets, the maximum number of extents:
- Per volume is 123
- Per component is 7257
The minimum allocation is one track per stripe.
The maximum allocation can exceed the 64K track limit.
All stripes must be able to satisfy the secondary space allocation (divided by the number of stripes) during extend processing or the allocation fails. Secondary space amount is divided by the number of stripes for both guaranteed and nonguaranteed space requests. OPEN, CLOSE, and EOV perform this calculation.
Non-VSAM multistriped data sets cannot be extended to additional volumes. Striped VSAM data sets can be extended to additional volumes.
Volume fragmentation information is not available to SMS at volume selection time. An allocation failure by DADSM because of fragmentation results in striping volume reselection.
Primary space
- For nonguaranteed space, the volume must be able to satisfy the primary space that is requested divided by the number of stripes. For example, if primary space is 15 MB and the number of stripes is three, the volume must be able to satisfy an allocation of 5 MB.
- For guaranteed space, for requests that contain specified volume serial numbers, each stripe must be able to satisfy the requested primary space (15 MB in above example).
Secondary space amount is divided by the number of stripes and rounded up for each volume.

The following is true for guaranteed space allocations:

If you explicitly specify volume serial numbers with guaranteed space and the target number of stripes is equal to the number of volume serial numbers that you specify, SMS must allocate the primary space requested on each of these volumes. If this is not possible, the allocation fails.
If you do not specify any volume serial numbers, then the target number of stripes is equal to the volume count. SMS tries to select the same number of volumes, but settles for less if this number is unavailable. If fewer stripes are allocated for the non-VSAM data sets, SMS increases the allocation per volume to compensate for the fewer stripes. For VSAM data sets, the primary space requested will be allocated on each stripe regardless of whether the target number of stripes is acquired or not.
For VSAM data sets, if the number of guaranteed space volumes exceeds sixteen (which is the maximum number of stripes for a VSAM data component), the number of guaranteed space volumes will be reduced to sixteen with the remaining volumes becoming candidates for secondary space.
If the target number of stripes is higher than the number of volume serial numbers you specify, SMS must select all the specified volumes plus additional ones. These nonspecific volumes are not mandatory and if none are available, SMS allocates the primary quantity on each of the specified volumes.

Recommendation: Ensure that you have a sufficient number of volumes behind each controller in the storage group to reduce volume overuse. An example of a problem would be if a storage group contains eight controllers and the average stripe count is four, each controller is selected approximately 50 percent of the time. If one controller contained only two volumes, each of the two volumes is selected for approximately 25 percent of all new striped allocations.