Conventional volume selection

Conventional volume selection is used for all nonstriped data sets, as well as for data sets allocated with an SDR of zero or blank.

Conventional volume selection uses the criteria specified in the storage class and the data class as preference attributes for volume selection. SMS uses the preference attributes to:

Classify the volume as primary, secondary or tertiary. Only volumes that meet all of the requested attributes can be classified as primary volumes.
Assign a selection weight to the volume.

SMS begins by classifying each volume in a storage group as primary, secondary, tertiary or rejected, as follows:

Primary

All the volumes in all the specified storage groups are candidates for the first, or primary, list.

Exception: When Multi-Tiered SG (Y) is specified in the storage class, only the volumes in the first storage group assigned by the ACS routine are eligible for the primary list. Also, volumes in Overflow and Extend storage groups are not eligible for the primary list.

The primary list consists of online volumes that meet all the requested preference attributes, are below their threshold, and whose volume status and storage group status are enabled. All volumes on this list are considered equally qualified to satisfy the data set allocation request. Volume selection starts from this list.

SMS chooses from the candidates on this list, preferring volumes that are not already allocated to a job or subsystem and that have the least I/O delay, as determined by the system resource manager (SRM). If no devices in the storage groups meet all of the primary list criteria, no volumes will appear on the primary list. It is possible that, even though there are devices on this primary list, the data set cannot be successfully allocated to any of these devices. That is, there is not enough space available when the actual allocation request is made.

For example, if you specify a 25 MSR in your storage class, volumes that are close to an MSR of 25 would be POOL1 volumes. POOL1 volumes are those volumes that meet your MSR requirement by a certain percentage. Each MSR percentage represents a range of MSRs. All volumes that fall within an MSR percentage are considered equal in performance. Volumes that provide a faster MSR by a larger percentage than POOL1 volumes are considered POOL2 volumes. POOL2 volumes include POOL1 volumes plus the volumes in the next higher MSR percentage. POOL1 volumes are preferred over POOL2 volumes.

Primary volumes include online POOL1 and POOL2 volumes that can meet the selection preference attributes listed in Table 1, in addition to guaranteed space requirement.

Volume selection from the primary list can result in skewing in extreme circumstances. These circumstances include the occasions when a VTOC index is disabled or when a new volume is added to SMS and selected most of the time. SMS uses the randomizing technique to avoid favoring one volume over others, such as an empty volume, a volume with a disabled index, or a volume under the allocation threshold. You can force SMS to always use the randomizing technique by specifying a nonzero IART value. This causes all fixed DASD volumes to be placed on the secondary list, which uses randomization between similar volumes.

Secondary

Volumes that do not meet all the criteria for the primary volume list are placed on the secondary list. If there are no primary volumes, SMS selects from the secondary volumes.

If a data set cannot be allocated on a primary volume, SMS continues to evaluate volumes on the secondary list until allocation is successful. SMS evaluates volumes in the secondary list based upon volume characteristics, performance, available space and system accessibility. See Table 1 for more information about volume selection preferences.

Tertiary

Volumes are marked for the tertiary list if the number of volumes in the storage group is less than the number of volumes requested. If there are no secondary volumes available, SMS selects from the tertiary candidates.

When a storage group does not contain enough volumes to satisfy the volume count, all volumes in the storage group are flagged as tertiary. Tertiary volumes are only selected when there are no primary or secondary volumes and the request is for a non-VSAM, non-GUARANTEED SPACE request. The concept of tertiary volumes does not apply to VSAM data sets. In other words, for all VSAM non-GUARANTEED SPACE requests, the volume count does not play a role in determining which storage group is selected.

Rejected

Volumes that do not meet the required specifications (ACCESSIBILITY = CONTINUOUS, AVAILABILITY = STANDARD or CONTINUOUS, ENABLED or QUIESCED, ONLINE...) are marked rejected and are not candidates for selection.

To improve performance, SMS limits the number of eligible volumes to those most likely to satisfy the request:

For non-best-fit allocation where the primary space must be gotten on one volume, SMS will exclude a volume from selection if the requested primary space exceeds its total capacity.
For best-fit allocations where the primary space is allowed to be spread out on multiple volumes, DADSM will be called until the primary space requested has been gotten or the maximum number of volumes has been used and requested space has not yet been completely gotten. In this latter case, SMS will reject a storage group after 59 volumes are rejected by DADSM for insufficient space. The remaining volumes in the storage group are not considered for further selection since they are less preferred and deemed to have less chance to fulfill the allocation request.
For non-best-fit allocations using 'fast' volume selection, SMS will perform volume selection from the prioritized list until 100 volumes have been rejected by DADSM for insufficient space. When that occurs, SMS will exclude, based on the volume statistics in the SMS configuration, all volumes that do not have sufficient free space. This 'fast' volume selection approach can greatly reduce the number of candidate volumes, and thus the number of retries. You can activate 'fast' volume selection by using the FAST_VOLSEL(ON) parameter in IGDSMSxxor SETSMS FAST_VOLSEL( ON) command.

After the system selects the primary space allocation volume, that volume's associated storage group is used to select any remaining volumes requested for the data set. If you specify an extend storage group, the data may be extended to the specified extend storage group. For information on how to specify an extend storage group using the Extend SG Name parameter, see Defining a pool storage group.