ABCs of z/OS System Programming Volume 3 - IBM Redbooks

► The VIO storage group uses system paging volumes for small temporary data sets.
► The TAPE storage groups are used to group tape volumes that are held in tape libraries.
► The OBJECT storage group can span optical, DASD, and tape volumes.
► The OBJECT BACKUP storage group can contain either optical or tape volumes within
one OAM invocation.
► Some volumes are not system-managed.
► Other volumes are owned by DFSMShsm for use in data backup and migration.
DFSMShsm migration level 2 tape cartridges can be system-managed if you assign them
to a tape storage group.
Note: A storage group is assigned to a data set only through the storage group ACS
routine. Users cannot specify a storage group when they allocate a data set, although they
can specify a unit and volume.
Whether or not to honor a user’s unit and volume request is an installation decision, but we
recommend that you discourage users from directly requesting specific devices. It is more
effective for users to specify the logical storage requirements of their data by storage and
management class, which the installation can then verify in the ACS routines.
For objects, there are two types of storage groups, OBJECT and OBJECT BACKUP. An
OBJECT storage group is assigned by OAM when the object is stored; the storage group
ACS routine can override this assignment. There is only one OBJECT BACKUP storage
group, and all backup copies of all objects are assigned to this storage group.
SMS volume selection
SMS determines which volumes are used for data set allocation by developing a list of all
volumes from the storage groups assigned by the storage group ACS routine. Volumes are
then either removed from further consideration or flagged as the following:
Primary Volumes online, below threshold, that meet all the specified criteria in the
storage class.
Secondary Volumes that do not meet all the criteria for primary volumes.
Tertiary When the number of volumes in the storage group is less than the number of
volumes that are requested.
Rejected Volumes that do not meet the required specifications. They are not
candidates for selection.
SMS starts volume selection from the primary list; if no volumes are available, SMS selects
from the secondary; and, if no secondary volumes are available, SMS selects from the
tertiary list.
SMS interfaces with the system resource manager (SRM) to select from the eligible volumes
in the primary list. SRM uses device delays as one of the criteria for selection, and does not
prefer a volume if it is already allocated in the jobstep. This is useful for batch processing
when the data set is accessed immediately after creation.
SMS does not use SRM to select volumes from the secondary or tertiary volume lists. It uses
a form of randomization to prevent skewed allocations in instances such as when new
volumes are added to a storage group, or when the free space statistics are not current on
volumes.
For a striped data set, when multiple storage groups are assigned to an allocation, SMS
examines each storage group and selects the one that offers the largest number of volumes
attached to unique control units. This is called control unit separation. After a storage group
has been selected, SMS selects the volumes based on available space, control unit
separation, and performance characteristics if they are specified in the assigned storage
class.
Chapter 5. System-managed storage 259