VSAM data set allocation

The ALLO and VOLUMES parameters control VSAM data set allocation. They are utility-specific because there are separate parameters for each of the BMC AMI Reorg and BMC AMI Load utilities. 

Related topics

Allocation-strategies

Overview-of-BMC-AMI-Utilities-concepts

Important

If you use SMS for Db2 data sets ( +VOLUMES(*)), then the most important aspects of an allocation strategy are already defined to SMS. You only have to define the +ALLO parameter to select the (PRQTY and SECQTY) allocation values.

Model AMS DEFINE

The values determined from the +ALLO and +VOLUMES parameters are used to build an IDCAMS DEFINE statement. These values are substituted in a model DEFINE statement that you can modify to conform to your installation requirements. 

Important

The variable &VOLUMES is included in the MANAGEMENTCLASS keyword. At run time, this variable is substituted with the value specified in the +VOLUMES parameter.

Example

To use a management class named DB201, you would specify the following statement. This statement is modified to support SMS where +VOLUMES(*) is specified.

+VOLUMES(DB201)

DEFINE CLUSTER -
 (NAME('&DSNAMEC.') -
  LINEAR  REUSE SPEED &ERASE.ERASE -
  MANAGEMENTCLASS( -
    &VOLUMES
    )
  &UNIT(&PRI &SEC)   SHAREOPTIONS(3 3)) -
DATA(NAME('&DSNAMED.'))

You can also effectively disable the VOLUMES parameter by changing the model DEFINE statement. If you remove the VOLUMES variable from the model statement, then any VOLUMES parameter specification will be ignored.

For more details about how to customize the model AMS DEFINE statement for your installation or for a specific Db2 subsystem, see Managing-the-model-AMS-define.

STOGROUPS

When a Db2 space is Db2-managed (that is, defined in a Db2 storage group), the following steps occur:

  • A BMC AMI Utility dynamically adds volumes when the data set needs to be extended.
  • If a BMC AMI Utility, while writing to a Db2 space, determines that the volume is full, the Db2 catalog definition for the storage group is accessed to find another volume.
  • The volume from the STOGROUP with the largest contiguous free space is dynamically added to the list of volumes for the space using the IDCAMS ALTER ADDVOLUMES command.
  • The I/O is then retried, allowing the data set to extend to the new volume.

You can specify a default list during installation. Specifying VOLUMES at execution time overrides the default. If you do not specify a volume override list, the new data set is defined on the same list of volumes used for the original data set, which is obtained from a LISTCAT of the original data set.

You can modify the model DEFINE statement during installation, for example, to support the use of SMS. If you use the variable &VOLUMES in a DEFINE keyword other than VOLUMES(&VOLUMES), for example MANAGEMENTCLASS(&VOLUMES), the value specified on the VOLUMES parameter is substituted for it.

A disadvantage of specifying multiple volumes for the DEFINE is that all the volumes after the first become VSAM candidate volumes, which are used when the data set is extended past the first volume. However, since the candidate volume number is set at the time of the original DEFINE, there is no certainty that those volumes will have space when the time comes to extend the data set.


 

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