ALTER SMDS (alter shared message data sets) on z/OS
Use the MQSC command ALTER SMDS to alter the parameters of existing IBM® MQ definitions relating to one or more shared message data sets associated with a specific application structure. It is only supported when the CFSTRUCT definition is using the option OFFLOAD(SMDS).
Using MQSC commands
For information on how you use MQSC commands, see Performing local administration tasks using MQSC commands.
Parameters not specified in the ALTER SMDS command result in the existing values for those parameters being left unchanged.
You can issue this command from sources
2CR. For an explanation of the source symbols, see Sources from which you can issue MQSC commands on z/OS®.
Parameter descriptions for ALTER SMDS
- Specify the queue manager for which the shared message data set properties are to be modified, or an asterisk to modify the properties for all data sets associated with the specified CFSTRUCT.
- Specify the coupling facility application structure for which the properties of one or more shared message data sets are to be modified.
Specify an override value
for the number of buffers to be allocated in the specified queue manager or queue managers for
accessing shared message data sets for this structure, as a number in the range 1 to 9999, or
specify DEFAULT to cancel a previous override and resume using the
DSBUFS value from the CFSTRUCT definition. The size of each buffer is equal to
the logical block size. SMDS buffers are allocated in memory objects residing in z/OS 64-bit storage (above the bar).
When this parameter is altered, any affected queue managers which are already connected to the structure dynamically increase or decrease the number of data set buffers being used for this structure to match the new value. If the specified target value cannot be reached, the affected queue manager replaces the specified DSBUFS parameter with the actual new number of buffers. If the queue manager is not active, the change will come into effect when the queue manager is restarted.
Specify an override
value to be used by the specified queue manager or queue managers to control expansion of shared
message data sets for this structure.
This parameter controls whether the queue manager should expand a shared message data set when it becomes nearly full, and further blocks are required in the data set.
- Expansion is supported.
Each time expansion is required, the data set is expanded by the secondary allocation specified when the data set was defined. If no secondary allocation was specified, or it was specified as zero, then a secondary allocation amount of approximately 10% of the existing size is used.
- No automatic data set expansion is to take place.
- Cancels a previous override.
If you used DEFAULT to cancel a previous override it resumes using the DSEXPAND value from the CFSTRUCT definition.
If an expansion attempt fails, the DSEXPAND override for the affected queue manager is automatically changed to NO to prevent further expansion attempts, but it can be changed back to YES using the ALTER SMDS command to enable further expansion attempts.
When this parameter is altered, any affected queue managers which are already connected to the structure immediately start using the new parameter value.