Allowing the Structure to be Altered

Use the ALLOWALTER parameter to indicate whether you permit the structure to be altered. If you specify ALLOWALTER=YES, you also must specify CFLEVEL=1 or higher because the structure must be allocated in a coupling facility that supports structure alter processing. For structure alter to occur, all connectors to the structure must specify ALLOWALTER=YES and CFLEVEL=1 or higher.

When you specify ALLOWALTER=YES, you can also specify:

Whether the entry-to-element ratio can be changed (RATIO)
Whether the percentage of event monitor controls (EMC) storage can be changed (RATIO)
The minimum number (as a percent value) of both entries and elements you want to be available at the conclusion of the structure alter process.
For list structures, this is a percentage of currently “in-use” entries and elements; for cache structures, this is a percentage of “in-use and changed” entries and elements.
- MINENTRY specifies the minimum level of available entries.
- MINELEMENT specifies the minimum level of available elements.
The minimum amount of storage (as a percent value) of storage allocated for event monitor controls that you want available at the conclusion of the structure alter process.
For keyed list structures, this is a percentage of “currently-in-use” EMCs.
- MINEMC specifies the minimum level of available EMCs.

With OS/390® Release 10 and higher, if ALLOWALTER=YES has been specified, an installation can specify that eligible structures are to be automatically altered by the system. See z/OS MVS Setting Up a Sysplex for a description of the automatic alter function.

The application must support alter for the structure to be allocated with support for storage-class memory. Consider the following conditions:

If the CFRM policy specifies SCMMAXSIZE and all other environmental requirements are met, and if the first connector specifies ALLOWALTER=YES, then the system allocates the structure with support for storage-class memory.
After the structure is allocated with support for storage-class memory, the system does not permit any connector that does not specify ALLOWALTER=YES to connect to the structure.

Comparing Structure Rebuild and Structure Alter

Structure alter and structure rebuild are complementary functions, each with its own purpose in a coupling facility environment. The structure rebuild function, introduced in SP 5.1, allows a connector to a structure to change many of the structure attributes, but requires the other connectors to participate in the rebuild process. The structure alter function, available in SP 5.2, allows an authorized user, not necessarily a connector to a structure, to change the structure's size, entry-to-element ratio, and percentage of storage for event monitor controls, without disrupting the structure's current connectors. The structure rebuild function physically relocates the structure, either in the same or a different coupling facility, thus requiring the installation to plan its CFRM policy to allow for coupling facility space to be left available for possible later rebuild use. The structure alter function does not relocate the structure, but changes it “in place”. Structure alter does not require additional coupling facility space to be reserved for a “new” structure, and does not disrupt the processing of connectors to the structure while it is being altered.