How MVS Initially Allocates the Structure

The system allocates storage for a coupling facility structure based on the level of the system requesting the allocation and the level of the coupling facility in which the structure is to be allocated. The amount of storage allocated is based not only on the connection parameters specified by the application but also on features supported by the coupling facility and the system performing the structure allocation.

Several values can affect the size of a structure — the STRSIZE value specified by the exploiter on the IXLCONN invocation and the SIZE, INITSIZE, Start of change MINSIZE, and SCMMAXSIZE End of change values specified by the installation in the CFRM policy. If a value is too small to satisfy the required control structure space, the connection attempt will fail. IBM® recommends that the policy SIZE value and IXLCONN STRSIZE value (if specified) be in a range of 1.5 - 2.0 times the INITSIZE value (if specified), to prevent situations in which the structure cannot be allocated at its initial size. As systems migrate to higher level coupling facilities that support additional features, it is possible that a value that was satisfactory for a lower level coupling facility might be unsatisfactory for a higher level.

The MINSIZE specification of the CFRM policy allows an installation to indicate the smallest size to which a structure can be altered, as well as to provide a minimum bound on all structure allocation requests. MINSIZE is an optional parameter in the CFRM policy, and its default value depends on the specification of ALLOWAUTOALT in the CFRM policy.

If ALLOWAUTOALT(YES) is specified and MINSIZE is not specified, its default value is 75% of the INITSIZE value or 75% of the SIZE value if INITSIZE is not specified.
If ALLOWAUTOALT(NO) is specified or defaulted to and MINSIZE is not specified, its default value is zero.

Structure size allocation uses the STRSIZE defined on the IXLCONN macro, if the application specified a value, and that value is less than the maximum size specified in the CFRM policy with the SIZE parameter and greater than the MINSIZE value. If STRSIZE is not specified on the IXLCONN macro, allocation uses the INITSIZE specified in the CFRM active policy. If INITSIZE is not specified (it is an optional parameter), then the SIZE specified in the CFRM active policy is used.

The following table shows the initial allocation size determination for a connection. The STRSIZE value specified on IXLCONN might or might not be present, depending on the application using the structure. The INITSIZE value from the CFRM policy also might or might not be present. The MINSIZE value, which is only available on an OS/390® Release 10 or higher system, is assumed to have a nonzero value in the table. In all cases, the initial allocated size of the structure will not be greater than the maximum structure size specified in the CFRM policy with the SIZE parameter nor less than the MINSIZE from the CFRM policy. Start of change (However, the structure size may be less than MINSIZE if the CFRM policy also specifies SCMMAXSIZE for the structure.) End of change

Table 1. Initial Structure Size Allocation
IXLCONN	CFRM Policy
IXLCONN	INITSIZE specified	INITSIZE not specified
IXLCONN STRSIZE specified between MINSIZE and SIZE	Target size = STRSIZE	Target size = STRSIZE
IXLCONN STRSIZE specified greater than SIZE	Target size = SIZE	Target size = SIZE
IXLCONN STRSIZE specified less than MINSIZE	Target size = MINSIZE	Target size = MINSIZE
IXLCONN STRSIZE not specified	Target size = INITSIZE	Target size = SIZE

To display the actual amount of storage allocated to a structure, issue the DISPLAY XCF,STRUCTURE command.