Understanding MINSIZE/INITSIZE/SIZE correlation

When requesting structure size, take the following into consideration.

INITSIZE, the size at which you want to initially allocate the structure.
SIZE, the maximum size of the structure.
MINSIZE, the minimum bound for structure allocation for structure allocation requests at OS/390® Release 10 and higher.
CFLEVEL, the level of CFCC in the coupling facility in which a structure is to be allocated.

When allocating the structure initially, whether INITSIZE is specified or not, the system attempts to build all control structures that will be required to support the maximum size of the structure. These control structures are built in the control storage allocation of the structure. For that reason, use care when specifying the maximum structure size with the SIZE parameter in the CFRM policy. IBM® recommends that the SIZE value be in a range of about 1.5 to 2.0 times the INITSIZE value. A SIZE value that is greater than twice the INITSIZE might cause the following:

It might be impossible to allocate a structure at a size of INITSIZE, because the amount of control storage that is required to support the SIZE value might actually be larger than INITSIZE.
If the allocation succeeds, it might result in a structure with a proportionally large amount of its storage allotted to structure controls, leaving too few structure objects to be exploited usefully by the associated application.

For example, if you have requested a maximum size that is very much larger than the initial size, the system will attempt to use a proportionally large amount of the allocated storage for its controls. The result could be that the allocated storage contains control structures for the future maximum size of the structure and insufficient storage might remain for the application's initial use.

The size of the control structures is affected by the CFLEVEL of the coupling facility in which the structure is to be allocated. Different CFLEVELs will have different control structure requirements, with the result that the same structure could have significantly different sizes depending on the CFLEVEL of the coupling facility. In some cases, a structure might actually become unallocatable in a coupling facility if the control structure allocation was so large that it occupied an amount of space larger than the INITSIZE value. Or, even if the initial allocation succeeds, there might remain in the structure only minimal space for the objects required by the application, thus preventing the application from successfully running. When rebuilding a structure from a coupling facility at one CFLEVEL to a coupling facility at another CFLEVEL, the amount of storage required for the control structures might increase to the point where the structure rebuild would fail because the structure is unallocatable. Increasing the INITSIZE value would be necessary in such instances.

MINSIZE is used primarily to limit the size to which a structure can be altered. However, MINSIZE also has implications at structure allocation time. MINSIZE serves as a minimum bound for the structure size, including the structure allocations during rebuild processing. The only exception to this is during system-managed rebuild processing, when the system will allow the allocation of the rebuild new structure with a size less than MINSIZE as long as there is enough space in the new structure to copy all the inuse objects from the old structure.

Specifying SIZE to be larger than INITSIZE provides flexibility when you need to dynamically expand the size of your structure. However, overspecifying SIZE or not taking into account the CFLEVEL of the coupling facility can result in the allocation of a structure with little usable storage for application data or the inability to allocate a structure at all.

See topic Allowing a structure to be altered automatically for more information about system-initiated alter processing. See topic CFRM parameters for administrative data utility for more information about specifying structure sizes in the CFRM policy.