Coupling facility data table server automatic structure alter

The coupling facility data table server monitors the total number of elements and entries in use in the structure, using information returned by the coupling facility on every request.

When the numbers in use exceed the specified warning thresholds, the server issues a warning message, and this warning message is repeated each time the number in use increases beyond further thresholds.

Each time the server issues a warning, it also tests whether an automatic structure alter for the entry-to-element ratio should be issued. If any form of alter has already been issued recently (by any server or through an operator SETXCF ALTER command) and the structure space usage has remained above warning levels since the previous attempt, any further structure alter attempt is suppressed until at least the minimum interval (specified through the ALTERMININTERVAL parameter) has elapsed.

The server checks whether an automatic structure alter should be issued, by calculating how many excess elements or entries will be left when the other runs out completely, based on the ratio between the current numbers of elements and entries in use. If the number of excess elements or entries exceeds the number specified in the ALTERELEMMIN or ALTERENTRYMIN parameter, and the same number expressed as a percentage of the total exceeds the value specified in the ALTERELEMPC or ALTERENTRYPC parameter, an IXLALTER request is issued to alter the entry to element ratio to the actual current ratio between the number of entries and elements in use.

Only one alter request can be active at a time for a given structure. This means a server may well find that another server has already started the structure alter process, in which case its own alter is rejected. However, the system automatically notifies all servers when the structure alter is completed, giving the new numbers of elements and entries so that each server can update its own status information.