VSAM RLS assigns locked resources to an entry value in the lock table, and uses this entry value to quickly check whether a resource is already locked. If the lock structure (and thus the lock table) is too small, many locks can be represented by a single value, making "false" lock contention possible. False lock contention occurs when two different locks on different resources attempt to use the same lock entry. The second lock requester is suspended until VSAM RLS determines that there is no real lock contention on the resource. False contention can be a problem for workloads with heavy R/W interests among systems.

To avoid false contention, you need to consider the size of the lock table. The lock table size is determined by the total size of the lock structure. When you define the size of the total lock structure, you should specify a value that is a power of two in order to maximize the lock table size: the lock table comprises 50% of the total space, and the record lock space the remaining 50%. Any memory in excess of a power of two value is allocated to the record lock space exclusively until the next power of two value is reached. At that time, the lock table space is doubled, and the two allocations are once again of equivalent sizes.

VSAM RLS uses the MAXSYSTEM value from the Couple Data Sets format utility to determine the size of each lock entry. Because smaller lock entry sizes imply a larger number of locks for the same memory allocation, it is very important to select an appropriate MAXSYSTEM value. The MAXSYSTEM value represents the maximum number of systems that can be connected into the Parallel Sysplex®. There is a lock table memory penalty when the MAXSYSTEM setting exceeds 7, and another penalty when it exceeds 23. Consequently, from a false contention point of view you want to select a MAXSYSTEM value that does not exceed 7 or 23.

To increase a MAXSYSTEM value that has already been specified, you must first format larger CDSs and switch them into use dynamically. Then, either manually rebuild the CF lock structure using the SETXCF START command or start system-managed duplexing rebuild using the SETXCF START,REBUILD,DUPLEX command.

To decrease a MAXSYSTEM value that has already been specified, you must shut down all of the VSAM RLS address spaces, and manually delete both the persistent connections and the lock structure. Then you must restart the VSAM RLS address spaces. If you decrease the MAXSYSTEM value without first shutting down, the decrease has no effect; the Parallel Sysplex continues to run using the old MAXSYSTEM value.