Storage estimate for the lock structure

For installation planning purposes, the initial size of the lock structure is based on how much updating you do.

Recommendation: If you do not specify a value for the LTE parameter in the IRLMPROC procedure, choose a value for INITSIZE that is a power of 2. This enables IRLM to allocate the coupling facility storage so that half is used for lock table entries and the remainder is used for record table entries. If a 1:1 split occurs and total size is not a power of 2, you might experience a severe shortage of space for the record table entries, resulting in Db2 or IRLM failures. Those failures occur because the number of lock table entries requested during connection to the group must be a power of 2. The record table is susceptible to storage shortages if the structure is too small or if the allocation of the lock table leaves too little storage for the record table.

When you specify a value for the LTE parameter in the IRLMPROC procedure, or when you issue the MODIFY irlmproc SET,LTE=nnnn command, you should monitor XES contention rates to determine the optimum value for your normal operating environment. If the contention rates appear to be too high, increase the LTE parameter value to the next power of 2. Remember, any increase in the size of the lock table causes a corresponding decrease in the record table, unless the structure size is also increased.

If you have little contention and want more storage available for record table entries, decrease the LTE parameter value by a power of two. Anytime the number of lock table entries is decreased, you should monitor contention rates for a period of time.

Because structure allocation is done during connection, any change that is made to the LTE parameter value does not take effect unless the lock structure is reallocated. You can reallocate the lock structure in one of the following ways:

  • Rebuild the lock structure. This is the simpler method.
  • Terminate the group, force deallocation of the lock structure, and restart the group.

The LTE parameter value of the first IRLM to connect to the newly allocated structure dictates the coupling facility structure sizes that are used by the group.

The following table shows initial size value recommendations.

Table 1. Recommendations for lock structure size
INITSIZE SIZE Condition
32 MB 64 MB For light sharing with a low amount of updating, or for a single-member data sharing group
64 MB 128 MB For medium sharing with a moderate amount of update activity
128 MB 256 MB For a high amount of sharing with a lot of update activity