The first IXGCONN request in a sysplex issued for a coupling facility log stream initiates the allocation of coupling facility space for the log stream.
If there are multiple coupling facility log streams assigned to one coupling facility structure, system logger divides the structure space evenly between each log stream that has applications connected to it via IXGCONN. This means that the first log stream that has an IXGCONN service issued against it gets 100% of the coupling facility structure space available. When a second log stream mapping to the coupling facility structure has an IXGCONN service issued against it, system logger divides the structure evenly between the two log streams. The first log stream's available coupling facility space will be reduced by half to reapportion the rest of the space to the second log stream. This can result in some of the data being off-loaded to DASD if the coupling facility space allocated to a log stream reaches its high threshold as a result of the reapportionment of the structure space.
For example, if an installation defines two log streams to a single structure in the LOGR policy, but only one of the log streams actually has a connected application, then that one log stream can use the entire coupling facility structure. When the second log stream connect occurs against the coupling facility structure, the space is gradually divided between the two log streams. Likewise, when the last disconnect is issued against one of the two log streams, all coupling facility structure space is again available to the first log stream.
Note that this process of reallocation of coupling facility space and subsequent offloading of coupling facility data might not take place immediately after the log stream connect. The reallocation of space might occur gradually, with an offloading taking place some time after the original log stream connect.