Distributed two-phase commit requires more request/response message exchanges than single-phase commit, which requires only a single message exchange. The overhead of protected conversations, even for read only applications, depends on the SNA LU 6.2 Syncpoint options supported by the server.
To support the LU 6.2 Syncpoint function, most of the control blocks used by the SPM are in global memory. The amount of global memory required is dependent on the number of protected conversations active at any one time. The amount of memory required can also be affected by the number of resynchronization connections that are pending.