*I* certainly didn't post this message to LSTSRV-L, and can't see the relationship between CTC problems and LINKSWT FILE, which is a LISTSERV data file defining those network lines that are particularly overloaded. However you should note that: - VM spool is not driven by a single virtual machine, but by CP. Spool I/O are, however, synchronous, and RSCS will therefore be 'held' until the spool I/O is complete. I've always said that RSCS ought to do its own private spooling. - Throughput is much better under MVS mainly because of the XA architecture, which (for example) allows a CCW to be processed by some subchannel and the next one by another subchannel, if the first one happens to be busy. This is another explanation of why I/O can degrade considerably under S/370 when the I/O subsystem is heavily loaded. - The RSCSV1 DMTVMC line driver is MUCH more efficient than DMTNJI/NJE. It takes a 4k spool buffer, sends it 'as is', etc. It works only between VM system of course, so it is irrelevant in this particular case. By doing some experimentation, I've found out that the speed of a CTC transfer is limited solely by the DASD speed/load using DMTVMC, whereas CPU contention can be observed with DMTNJI/NJE if the system is heavily loaded. Eric