On Tuesday, 08/02/2011 at 10:08 EDT, peter.w...@ttc.ca wrote: > Yes, it can and has been done. As you say, it works well for forward > adjustments; CP abends on backward adjustments. For our old Multiprise 2000, > which ran slow, it was fine. Our z9 runs fast, so we no longer use it.
I don't deny that you have had success with it, but hopefully folks recognize that adjusting the time in this fashion is kind of like bending space-time. If it were that simple, trust me, it would have been done already. - There are timers active throughout CP that must be revisited when the clock is set. But what to do? Was the timer meant to represent a delay interval or a specific time of day (e.g. midnight)? - Some of those timers represent *guest* timers. CP has no idea what kind of time that represents (interval or absolute), so.... - Virtual machine epochs must be updated in order to avoid time jumps. (CP doesn't virtualize STP/ETR timing signals.) - CP time services (DIAGs) will "jump", putting them out of sync with VTOD. Sir Rob's reference to CMS' use of both time sources now comes into play. - Timers that were scheduled to pop within the compressed time interval will now pop all at once. - Missing interrupt timers may be triggered. On current machines, normal oscillator-induced drift and small external time reference changes are automatically corrected by the TOD Clock Steering Facility (see Principles of Operations for gory details). Large changes in time (for small values of "large") are handled through time sync checks in which the host is notified of a problematic difference in time. That is, one that cannot be steered in a reasonable amount of time. It is then the host's responsibility to evaluate what it needs to do and do it, including issuing the necessary PTFF or SCK instructions. So, no surprise, IBM recommends AGAINST any use of such a mod on your VM system. (Of course, don't let that stifle your curiosity and experimentation to determine the possible effects of malleable space-time on virtual machines!) Alan Altmark Senior Managing z/VM and Linux Consultant IBM System Lab Services and Training ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 alan_altm...@us.ibm.com IBM Endicott
