On Saturday, 07/21/2012 at 03:39 EDT, Rob van der Heij <[email protected]> wrote: > Ever since my presentation in 2005, I find customers who were told by > their friendly IBMer not to get STP/ETR on the machine because "it's > not supported by z/VM" The STP/ETR feature is a priced feature that > is best negotiated into the deal early rather than acquired separately > later. When the LPAR TOD is kept in synch with the world, by pure > magic CP, CMS as well as Linux guests will see accurate time. There is > no need for ntpd in Linux (in fact, doing that will make things > worse). > > The only disruptive time shift would be when you POR with the feature > on. Since the LPAR also picks up the actual time at activation, there > should not be a jump. And STP/ETR will nicely speed up TOD increments > to make it match real time. Except when the leap second meets a > software bug and takes the system down :-)
While one can get this information from the published IBM Redbooks on STP and the Principles of Operations, I will summarize the best I can. Warning: My terminology is sloppy in the interests of understandability. - If you have STP enabled and configured, TOD clock steering will occur. It doesn't matter if the OS "supports" STP or not. - On modern machines, STP steers the TOD at a rate of approximately 1 second in 7 hours (ETR takes 11 hours). - That means it will take about 7 hours for QUERY TIME to match the external time source after a leap second has been added. - Good news, that means QUERY TIME will be correct when you come in a 8am (leap seconds are added at 11:59:60). - If your Linux guests can't live within that tolerance, then you need to consider ntp for Linux. - When the difference between the LPAR TOD and the time source is +/- 50 ยต s (model dependent), an STP-sync-check interrupt is raised. - When I/O timestamps are being used, CP uses the STP-sync-check interrupt to re-calculate the correct time adjustments to apply to the TOD used in the I/O. - The oscillators are extremely stable, so it doesn't take much pull on the wheel for STP to steer out the drift, as it were. 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 [email protected] IBM Endicott ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
