On 10/11/06, McKown, John <[EMAIL PROTECTED]> wrote:
Why does Linux/zSeries need STP? I know that my home system can use XNTP to set its clock (which I do daily). Can't XNTP be used for Linux on zSeries just as it is for all other Linux platforms? If not, why not?
I did some work on that in the past. The biggest concern I think is that the way this works in the kernel causes extra work that keeps the server in queue even when it does not have real work, so it affects scalability of the configuration. The quality of the solution is also not very good in a virtualized environment (the hardware TOD is more stable than XNTP can achieve with shared CPU's - and who runs dedicated CPU on zSeries). http://www.vm.ibm.com/events/nl77.pdf When the zSeries TOD is set by the Operator's $5 Mickey Mouse watch, it will be different from other systems that use NTP to keep their clock synchronized. Stable, but different. For some applications this may be a problem, for many sysadmin things it is a nuisance. Suppose your TOD (and z/VM) are 1 minute early. When you start your Linux server with XNTP, it will probably yank the clock backwards short after startup. Ask Neale what this does to the 'make' you already started... My suggestion has always been to get a 9037-2 and use that to set the hardware TOD at POR and let it steer the clock (so it will make the TOD run a weeny bit faster or slower to catch up). Even then, there was confusion in how z/VM supports that. The fact is that a z/VM system *can* enjoy the fact that the clock is kept in synch, it just did not support the protocol that makes it aware of when such corrections are made, so you may not understand why the same job took more or less time than before. For the 9037-2 the maximum steering was 10 ppm, so if your program runs for a day you may be up to 1 second short.
From what I understand of the announcement, it appears to me this is
sort of the built-in version and uses other references than a dial-up to Boulder. If that's correct, then z/VM and Linux would still be able to take advantage in the same way. But I might need to spend more time reading (no pun intended). Rob -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/ ---------------------------------------------------------------------- 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
