On 10/12/06, John Summerfield <[EMAIL PROTECTED]> wrote:
I'm feeling a little slow; is the TOD set by the operator for each VM guest? Or managed by VM? Or, (if you have one of these features) by the underlying hardware?
There's a system TOD that is set at POR time (from the clock of the PS/2 or so?) Unless you have the gear that will synch that from true time, it will be off some amount. When you IPL z/VM in the LPAR, it will pick up the TOD and allow the operator to correct that. If he does, z/VM will compute the offset and set the LPAR offset to the hardware TOD. From that point on the LPAR clock runs along with the hardware, but with fixed offset. So LPAR virtualized the TOD clock by letting each LPAR have its own. When a virtual machine reads the TOD, it will get the LPAR TOD plus a per-user fixed offset. This offset can be set (when TODENABLE set for that user) by the operating system again. So z/VM virtualized the TOD clock to let each virtual machine have its own - but they all keep that fixed distance of the real TOD. Linux on zSeries does not use this ability to set the VTOD offset. An ETR would steer the real TOD and thus also adjust all the others that are with fixed offset from it. In addition to the virtual machine set its own offset, you can also define the offset to be taken from some other user when the virtual machine logs on. This is neat for simulating parallel sysplex in virtual machines, or for Y2K testing. You could imagine at IPL of z/VM have a single virtual machine play with NTP and obtain true time, and set VTOD for that. All Linux servers could then want that time. So they would all run the same time. Good, except for drift. Back then I suggested to enhance the VTOD to allow steering - when the virtual machine is not dispatched you can adjust the clock provided you don't yank it over a set timer. Never happened, maybe because we knew the new ETR/STP stuff would come. Rob ---------------------------------------------------------------------- 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
