On Tuesday, 03/11/2008 at 10:41 EDT, Lee Stewart <[EMAIL PROTECTED]> wrote: > In addition to all the good advice already, I will add: Do not try to > run your VM system 2nd level if you have Linux guests. Performance of > the Linux guests will be terrible. There are only two levels of > hardware virtualization. That leaves your Linux guests being > virtualized in software simulation -- which is way slow...
The Interpretive Execution Facility (SIE) is not really simulated so much as there is a "pancake effect" as the description of the nth level virtual machine is transformed by each layer of CP into a successively smaller SIE descriptor. E.g. 50% of the 3rd-level guest pages appear to be resident, as far as the 2nd-level CP is concerned. But not really. Only half of the 2nd level CP's pages are, in fact, resident. Maybe that means only 25% of the 3rd level guest pages are really resident. Add to that, the 3rd level guest is getting a slice of a slice of the CPU. And, finally, I/O is going through multiple address translations. So if you want to use z/VM only as a thin "RDEV translator" you need to very specifically size and configure the 1st-level system to do that. This could mean each hosting z/VM LPAR has only a single second-level z/VM, with locked pages, dedicated XSTORE, dedicated CPUs, elongated time slices, and so on. These kinds of thing don't eliminate the "pancake effect", of course, but they can help minimize shrinkage (ahem) of the virtual machine as it glides towards the hardware. Alan Altmark z/VM Development IBM Endicott
