I agree. My point remains that most of the virtual machines' use of the instruction set is handled in SIE. And, while not getting carried away, great strides of been made in CP I/O handling, like fast path, etc. And avoidance with using MDC, shared segments, etc. also contributes to our sunny io disposition. David
________________________________ From: The IBM z/VM Operating System on behalf of Alan Altmark Sent: Fri 10/31/2008 11:37 AM To: [email protected] Subject: Re: [IBMVM] I/O Overhead - z/VM versus VMWARE On Thursday, 10/30/2008 at 10:29 EDT, David Kreuter <[EMAIL PROTECTED]> wrote: > Is non-insignifcant a mutated way of saying significant? The i/o code in vm is > like much of CP highly optimized. Overhead has been reduced greatly since the > XA introduction of SIE emulation. VM has low overhead due to: > 1. avoidance. Let SIE handle it. > 2. highly optimized code paths. While I will grant you the "optimization" point, let's not get too carried away. In an LPAR, SIE handles guest I/O only for dedicated OSA and FCP adapters. All other I/O is virtualized by CP. SIE *does* handle CP's I/O! The emphasis on the use of a virtual switch rather than dedicated OSAs leaves us with primarily FCP adapters for SCSI. But with all that said, as others have pointed out, the word "overhead" has no meaning. Yes, there is overhead and sometimes, yes, it can be "not insignificant". The question is whether the applications are meeting their SLAs and whether the IT provider is meeting its expense goals. "Can I get acceptable application performance at a cost I can afford?" As was mentioned, you may have more overhead handling an I/O request, but if you can satisfy it from MDC, it was time well-spent. Assuming, of course, you've got the CPU available to handle the I/O request! Alan Altmark z/VM Development IBM Endicott
