Price of hardware is only a small percentage of the total cost. It's
just easy to focus on when politics is playing a big part in the
comparison.
What the real costs are: software, people, management, floorspace and
other environmentals. If it costs you an extra $50,000 for storage, but
you save 2 people, the "2" people don't like it for some reason. If you
reduce your Oracle license costs by $500,000, maybe someone at Oracle
doesn't like it. If you don't build the extra data center for $15M, the
electricians union doesn't like it. IBM or your business partner should
be able to help you with a less biased "opinion" than just the price of
storage. It would be nice to see a fair AND validated TCO study
comparing VMWare to z/VM.
Mark Post wrote:
On 2/11/2009 at 8:47 AM, Erling Ringen Elvsrud <erlin...@gmail.com> wrote:
-snip-
First, thanks for your informative reply. One argument against linux
on z/VM I have heard a lot is that memory on a mainframe is expensive
compared to memory for vmWare ESX-hosts.
Do you know any rough estimates for how much physical memory that is
needed for each linux host on z/VM that is running WAS? I know z/VM
implements various techniques to reduce overall physical memory
requirements among a set of hosts, but how effective is it?
Yes, it's something that has been a point of contention for quite some time.
Even with the price reduction that came with the z10, it's still more expensive
than a lot of people like.
The effectiveness is going to vary quite a bit with the particular workload an
application. Tuning application code for any particular performance problem
will usually provide 80% of any relief from that problem. But, from what I've
been told by people doing the work, an over-commit ratio of 1.5 is fairly
typical, and some people get as much as 2.0. Just to be clear, the 1.5 means
that if you have 100GB on the box, you can have 150GB of virtual storage in use
by the guests.
Mitigating all of this is that z/VM is capable of paging much more than distributed systems without
a performance impact because the I/O subsystem is so much better. The other factor is that Linux
for System z and z/VM can cooperate in memory management to reduce the amount of "double
paging" that you run into on any virtualization platform. There are other facilities in z/VM
that Linux has been "taught" to use, such as DisContiguous Saved Segments that can reduce
disk I/O as well as real memory consumption.
All in all, the big key is to not give in to developers' perceptions that they
need as much memory for their virtualized guests as they (think) they do on
discrete hardware. We've seen any number of cases where Oracle databases run
quite nicely on 4-8GB of virtual memory when the developers and DBAs thought
they needed 32-64GB because that's what they had on Intel/AMD. Each case is
different, of course, and may require some application rework to get SGA sizes
down to a more reasonable level without impacting performance of the
application. But, with a little time and effort, it can _usually_ be done.
All that said, even with the price of real storage for the z10 being as high as
it is, it almost always turns out cheaper to run the kind of workload you say
you have on Linux for System z than it is on VMware. If you talk to your IBM
rep, or one of IBM's Business Partners, they have tools that can help you
figure out just how much that might be before you make any commitment to buy a
thing. They've been refining it for a number of years now, and if they've got
good numbers to put into them, they can get fairly close.
Mark Post
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