[CentOS] OT: Why VM?

Rob Lines rlinesseagate at gmail.com
Fri May 27 18:55:10 UTC 2011

On Fri, May 27, 2011 at 2:33 PM, James B. Byrne <byrnejb at harte-lyne.ca> wrote:
> I have been working off and on with Xen and KVM on a couple of test
> hosts for that past year or so and while now everything seems to
> function as expected, more or less, I find myself asking the
> question: Why?
> We run our own servers at our own sites for our own purposes.  We do
> not, with a few small exceptions, host alien domains.  So, while
> rapidly provisioning or dynamically expanding a client's vm might be
> very attractive to a public hosting provider that is not our
> business model at all.
> Why would a small company not in the public hosting business choose
> to employ VM technology?  What are the benefits over operating
> several individual small form factor servers or blades instead?  I
> am curious because what I can find on the net respecting VM use
> cases, outside of for public providers or application testing, seems
> to me mostly puff and smoke.
> This might be considered OT but since CentOS is what we use it seems
> to me best that I ask here to start.

I would have to say that the usecase really has to do with what you
are doing with servers.

For us when our facility was built ~6 years ago they bought racks and
racks of servers from Dell.  There were racks with 1U dual CPU boxes
and then racks with 2U dual CPU and then the racks with 4U Quad CPU
boxes with varying amounts of ram and drive space in them.  That was
the way to have resources available to spin up a new box for someone
in a timely manner rather than waiting a few weeks for one to be
delivered... but at year 2 it became obvious that many of the boxes
were not being used or they were really underutilized as the software
didn't need that much power but it was a Windows App that wanted to
have the box to itself.  So virtualization came into play.  Now
instead of 10 racks of random servers of varying specs we have 1 rack
with some really beefy servers and then a couple racks with individual
boxes for specific applications that were to resource intense to make
sense to virtualize.

The advantages we have been able to realize is the ability to spin up
a 2 processor 1gb VM for someone to start their project and then if it
gets off the ground and they need more we can bump it up to what they
need by editing a configuration rather than having to add memory to a
box or worse yet have to migrate it from one box to another.  This has
resulted in much better utilization of the hardware that we have.
Many servers don't need lots of resources they just need to have the
resources they have all to themselves or atleast think that they do.
We have many servers that only see a spike in workload periodicaly
during the week/month when a researcher has collected the data they
need to work on.  The rest of the time it sits there waiting.

The other real advantage for us is the DR solutions that virtualizing
gives us.  Previously DR ment having a stack of machines at the DR
site waiting to be recovered to and deciding what servers were
important enough that we needed a box with their name on it and what
could wait until we could call Dell and get a new one shipped out.
Now we just have a smaller VMware cluster at the other end running
some hot DR servers and ready to bring up the replicated LUNs from our
primary site in case of a failure.  We can bring those VMs back up in
a short time instead of having to do baremetal installs then restore
from backups.  Going foward we will be scaling so that we can share
our DR at the remote site with the group that is there as their
primary and they will be able to use our cluster here as their DR
allowing us to consolidate the number of hosts needed.  That is
something that just isn't possiable with physical boxes.

I can't say that you don't take a monitary hit to buy the shared
storage, networking, and software required to support a Virtual world
but I personaly see it as worth it.

On the other hand there are those systems that just make no sense to
virtualize.  We run a HPC cluster with hundreds of nodes.  There is no
reason to virtualize it as we are already trying to squeeze every bit
of performance out of the boxes and adding the hypervisor would defeat
that and we don't need/want more than one OS install running on a
physical box.

So really look at your workload and your business case.
Virtualization isn't the end all be all silver bullet that will make
everything in IT perfect.  It is just another tool in your toolbox
that can help in some cases but much as showing up with a hammer and
finding a screw you might be better off going back to get a screw
driver than trying to just pound it in.

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