[CentOS] OT: Why VM?

Fri May 27 19:20:00 UTC 2011
Les Mikesell <lesmikesell at gmail.com>

On 5/27/2011 1:33 PM, James B. Byrne 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.

It is fairly difficult to avoid applications that require specific OS 
versions or conflict with certain other applications, especially as 
things evolve over time.  This means you are likely to end up with 
machines (and their backups) dedicated to specific legacy apps even 
though you could otherwise consolidate them on newer/faster hardware and 
reduce the maintenance/power/space requirements. Putting them on VMs 
lets you separate the physical resource concerns from the applications 
you support.  And in some cases you might set up backup/failover 
instances as VMs even where the normally-live host is a physical machine.

> This might be considered OT but since CentOS is what we use it seems
> to me best that I ask here to start.

Aside from consolidating things that don't want to be consolidated (a 
somewhat odd concept for an OS that should theoretically be able to have 
multiple versions of most libraries and applications running at once but 
yum and rpm won't like it), there is also the issue of moving to 
different hardware if an older machine crashes.  While Centos is pretty 
good about detecting hardware during an install, if you want to restore 
your backup onto something different you'll need to know as much as 
anaconda does to make it work, where a VM image will work the same way 
regardless of the physical hardware.

   Les Mikesell
    lesmikesell at gmail.com