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Overhead with VM's

Posted on 2010-08-30
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Last Modified: 2012-08-14
Hi

We have an application (Finance1) that we are looking to deploy within our network. As is our current strategy for most things, we will deploy the app on VM ESX 3.5 guests rather than physical boxes.

The vendor has various performance calculators, hardware calculators, storage calculators to gauge the hardware necessary to run Finance1. We enter in data for some fields related to usage (number of users, level of activity they are carrying out etc) and also the hardware specs of the machine (physical memory, CPU types etc) and the calculators will provide info back on whether this would work adequately or not.

Problem is, their calculators are all based on physical servers not virtual ones. I'm going to get back to them and ask them, but in the meantime I was wondering - should this sort of calculator apply to both physical and virtual servers? Or do VM guests' resources have an overhead for running as VM's (e.g. some percentage of resources being eaten up by the host etc) that would make them not accurate?
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Question by:kam_uk
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jlanderson1 earned 100 total points
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The values should be the same for VM's and physical servers.  
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by:Zupreme
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ID: 33560653
The exact same considerations apply whther you are talking about physical or virtual machines, with one caveat:  if this is a critical application make sure that you atually provision those resources as dedication for your VM (as opposed to only making the resources available to the VM) otherwise an activity storm on any other VM hosted on the same box will degrade performance of your critical application.
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by:kam_uk
ID: 33560839
Thanks guys..

Ok - let's say the calculator asks me to enter in the type of processor the server is using. I enter the type of processor used on the ESX host, and since I am only giving the guest 1 CPU, I enter "1" as the number.

Surely, this is not correct? The virtual CPU that I have assigned the guest and the actual CPU of the host cannot be compared in terms of resources to the guest?
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by:Andrew Hancock (VMware vExpert / EE MVE)
Andrew Hancock (VMware vExpert / EE MVE) earned 100 total points
ID: 33560909
The first thing you need to establish, is whether the vendor will SUPPORT the application on a Virtual Platform, VMware, Microsoft, Citrix, Sun etc.

If not you'll then need to make a decision, as too whether to risk this move or not.

Some Vendors I work with at present will NOT SUPPORT their applications on any Virtual Platform. This make is very difficult when their application (DB etc), has faults, because they refuse to support it, unless we can prove the same fault occurs on a physical platform.

Sometimes this is because the vendor does not have any specfic knowledge, training or testing of their product/application on a virtual platform. Ask the vendor if they have any clients that currently run on a virtual platform, and what specs are they using, ask for their telephone number and talk to them about any issues they have!

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by:kam_uk
ID: 33560997
Yep, it's def supported to run on virtual :)
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by:SysExpert
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ID: 33561007
actually, that depends on whether you have multiple processors/cores.

If the core is not shared by any other guest, then the value should be the same whether a VM or Physical since the Host does not use much capacity.

I hope this helps !
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by:ddawson100
ddawson100 earned 100 total points
ID: 33564074
The guest OS isn't going to be aware of whether it's running on physical or virtual hardware so you can use the same formulas when planning.

As you may be installing this guest on a common pool or common host you'll also want to be sure you're not already pegged on performance. Now is the time to check your utilization using the charts or esxtop/resxtop for realtime counters. You'll want to ensure a smooth implementation rather than troubleshoot performance later.

Finally, since your vendor will support this on a virtualization platform they may have guidance on how to reserve resources (for CPU and memory) or target the best disk array (RAID levels and current utilization levels).
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