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I am working on an infrastructure refresh for the company I work for and I need to do capacity planning. My boss felt somewhat burned by VMs he purchased from a hosting company not living up to the performance level expected, so any capacity numbers for KVM VMs that I can use as a basis for my calculations are appreciated.

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capacity-planning would be a good tag for this but I don't have the rep to create it. –  Larry Smithmier Apr 27 '11 at 13:47
    
Have you checked this serverfault.com/questions/61414/… –  desgua Apr 27 '11 at 14:02
    
I have not, but the link to 'The Art of Capacity Planning' is a good one and I will be taking a look at it. –  Larry Smithmier Apr 27 '11 at 14:14

2 Answers 2

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It depends on how busy they will be and how much ram you plan on allocating to them. For a basic setup, I would consider 1-2 gb for each vm, so you should be able to run 10-20 if they aren't heavily cpu bound.

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Thanks! They aren't CPU bound and actually don't have much external traffic load. The 1-2 GB Ram was the missing piece. –  Larry Smithmier Apr 27 '11 at 16:36

The art of capacity-planning is not an exact science - it does depend on several inputs - chief amongst which are:

  1. What OS are you intending to run
  2. What will the OS be running 50% of the time
  3. What is the expected memory usage for that 50%
  4. What is the expected CPU usage for that 50%
  5. How much Host RAM
  6. How much Host CPU - roughly cores multiplied by number of processors

The target you should be looking at is roughly 75% CPU & Memory Usage for all VM instances.

KVM does have fairly good memory management and as such you can overallocate [balloon] memory that would be made available by the host.

Red-Hat/Cent OS have a similar feature as VMWare ESX where KVM can page in/page out memory segments shared by different OS's - hence you can squeeze even more VMs.

Thus - lets assume you are running 11.04 desktop for each VM instance. The majority of the work is users using Firefox & LibreOffice - taking up, say half of the required memory (Ubuntu minimum is a recommended 1GB), but little in the way of CPU usage. I would expect (wet finger in the air) for a 24GB host to be able to run 32 VM's. On Red-Hat/Cent OS - you perhaps could increase this to 48 VM's.

Again - not an exact science, but not knowing what the chief areas (1 to 6 above) are, just an educated guess.

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