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I got a high spec server. (128G memory, 1.2T harddisk and 12-core cpu x 2 )

I would like to know how many KVM instances can work on one server. For example, If 200 instances work on one server, one instance spec is

  • memory: 512M
  • hard disk: 5G
  • cpu: 1 core

In this case, VMs work correctly? I googled and got a IBM report about kvm limitation that was good for me, but I would like to know another view .

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migrated from stackoverflow.com Nov 11 '11 at 10:15

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So funny that the memory and CPU are sure impressive, but the hard drive seems quaint. :) –  sarnold Nov 11 '11 at 4:49

1 Answer 1

up vote 2 down vote accepted

This is never about only numbers. You need to know what your VMs are going to be doing, and if they aren't going to be consuming much CPU, you definitely go overcommit to very high rates (I've seen 1:50 work quite well for that particular environment specific conditions).

The general rule of thumb is to try to avoid giving one VM more cores than there are physical cores available, but otherwise, it's all a matter of the loads within the VMs themselves. What you need to understand is that a VM's CPU is really a thread to the host OS, tended to by the hosts' CPU scheduler.

EDIT: and yes, a VERY valid comment - you need pretty powerful storage for that many VMs, disk IO will probably your bottleneck here and not the phy/virt CPU ratio.

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