Lets Design, Implement and do Administration of ESX3

Virtualization with VMWare Infrastructure 3.0

Resource Mgmt Guide -01

Posted by Preetam on March 31, 2007

Resource Management Guide points filtered out. It is must to read resource management guide for VCP. Below are the points I feel are important & gives quick glimpse of Resource mgmt in VI3


The amount of physical memory VMs can used is less than what is present on physical host because Virtualization layer uses some part of it.

 e.g. Physical Host with 3.2 GHz with dual core and 2 GB RAM, will make 6 GHz and 1.5GB RAM to VMs 

Reservation [GHz/MB]:

By default no reservation is specified on CPU and Memory on VMs, however it is good idea to specify reservation for Memory/CPU for VMs, so that resource is always available for VMs.

  • Use reservation to specify minimum amount of cpu/memory, host assigns additional resources as available based on the limits and number of shares. Don’t set reservation too high because it will restrict VM to that resource pool only.
  • Reservations are used by running virtual machines and resource pools with reservations.
  • Reserved and unreserved CPU and Memory change only if the virtual machine is powered on.


E.g. Consider VM with 2GHz reservation, but it is not using this reservation. This reservation cannot be used for reservation for other VMs however this 2GHz can be used by other VMs so the idle CPU resource is not wasted.

 E.g. VM-A and VM-B have 1GHz of CPU reservation, if VM-A is using 500 MHz of CPU, then VM-B can use 1.5 GHz of CPU to use idle resources. 

The ESX server will allocate number of MB reserved for VM directly, anything beyond the reservation is allocated using the host’s physical resources or when physical resources aren’t available, handled using techniques like balloning or swapping.


Limit [CPU/MB]:

By default no limit is specified on CPU/MB, in this case whatever memory/cpu you assign to Virtual Machine becomes its limit.


Each VM is entitled to resources in proportion to its specified shares, bounded by reservations and limits. Shares are typical specified as high, normal and low. They are in the ratio of 4:2:1. If you expect frequent change in the resources, then use shares than reservations to allocate resources fairly across VMs.


  • High — 2000 shares per virtual machines, 20 shares per MB of VM memory
  • Normal — 1000 shares per virtual machines, 10 shares per MB of VM memory
  • Low — 500 shares per virtual machines, 5 shares per MB of VM memory 

In short high means twice as much as normal, normal means twice as much as low. You can also specify custom shares

e.g. VM with 2 Virtual CPU & 1 GB RAM, with shares set to normal for CPU/RAM, then2×1000=2000Ghz shares of CPU and 1024×10=10240 MB memory shares.  

By default all VMs associated with single ESX host received

q       Equal share of CPU per Virtual CPU. E.g. If VM-A has one 1 CPU and VM-B has two CPU, then VM-A will receive half of CPU of VM-B

q       Equal MB of per share of virtual memory size.


When you power on VMs, system checks amount of CPU/Memory unreserved available, depending upon reservation set on VMs, it compares and checks if this VM can be powered on, this process is called Admission Control.



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