Virtual Machines are Insulated from Hardware Changes

[Ranjna Aggarwal]

In one statement we can say that Virtual Machines are Insulated from Hardware Changes and other statement says only that for example we have 4 LCPU on host then to a vm we can maximum allocate 4 vCPU. Then How they come to know that these number of CPU's are available at the host level?

[weinstein5]

It is not so much the VM knows but the vmkernel is aware of the number of resources iit has available to schedule the vcpu to  run on - so it is the vmkernel that will prevent assigining more than 4 vcpus -

[rickardnobel]

Ranjna Aggarwal wrote:

In one statement we can say that Virtual Machines are Insulated from Hardware Changes and other statement says only that for example we have 4 LCPU on host then to a vm we can maximum allocate 4 vCPU.

As weinstein noted that number of vCPUs could not be higher than the amount of logical CPUs available, since it would be impossible to schedule them all if so.

For the first statement it means basically that the VM is always runnings the same Virtual Hardware, meaning it would not matter if you change the type of physical NIC or physical SCSI controller or any other hardware device. This will also be true when vMotion a VM from one type of server to another, like from a HP to a IBM server (as long as the CPUs are compatible on feature level too.)

[admin]

Ranjna Aggarwal wrote:

In one statement we can say that Virtual Machines are Insulated from Hardware Changes...

You can say this, but it wouldn't be entirely accurate.  Install bad memory, and your virtual machines are not going to be insulated from the effects.  Replace a 1GB NIC serving your VM Network with a 100MB NIC, and network bandwidth in your virtual machines will be impacted.  Replace a 1.8GHz Xeon with a comparable, but faster, 2.6GHz Xeon, and your virtual machine's CPU performance is going to improve.

[sparrowangelste]

Thanks jmattson, you make an excellent point.

One of the things that are said about virtual machines are that they are hardware independent, but at the same time, with the cases you have presented, they truely arent 100% independent from the host..

[AKostur]

Depends to what level you're looking at.  The VMs are insulated from hardware changes in such things like if you changed the 1 GBit ethernet for 10 GBit ethernet, the VM doesn't see a change in the hardware.  It's still seeing the same Intel E100 card (or VMXnet3, or whatever the virtualized card you've presented is), only that it is now magically faster.   Same if you changed the video card.  Your VM doesn't really know that the hardware has changed (it's still a VMware SVGA II card).  Or that you've moved your virtual disk from a 5400 rpm disk to an SSD.  The VM still sees it as the same hardware, just that it is now faster.  (Or slower if you go the other way).  CPU is one of the few things where certain parts of the CPU hardware may leak through into the VMs perception, and even some of that is erased by using EVC modes in vSphere.

I'm not sure that most people would want to try to achieve this "100% independance" of the VM.  Most people want sufficent independance such that when swapping around the physical hardware, the OS inside doesn't need to reconfigure itself to deal with the new hardware.  It's kinda one of the advantages of virtualization in that you can upgrade the underlying hardware for faster resources, and you have to do nothing with the VM to take advantage of the new speeds.

(This, of course, does not consider the pass-through features where the admin is intentionally piercing the virtualization layer to present actual physical hardware to the VM.  Of course if that hardware changes, the VM will notice.)

[TomHowarth]

Hardware independance relates to the standard driver set that is in a guest,  for example you could have a Broadcom NIC on the ESX Host but you guest may be utilsing a E1000 driver.

this is hardware independance.

[admin]

Tom Howarth wrote:

Hardware independance relates to the standard driver set that is in a guest,  for example you could have a Broadcom NIC on the ESX Host but you guest may be utilsing a E1000 driver.

this is hardware independance.

Right.  The phrase "hardware independence" has a very specific meaning, which falls far short of "insulation from hardware changes."

[sparrowangelste]

Thanks for the clarification guys

How does this sound: just to further anyone else's udnerstanding and confirm my own thoughts.

Hardware independence relates to our vms  in the aspect that virtual machines are able to utilize the virtual  machine hardware without awareness of of the specifics of the underlying  host hardware.  As long as a 1 GB hardware nic is on the HCL, the vm  isn't concerned with what nic the physical nic is.

Hardware isolation relates  to a vm being free from the effects of hardware problems or changes. If  a host has a bad memory module, the vm would be affected. So it is not  hardware isolated..