Physical vs Virtual Machine in a Guest OS's perspective

[bluegene]

Hi,

I have a Windows Server 2003 installed on a Virtual Machine running on VMware ESX server. My question is, in what ways a virtual environment affect the functionalities of the guest OS paricularly Windows Server? For example, disk defragmentation is a good functionality available in Windows to reclaim disk space from a fragmented physical disk but it is no longer relelvant in Virtual environment. I'm looking for a list of various functionalities or activites in Windows that can/can't be done in a physical machine now can't/can be done in a VM.

[VMmatty]

The most common things that a VM can't do that a physical server can relate to connecting to physical hardware resources. You can't connect directly to USB devices, certain PCI cards like fax controllers, etc. VMs also have limited graphics abilities compared to physical servers with graphics cards.

Disk defragmentation is still relevant in a virtual machine. The thing to remember is that when you defragment the disk you will potentially affect performance of all VMs in the same datastore.

As for what can be done in a virtual machine, the list is pretty long. Easily backup entire servers as a single file, use VMotion to move running VMs to different hardware, take snapshots to easily test and undo changes, just to name a few.

[AndreTheGiant]

A big difference in a VM could be (v)RAM:

In physical machine RAM is always RAM and Virtual Memory could be RAM or disk (swap/pagefile).

In a VM, using memory overcommit in a very memory load host, the (v)RAM could be host RAM or swap file!

For this reason is very important look for RAM usage, and using in a right way resource management (shares, limits and reservation).

Andre

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[mnasir · Accepted solution]

Let me try to answer your question in a more practical manner, what I mean by practical manner is what you will come across in your day to day management of Virtual servers vs. physical servers.

Disk:

Physical: Physical servers usually uses scsci disk in raid configuration, so adding space or expanding drive is a nightmare.

VM: If you are using vmdk files, then you can actually extend the disk on the fly, given you have configured your data drives as dynamic disks. Yes, disk defragmentation is still happening on the os level, you should run jkdefrag on the virtual machine disk volume to maintain optimal disk performance.

Ram:

Physical VS virtual, I don’t see any major difference for the guest OS, but I have seen windows require allot less memory to run smoothly on virtual machines.

Page File: You should do the same thing in a virtual machine to optimize page file usage that you will do in a physical machine, I.e.: move the page file to a separate disk from the core OS partition.

Only thing that was an issue when I was vrtualizing my datacenter was servers that have external peripherals, like, serial connection or RJ11 connection (like my paging server). My rule of thumb is, if it all ip based, virtualize it.

Performance: My experience is that windows run smoother, faster and more efficiently in a virtual environment. My reboot time in virtual servers are anywhere between 9 to 13 seconds vs. 3 to 4 minutes in physical servers. Also, make sure to provide single processor, since most of your app may not need dual processor.

If you need me to address any specific question, please free to drop me an email. I try to address it to best my ability.

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