What is a virtual hard drive (VHD)?

Question

I will hear the term "virtual hard drives" (VHD) used and read articles (such as Lifehacker) mentioning them, but I have long remained in the dark, so to speak, as to what that term really means?

Questions/ Points of clarification:

• What are VHDs exactly?
• What purpose/ advantages do they have?
• Do they slow down or speed up performance? (read/write speeds, caching, etc)
• How are they better or worse than traditional/solid state hard drives?

Clarification/explanation on any of these points would be very helpful! Thanks!

Answer 1

A very basic way to get the idea of virtual hard disks is to remember that two computers cannot share the same physical hard drive. So, they are essential for virtual computers to work, as two machines cannot share the same low level access hard drives - they need their own.

A physical host computer will only be able to access any guest OS's virtual hard drives through network, just like another computer on the LAN, even though that virtual hard disk resides on one of its own physical drives.

There are no performance advantages, as they can never be faster than the physical disks they are on. An advantage that they do have is that they are portable, being easy to transfer to another physical disk or machine.

.VHD is Microsoft's format for virtual drives and .VMDK is VMwares. It's quite easy to convert between different formats.

You can mount .VHDs in Windows 7 and Server 2008 R2 Disk Management, and you can even boot physical computers from .VHD disks, which I have been finding useful for old OS installations, and as a neater alternative to dual-booting.

Answer 2

I don't know where you got the acronym VHM from "Virtual Hard Drive", but typically a virtual disk is coupled with a virtual machine as a simulated hard disk. It is essentially just a big file (or multiple split files) that act as a hard drive for a VM. The advantage is that the virtual environment can be easily moved to different machines and still run. The performance will typically be slower than using a physical disk itself because of the overhead involved, and the disk may also have to take care of operations on the host OS as well (depending on your setup). I think that answers the question regarding solid state drives as well.

For further information: VHD file format

That covers Microsoft's vhd format, but the principals are similar for vmware and virtualbox.