Virtual Machine

Virtual machine (VM) can be defined as a software based emulation of a computer program or whole computer system, emulates to provide best possible substitute for the required program, operating system or a particular real machine with all necessary controls required for execution.

Classification

Virtual machines can be classified as "Full Virtualization" and "Partial Virtualization" considering the controls and functionality towards real machine. Full Virtualization VMs provides complete emulation for required Operating System or machine while Partial Virtualization is designed to provide emulation for a single software program.

A program that manages Virtual Machines is called "Hypervisor"

Hypervisor

A Hypervisor (virtual machine monitor (VMM)) is a piece of software programmed to support multiple operating systems on a single hardware. Hypervisors provide emulation in such a way that multiple operating systems appear to have its own memory, processor and other hardware resources.

A computer on which a hypervisor is running is called as a "host machine".

And

Virtual machine running by a hypervisor is called a "guest machine".

Hypervisor types

Based on the control over hardware they are classified as type 1 hypervisor and type 2 hypervisor.

Type-1 Hypervisor

These hypervisors are directly placed over host's hardware and then guest operating systems are placed over the hypervisor layer. The hypervisor layer controls both the hardware and guest operating systems. Because they are directly installed over the system hardware they are often called bare metal hypervisors.

Few examples of Type-1 Hypervisor that are in cope with current technology are:

  • Microsoft Hyper-V 2008/2012
  • VMware ESX/ESXi
  • Citrix XenServer
  • Oracle VM Server for SPARC

Type-1 Hypervisors are also termed as "Bare Metal Hypervisors".

Type-2 Hypervisor

Type-2 Hypervisors are installed on host operating system unlike Type-1 Hypervisors which are installed directly on host hardware. Type-2 Hypervisors are installed and run like other software programs on the Operating System. They are easy to operate, handle and manage but they lack in performance as compared to Type-1 Hypervisor.

Few examples of Type-2 Hypervisor that are in cope with current technology are:

  • QEMU
  • VMware Workstation
  • Virtual Box
  • VMware Player

Type-2 Hypervisors are also called "Hosted Hypervisors".

Microsoft Hyper-V and Virtual Machine Files

Hyper-V or Windows Server Virtualization is a virtualization product developed by Microsoft for virtualization of their windows machines. Hyper-V comes preinstalled starting with Windows 8, Server 2008 and versions above.

Hyper-V machine files

Hyper-V XML file: XML file contains the configuration details of virtual machine such as location of snapshots.

Hyper-V BIN file: The file is located inside the globally unique identifier folder and corresponds to the memory of a virtual machine.

Hyper-V VSV file: like bin file it is also located inside GUID folder and corresponds to the state of devises of virtual machine.

VHD & VHDX file: These files are main storage units of virtual machine and works same as a hard disk to virtual machine. These files interact the most to Hyper-v while virtualization proceass, data is read out and write in all the time from these disks and thus have maximum possibility towards corruption, a small shortage in RAM can lead to corruption and makes the drive unreadable.

Hyper-V AVHD file: These are the snapshots files of Hyper-v virtual machine and located in the same path of vhd.

"Bin file" and "VSV file" are present when virtual machines are running.

How to fix a corrupted VHD/VHDX file?

Vhdx and Vhd files are the main data storage blocks and consequently more prone to corruption. To repair a corrupted or damaged vhd/vhdx files we have to take a look for corrupt or damaged sector. The architecture of Hyper-V .vhd file consists of header, footer, block allocation table and data blocks. Headers and footers stores information such as size and geometry. Data blocks stores raw data and user data files with them and block allocation tables are required by dynamic and differentiating disks to map the offsets.

If the footer of a dynamic .Vhd or .vhdx file is corrupt and Header is unaffected then Hyper-V can automatically recover it by performing an auto repair over the .vhd file. But if footer and header both are corrupt or in case of a fixed vhd or vhdx file (there is no mirror of footer in fixed vhds) there are no manual methods to recover them the only way to recover them is to take help of VHD or VHDx repair software. The VHD or VHDx repair software will allow you to mount your corrupted .vhd or .vhdx file and allows you to explore and retrieve the files, documents from the data blocks.

Header of a Vhd is mirror image of its footer.

If incase data block is corrupt, the virtual machine boots without any error but there is no data inside of virtual machine then we can take help of a data recovery software and run it inside a virtual machine or can take help of a VHD or VHDx recovery software.

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The software is unbelievable, My Hyper-V virtual machine was unable to boot .vhdx file, shows error message again and again I was getting frustrated then I tried to mount it on VHDX recovery tool and it works, software successfully recovered my important files.

- Jordan Miller, Belgium