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Virtualization software: the virtual disks


Offering an opportunity to increase efficiency in computer use virtualization technology gained wide popularity recently. The technology of virtual machines suggests the idea of creating a fully operational isolated system within the main system based on the resources of the same hardware.

Nowadays virtual machines are available not only for corporate but also for home use. They allow users to leverage hardware resources and are quite reliable in back-up without affecting 'home' system in case of failures.

What is virtual disk?

Virtual machine is special software emulating operation of a physical machine. A virtual machine is capable of running its own operating system and applications isolatedly from the physical one.

Being based on the resources of a physical machine, virtual machine remains completely independent and uses its own software-based components (CPU, motherboard, video adapter, network adapters and hard disks) which may differ from that of the 'real' machine it is installed on. However, virtual machine is unable go beyond the limits of its virtuality and should be located within real physical machine and resources of its operating system. The operating system where the virtual machine is installed on is called host OS, while the operating system of the virtual machine is referred to as guest OS.

Virtual disk is a file located on a host OS containing all information about data of a guest OS and is used to isolate virtual machine from the physical one. More than one virtual disk can operate simultaneously on a single physical drive, each isolated from one another. A virtual disk may be a part of a virtual machine or may be mounted on the OS of a physical machine.

How to access the data?

A usual way to exchange information between host OS and guest OS is to run both of them and use virtual network transport. Virtualization software often offers transport wrapper software for guest OS allowing files exchange with simple drag-and-drop procedure.

However, data exchange may require much time or be simply impossible in the following situations:

  • Getting data from a historical virtual machine snapshot:
    - running virtual machine is not recommended in order not to modify the snapshot. It's required to copy the virtual disk with setup and boot of a new virtual machine.

  • Utilities of the virtual machine are not installed for some reason:
    - either because of isolation of the virtual machine or non-availability of utilities for guest OS.

  • No special networking protocols installed on the guest OS:
    - isolation of the virtual machine does not allow file transfer.

  • File size limit:
    - the software may have problems to copy very large files from guest OS to host OS.

Yet, there is a better solution of files exchange between the guest and host OS much faster and fluently. As required files are already stored on host computer inside the virtual disk, it's possible just to extract these files from a virtual disk at a logical level.

We offer UFS Explorer Standard Access as a perfect solution to open such virtual disk files, browse files and folders on them and copy files out of virtual disk to host OS storage. In case of data loss from a virtual machine our data recovery products help to get your data back with maximum possible result. All products support both flat and sparse virtual disks as well as virtual disk formats of major virtualization software vendors.


Some virtualization products implement specific techniques for virtual disks for optimization and compatibility with a host OS complicating data extraction from virtual disks. Among them are:

Chunked virtual disks introduced by VMware to create FAT-compatible virtual disks. Since FAT has limitations to file size, VMware products support splitting of virtual disk to fragments (chunks) not larger than 2GB each. To read such chunked virtual disks assembling chunks into one virtual image is required. The function is available in UFS Explorer Professional Recovery and UFS Explorer RAID Recovery through tools for RAID-building or via RAID Access Plugin - RAID Builder for all standard UFS Explorer software.

Virtual machine package introduced by Parallels for their Paralles Desktop for Mac product. Virtual disk is packed into single file along with its snapshots. To get data from this kind of a virtual disk one has to extract it from package first.

ESX Server Storage virtual disks could be shared over network from VMware ESX Server, however, known issue of file sharing software for ESX is that virtual disk files are often shared as blank. To access file from such virtual disk, the file should be transfered from ESX server to 'local' storage first.