What is virtual disk?

Each virtual machine created with virtualization software acts as a different computer with its own hardware. This includes own virtual motherboard, video adaptor, network adaptors and disk storage devices. Virtualization products support use of real physical drives to store information of virtal machine, however it is not recommended because it brings portability problems as well as possible conflicts with host OS (the OS, running virtualization software).

Since a virtual machine acts like a real computer, it requires the guest OS (the OS run on a virtual machine) and specific software to be installed on it. To isolate this information from a host OS, virtualization software uses so-called virtual disks.

Virtual disk - is a large file located on a host OS that contains all information about disk data of a guest OS. These files could be plain bit-to-bit disk images (flat disks) or optimized by size sparse disks. The latter will contain only information about used fragments of the disk.


How to access the data?

The natural way to exchange information between host OS and guest OS is to run both of them and use virtual network transport for it. Virtualization software may often install transport wrapper software for major guest OS to allow files exchange with simple drag-and-drop.

However, there is a number of situations where it may not work or require more time:

• Try to get data from historical virtual machine snapshot:
  - running of virtual machine is not recommended so as not to modify snapshot. It requires to copy virtual disk with new virtual machine setup and boot.
• Virtual machine utilities are not installed for some reason:
  - reason could be either virtual machine isolation or utilities are not available for a guest OS.
• No specific networking protocols installed on guest OS:
  - virtual machine isolation does not allow file transfer.
• File size limit:
  - the software may have problems to copy very large files from guest OS to host OS.

As required files are already stored as a part of virtual disk file on host computer, the logical solution is just to 'extract' these files from a virtual disk. The file extraction will work much faster than entire virtual disk transfer, virtual OS setup or even virtual OS boot.

Our software products (like UFS Explorer Standard Access) can open such virtual disk files, browse files and folders on them and copy files out of virtual disk to host OS storage. The products support both flat and sparse virtual disks as well as virtual disk formats of major virtualization software vendors.


Specifics

Some virtualization products use specific virtual disk storage schemas for optimization and compatibility with a host OS. These schemas could make process of data extraction from virtual disks more difficult.

Chunked virtual disks - was 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), no more 2GB each. Yet UFS Explorer products can read such chunked virtual disks, however it requires assembly of chunks to single virtual image. It works in UFS Explorer Professional Recovery through RAID creation tool as well as can be enabled for all UFS Explorer version 4 products with RAID Access Plugin - RAID Builder.

Virtual machine package - was introduced by Parallels for their Paralles Desktop for Mac product. The virtual disk is packed into single file along with its snapshots etc. To get data from this kind of a virtual disk you have 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 'empty' (blank). To access file from such virtual disk, the file should be transfered from ESX server to 'local' storage first.