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What is a Virtual Machine?

Written by . Posted in What is

A virtual machine is a software computer that, like a physical computer, runs an operating system and applications. An operating system installed on a virtual machine is called a guest operating system.

Every virtual machine has virtual devices that provides the same functionality as physical hardware. Virtual machines get CPU and memory, access to storage, and network connectivity from the hosts they run on.

Virtual machines run on a host or cluster. Muliple virtual machines can run on the same host or cluster at the same time.


Benefits of Virtual Machine

A physical machine is hard to move. Difficult to copy, and bound to a specific set of hardware. Virtual machines are easy to copy and move because they are indepented of physical hardware. Also they are easy to manage because they are isolated from other virtual machines running on the same physical hardware and insulated from physical-hardware changes.

Virtual Machines enhance infrastructure by providing:


Freedom from physical hardware constraints
Virtual machines allow the operation of heterogeneous operating systems running across heterogeneous hardware.

Backup and recovery with little or no down-time
You can configure virtual machines with operating systems and applications once and then clone them many times. Backing up a virtual machine is as easy as backing up a few files. In this way, virtual machines ensure fast deployment and reliability.

Greater resource utilization
Muliple virtual machines can run on the same physical server. In addition, consolidating computing power to fewer physical computers cansubstantially increase power savings in your enterprise.


What Files Make Up a Virtual Machine in Vmware?


A virtual machine typically is stored on the host computer in a set of files, usually in a directory created by ESX for that specific virtual machine.


The key files are listed here by extension. In these examples, <vmname> is the name of your virtual machine



File Name Description




This is the file that keeps a log of key VMware Workstation activity. This file can be useful in troubleshooting if you encounter problems. This file is stored in the directory that holds the configuration (.vmx) file of the virtual machine.




This is the file that stores the state of the virtual machine's BIOS.
.vmdk <vmname>.vmdk

This is a virtual disk file, which stores the contents of the virtual machine's hard disk drive.

A virtual disk is made up of one or more .vmdk files. If you have specified that the virtual disk should be split into 2GB chunks, the number of .vmdk files depends on the size of the virtual disk. As data is added to a virtual disk, the .vmdk files grow in size, to a maximum of 2GB each. (If you specify that all space should be allocated when you create the disk, these files start at the maximum size and do not grow.) Almost all of a .vmdk file's content is the virtual machine's data, with a small portion allotted to virtual machine overhead.

If the virtual machine is connected directly to a physical disk, rather than to a virtual disk, the .vmdk file stores information about the partitions the virtual machine is allowed to access.

Earlier VMware products used the extension .dsk for virtual disk files.

<diskname>-<###>.vmdk This is a redo-log file, created automatically when a virtual machine has one or more snapshots. This file stores changes made to a virtual disk while the virtual machine is running. There may be more than one such file. The ### indicates a unique suffix added automatically by VMware Workstation to avoid duplicate file names.
.vmem <uuid>.vmem The virtual machine's paging file, which backs up the guest main memory on the host file system. This file exists only when the virtual machine is running, or if the virtual machine has crashed.
<snapshot_name_and_number> Each snapshot of a virtual machine that is powered on has an associated .vmem file, which contains the guest's main memory, saved as part of the snapshot.
.vmsd <vmname>.vmsd This is a centralized file for storing information and metadata about snapshots.
.vmsn <vmname>-Snapshot.vmsn This is the snapshot state file, which stores the running state of a virtual machine at the time you take that snapshot
<vmname>-Snapshot<###>.vmsn This is the file which stores the state of a snapshot
.vmss <vmname>.vmss

This is the suspended state file, which stores the state of a suspended virtual machine. Some earlier VMware products used the extension .std for suspended state files

.vmtm <vmname>.vmtm This is the configuration file containing team data.
.vmx <vmname>.vmx This is the primary configuration file, which stores settings chosen in the New Virtual Machine Wizard or virtual machine settings editor. If you created the virtual machine under an earlier version of VMware Workstation on a Linux host, this file may have a .cfg extension
.vmxf <vmname>.vmxf This is a supplemental configuration file for virtual machines that are in a team. Note that the .vmxf file remains if a virtual machine is removed from the team.


Source: Vmware