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Following the idea at What are the benefits of using a proxy? I've used VMs personally, but can't seem to understand why it exists for any real world need. So I ask: why are virtual machines needed?

I can think two things. Testing compatibility support for different operating systems and 'isolation', a quarantine for someone to reverse engineer a virus or something. But are these really enough to justify the multiple projects dedicated to making good VM software? Where is the demand for these tools coming from?

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marked as duplicate by Ƭᴇcʜιᴇ007, Excellll, James, Heptite, Moses Dec 31 '13 at 0:32

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There are lots of uses for virtualization of hardware. Being able to on the fly, configure a machine, that can be configured in basically infite number of different combinations has several dozen uses including being able to use said virtualized hardware as a server( VPS ). –  Ramhound Dec 30 '13 at 19:34
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One advantage overlooked here is Disaster Recovery. If a server fails, you can shift backups of its VMs to another server, or to the repaired server once fixed, and be up and running much faster. When you are extremely budget-limited, this can be highly useful, as well as when physical disasters strike and you HAVE to use different hardware.

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I agree that this is easier with VMs, but if the service is important then you should already have a way to recover. Even without using virtualisation. –  Hennes Dec 30 '13 at 19:47
    
"Easily" as long as you have a backup of the VM someplace that isn't on the now-dead host. ;) –  Ƭᴇcʜιᴇ007 Dec 30 '13 at 19:50
    
@Hennes That doesn't make any sense. If the service is important, uses virtualization, and your recovery method involves that virtualization, why should you already have a way to recover that doesn't use virtualization?! Are you saying that one way to recover that uses virtualization isn't enough for some reason and you should also have one that doesn't? Why the duplication? –  David Schwartz Dec 30 '13 at 20:04
    
I tried to say that if your service is important then you have a way of recovery. Either via visualization or some other way. E.g. by having a spare host or a properly written (and tested) document for reinstalling on new hardware. –  Hennes Dec 30 '13 at 20:22
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A couple years ago we converted most of our servers to virtual machines. Most of our servers were either running idle most of the time but needed, or not using a lot of CPU/RAM (license servers, AD servers, etc.). We were able to reduce the number of physical systems running. This lowers chance of hardware failure as well as the energy cost to run it.

Legacy software that requires very specific operating systems and cannot be migrated to more modern operating systems can be thrown to a VM. The old software can then outlive its physical system, and it's easier to secure since there are additional layers between the VM and the network.

One thing most virtualization software does is let you take snapshots of VMs. If a system later becomes infected or otherwise experiences some sort of software failure, you can revert to the earlier snapshot and recover very quickly from it. This lowers costs and provides some inbuilt protection against unexpected events.

If you provision hardware to run more VMs than what you need currently, then provisioning new servers for whatever purpose is needed is quick and easy.

Since VMs are not tied to any specific hardware, its very easy to move them around should the need arise. So, if you outgrow your physical hardware, migrating it to more powerful hardware is easy and won't disrupt anything installed within. With the proper hardware you can do this without powering off any VMs. Things like live failover and such are also possible.

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