Questions tagged [virtualization]

Virtualization is a group of software technologies that allow abstraction between layers of a system. This allows separation between the logical layers of the system, providing isolation, flexibility, and/or the ability to run more than one at a time. This differs from most traditional systems where the various layers are inherently tied.

Virtualization commonly refers to three distinct technologies: Hardware Virtualization, Software Virtualization, and User Experience Virtualization. (Most commonly the first, Hardware Virtualization)

Hardware (sometimes called Operating System virtualization) is the use of software to allow a piece of hardware to run multiple operating system images at the same time. Traditional servers have a 1:1 ratio (One operating system running on one piece of hardware) and virtualization allows 1:many, making efficient use of available hardware. With the use of the hypervisor many operating systems can be run on top of one piece of physical hardware.

There are three main types of hardware virtualization, hypervisor, paravirtualization, and emulation. The bare metal hypervisor, or type 1, itself runs directly on the computer hardware. Hypervisors are generally thought to be enterprise level solutions to virualization as they make the most efficient use of available hardware resources.

Paravirtualization, or type 2, installs on top of a pre-existing operating system. Type 2 solutions are not as efficient because resources are also going to the host operating system, therefore type 2s are possibly better for hobbyist or development. Paravirtualization also requires the guest operating systems to be aware of the virtualization system and be designed to work with it.

Emulation also runs atop an existing system like paravirtualization; unlike its more efficient siblings, every instruction issued by the guest operating system must be interpreted by the emulation system. Emulation is notably less efficient than the other two, however it can enable a guest operating system to run on a host processor that it completely different than it was intended for.

Application Virutalization allows applications, which normally require installation, to run on system where they not actually installed. The virtualization layer simulates the installed prerequisite components, allowing the application to run normally.

There are two main types of User Experience Virtualization: Presentation and Data Location. Presentation Virtualization is commonly implemented by running a program on one system and producing the GUI at another. This may be as simple as a VNC or Remote Desktop Connnection, or a more complicated Virtual Desktop Infrastructure. Data Location Virtualization allows users a consistent view of the logical location of data across multiple distinct systems. The primary advantage of these systems is allowing users to access data in a consistent manor regardless of the physical location of the user or data.

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Difference between KVM and QEMU

I have been reading about KVM and Qemu for sometime. As of now I have a clear understanding of what they do. KVM supports hardware virtualization to provide near native performance to the Guest Operating sytems. On the other hand QEmu emulates the…
Abhishek Gupta
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What does Virtual memory size in top mean?

I am running top to monitor my server performance and 2 of my java processes show virtual memory of up to 800MB-1GB. Is that a bad thing? What does virtual memory mean? And oh btw, I have swap of 1GB and it shows 0% used. So I am confused. Java…
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Why do we use a OS Base Image with Docker if containers have no Guest OS?

I've just started to study Docker and there's something that's being quite confusing for me. As I've read on Docker's website a container is different from a virtual machine. As I understood a container is just a sandbox inside of which an entire…
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How do you increase a KVM guest's disk space?

I setup an Ubuntu guest on a CentOS KVM host with initially 6GB of disk space. How do I go about increasing the Ubuntu guest's disk space from the command line? EDIT #1: I'm using a disk image file (qemu).
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What range of MAC addresses can I safely use for my virtual machines?

I want to assign my virtual machines MAC addresses so that I can configure DHCP reservations for them so that they always get the same IP address regardless of which host hypervisor they are running on or operating system they are running. What I…
Chris Magnuson
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Why do systems generally disable virtualization by default in BIOS settings?

I have yet to see a system whose default configuration enables MMU and directed I/O virtualization. Often this necessitates rebooting and going into the BIOS to enable it if you want, e.g., 64-bit support on your VMs. Is there some kind of…
John Feminella
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Why is TCP accept() performance so bad under Xen?

The rate at which my server can accept() new incoming TCP connections is really bad under Xen. The same test on bare metal hardware shows 3-5x speed ups. How come this is so bad under Xen? Can you tweak Xen to improve performance for new TCP…
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What is the difference between PV and HVM virtualization types in ec2?

AWS EC2 offers two types of virtualization of Ubuntu Linux EC2 machines - PV and HVM. PV: HVM: What is the difference between these types?
Adam Matan
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How do I know if I'm working on a Virtual Machine or not?

Is there a way to know if the Windows machine I'm working on is virtual or physical? (I'm connecting with RDP to the machine. If it's a virtual machine it is working and handled by VMWare).
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What are the drawbacks of running a database inside a virtual machine? How do I overcome them?

Running anything inside a virtual machine will have some level of performance hit, but how much does it really impact the performance of a database system? I found this academic reference paper with some interesting benchmarks, but it was a limited…
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Creating a virtual machine in VirtualBox from a physical one

Is there any way to create a virtual machine that you can use in VirtualBox from a physical installation that you have? For instance, if I have Windows XP installed on a physical computer and want to have a virtual version of that machine on a…
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How to [politely?] tell software vendor they don't know what they're talking about

Not a technical question, but a valid one nonetheless. Scenario: HP ProLiant DL380 Gen 8 with 2 x 8-core Xeon E5-2667 CPUs and 256GB RAM running ESXi 5.5. Eight VMs for a given vendor's system. Four VMs for test, four VMs for production. The four…
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How to zero fill a virtual disk's free space on windows for better compression?

How to zero fill a virtual disk's free space on windows for better compression? I would like a simple open source tool (or at least free) for that. It should probably write an as big as possible file full of 0and erase it afterwards. Only one pass…
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Is virtual machine slower than the underlying physical machine?

This question is quite general, but most specifically I'm interested in knowing if virtual machine running Ubuntu Enterprise Cloud will be any slower than the same physical machine without any virtualization. How much (1%, 5%, 10%)? Did anyone…
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vSphere education - What are the downsides of configuring VMs with *too* much RAM?

VMware memory management seems to be a tricky balancing act. With cluster RAM, Resource Pools, VMware's management techniques (TPS, ballooning, host swapping), in-guest RAM utilization, swapping, reservations, shares and limits, there are a lot of…
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