Information Technology Reference
In-Depth Information
ferent schedules for different services. In all these cases, virtual machines permit isolation
at the OS level.
Benefits of Virtual Machines
Virtual machines are fast to create and destroy. There is very little lead time between when
one is requested and when the virtual machine is usable. Some systems can spin up a new
virtual machine in less than a minute. Consequently, it is easy to create a virtual machine
foraspecifictaskanddeleteitwhenthetaskiscompleted.Suchavirtualmachineiscalled
an ephemeral machine or short-lived machine. Some systems create hundreds of ephem-
eralmachinesacrossmanyphysicalmachines,runaparallelcomputejob,andthendestroy
the machines when the job is complete.
Because virtual machines are controlled through software, virtualization systems are
programmable. An API can be used to create, start, stop, modify, and destroy virtual ma-
chines. Software can be written to orchestrate these functions on a large scale. This is
not possible with physical machines, which have to be racked, cabled, and configured via
manual labor.
Virtual machine functionality is provided by a combination of virtualization support
in modern CPUs and virtualization control software called the virtual machine monitor
(VMM).ModernCPUshavespecialfeaturesforpartitioningmemoryandCPUtimetocre-
ate virtual machines. Access to disk, network, and other I/O devices is handled by emula-
tion at the chip or device level.
Some virtualization systems permit a virtual machine to be moved between physical
machines. Like a laptop that is put to sleep and woken uplater,the VMMputs the machine
to sleep, copies its memory and all other state to a different physical machine, and contin-
ues its activities there. The process can be coordinated so that most of the copying happens
ahead of time, so that the machine freeze lasts for less than a second. This permits a vir-
tual machine to be moved to a different physical machine when the current one needs to
be upgraded or repaired, or moved to a different failure domain in advance of a planned
maintenance outage.
Search WWH ::




Custom Search