Information Technology Reference
In-Depth Information
IaaS generally provides the widest variety of options. PaaS generally obscures what is
used,astheuserworksinaframeworkthathidesthedistinction. Thatsaid,mostPaaSpro-
viders use containers.
3.2.1 Physical Machines
A physical machine is a traditional computer with one or more CPUs, and subsystems for
memory,disk,andnetwork.Theseresourcesarecontrolledbytheoperatingsystem,whose
job it is to act as the traffic cop coordinating all the processes that want to share these re-
sources. The resources allocated to a running process (a program running on the system)
are actual hardware resources. As a result their performance is relatively predictable.
3.2.2 Virtual Machines
Virtual machines are created when a physical machine is partitioned to run a separate op-
erating system for each partition. Processes running on a virtual machine have little or no
awareness that they are not on a physical machine. They cannot access the resources, such
as disk or memory, of other virtual machines running on the same physical machine.
Virtual machines can make computing more efficient. Physical machines today are so
fastandpowerfulthatsomeapplicationsdonotneedthefullresourcesofasinglemachine.
The excess capacity is called stranded capacity because it is unusable in its current form.
Sharing a large physical machine's power among many smaller virtual machines helps re-
duce stranded capacity by permitting the creation of virtual machines that are the right size
for their requirements.
Stranded capacity can also be mitigated by running multiple servers on the same ma-
chine. However, virtualization provides better isolation than simple multitasking.
For example, when two applications share a machine, when one application gets over-
loaded or has a problem that causes it to consume large amounts of CPU, disk space, or
memory, it will affect the performance of the other application. Now suppose those two
programs each ran on their own virtual machines, each with a certain amount of CPU, disk
space, and memory allocated. This arrangement provides better isolation for each applica-
tion from problems caused by the other.
Sometimes the reason for using virtual machines is organizational. Different depart-
ments within an organization may not trust each other enough or have sufficient cross-de-
partmentbillingoptionstorunsoftwareonthesamemachine.Nevertheless,theycanshare
a pool of physical machines if each is able to create its own virtual machine.
Sometimes the reason for using virtual machines is logistical. Running five services on
one machine requires that any OS patches or upgrades be approved by all five services. If
eachservicerunsinitsownvirtualmachine,thenupgradesandpatchescanbedoneondif-
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