Operating system-level virtualization imposes little overhead as guest programs use the host OS interface without emulation. However, it is not as flexible as other approaches as it can only host the same OS as the host and not different OSes. Some OS virtualizers provide file-level copy-on-write to back up data more efficiently than block-level schemes used by whole system virtualizers. Restrictions are placed on containers to prevent modifying the kernel or accessing certain resources. Virtualization has matured and is used for server consolidation, operational agility through live migration, high availability, and improving responsiveness of IT operations.

1. 4. EVALUATION
Overhead
Virtualization usually imposes little or no overhead, because programs in
virtual partition use the operating system's normal system call interface and do not need
to be subject to emulation or run in an intermediate virtual machine, as is the case with
whole-system virtualizers (such as VMware andQEMU) or para-virtualizers (such as Xen
and UML). It also does not require hardware assistance to perform efficiently.
Flexibility
Operating system-level virtualization is not as flexible as other
virtualization approaches since it cannot host a guest operating system different from the
host one, or a different guest kernel. For example, with Linux, different distributions are
fine, but other OS such as Windows cannot be hosted. This limitation is partially
overcome in Solaris Containers by its branded zones feature, which provides the ability to
run an environment within a container that emulates a Linux 2.4-based release or an older
Solaris release.
Storage
Some operating-system virtualizers provide file-level copy-on-write
mechanisms. (Most commonly, a standard file system is shared between partitions, and
partitions which change the files automatically create their own copies.) This is easier to
back up, more space-efficient and simpler to cache than the block-level copy-on-write
schemes common on whole-system virtualizers. Whole-system virtualizers, however,
can work with non-native file systems and create and roll back snapshots of the entire
system state.
Restrictions inside the container
The following actions are often prohibited:
• Modifying the running kernel by direct access and loading kernel modules.
• Mounting and dismounting file systems.
• Creating device nodes.

2. • Accessing raw, divert, or routing sockets.
• Modifying kernel runtime parameters, such as most sysctl settings.
• Changing secure level-related file flags.
• Accessing network resources not associated with the container.
5. Conclusion and Future Work
Virtualization technologies have matured to the point where the technology
is being deployed across a wide range of platforms and environments. The usage of
virtualization has gone beyond increasing the utilization of infrastructure, to areas like
data replication and data protection. This white paper looks at the continuing evolution of
virtualization, its potential, some tips on optimizing virtualization as well as how to
future proof the technology.
After all, server virtualization's value is well-established. Many, many
companies have migrated significant percentages of their servers to virtual machines
hosted on larger servers, gaining benefits in hardware utilization, energy use, and data
centre space. And those companies that haven't done so thus far are hatching plans to
consolidate their servers in the future. These are all capital or infrastructure costs, though.
What does server virtualization do for human costs—the IT operations piece of the
puzzle?
Base level server consolidation offers a few benefits for IT operations. It
makes hardware maintenance much easier, since virtual machines can be moved to other
physical servers when it's time to maintain or repair the original server. This moves
hardware maintenance from a weekend and late night effort to a part of the regular
business day—certainly a great convenience.
The next step for most companies is to leverage the portability of virtual
machines to achieve IT operational agility. Because virtual machines are captured in disk
images, they are portable, no longer bound to an individual physical server. The ease of
reproducing virtual images means that application capacity can be easily dialed up and
down with the creation or tear down of additional virtual images. Server pooling allows
virtual machines to be automatically migrated according to application load. More
sophisticated virtualization uses include high availability, where virtual machines can be
moved—automatically, by the virtualization management software itself & mdash: when
hardware failures occur. Seeing the magic of a virtual machine automatically being
brought up on a new server after its original host is brought down, all without any human
intervention vividly demonstrates the power of more sophisticated virtualization use.

3. Certainly these kinds of uses of virtualization demonstrate its power to
transform IT operations, enabling IT organizations to offer the kind of responsiveness
and creativity that could only be dreamed of a few years ago. The deftness with which
applications can be migrated, upsized, down sized, cloned, etc. is something that will
forever change the way IT does its job.