Datasheet

Chapter 1: Overview of Virtualization
23
storage requirements of those servers, as well as the way in which they use that storage, requiring
additional storage space, changes to how and where swap and paging space is allocated, and so on.
You also need to consider software licensing issues when using virtualization to host multiple user
accounts on a single server or when cloning virtual machines. If you cannot afford site - wide licenses for
software that your organization depends on, the use of flexible licensing schemes such as floating
licenses is critical for servers that host large numbers of users. Another possible licensing problem can be
caused by software vendors who do not support their software in virtualization environments — you
should check your software license to be sure.
Even areas of virtualization such as desktop virtualization are not appropriate for all users. For example,
remote and laptop users will need significantly more local software installed on their systems to
guarantee their ability to get work done without continuous dependencies on remote access to your
centralized systems.
Per - Server Network Congestion
Most full virtual machines use virtual network interfaces, subnets, and bridging packages to map those
virtual interfaces (and virtual networks) to the physical hardware. If your physical host system provides
only a single network interface, running multiple virtual machines that are performing network -
intensive tasks can make too many demands of your physical network hardware. This can result
in network performance problems for a single host or for all hosts that are sharing the same physical
network interface. One obvious solution is to install multiple network interfaces in your physical host
system and assign these to specific virtual machines. Unfortunately, this type of configuration can make
it more complex to migrate these virtual machines from one host to another in order to work around
emerging hardware problems or general performance issues.
Increase in Networking Complexity and Debugging Time
Networking on full virtual machines uses virtual network interfaces, subnets, and bridging software to
make each host appear to have a unique network interface. Using full virtual machines that each have
their own network interface (virtual or physical), IP address, and so on can be more complex than
managing multiple physical hosts with the same characteristics. This is not only because of the extra
level of software that bridging or other approaches to virtual networking require, but also because
firewalls and other network control mechanisms need to be configured to allow specific traffic in specific
directions and between specific virtual hosts.
Many sites set up virtual machines on their own subnets, which require separate DHCP servers in order
to manage the associated ranges of IP addresses. These subnets also simplify firewalling and routing
issues because the firewall or router can easily be configured to handle the blocks of IP addresses
associated with those subnets differently.
Other potential issues that you must take into consideration are possible problems such as hardware
MAC (Media Access Control) address collisions. In the real world, each physical Ethernet device has a
unique MAC address, but most full virtual machines can be assigned specific MAC addresses. Unless you
are careful when cloning and customizing your virtual machines’ configuration data, it is easy to
accidentally bring up multiple hosts whose network interfaces have the same MAC address. This can
c01.indd 23c01.indd 23 12/14/07 3:57:27 PM12/14/07 3:57:27 PM