Hardware or OS virtualization software for servers? Server virtualization: the capability of partitioning a physical server into smaller virtual servers.

Server virtualization has recently been gaining greater attention by enterprise IT professionals. There's a good chance that you have come across someone using, promoting or writing about the wares of consolidating servers or adding virtualization capabilities to their enterprise. What has been missing from the conversation is the fact that there are generally two different approaches to server virtualization: hardware emulation (VMware and Microsoft) and OS partitioning (such as Virtuozzo and VServer). Each technology has its advantages, and each was developed to address particular IT organizational needs.

Hardware Virtualization

Hardware emulation or virtualization is currently the best known virtualization technology. VMware is well known and Microsoft has just introduced a similar technology, its Virtual Server solution purchased from Connectix last year. Hardware emulation technology creates a full emulation of hardware at the host OS level, usually running as a single process inside the host OS. Full emulation typically runs some part of the guest OS code on native CPU, but some privileged instructions are emulated and handled separately. In addition to CPU, the software has to emulate BIOS, video adapters, network adapters, storage and input/output devices to provide a normal environment for the guest OS operation. The extent of the emulation allows almost any existing OS to run inside the virtual environment. Each virtual environment (generic term used across all technologies for virtual machine, virtual server, and virtual private server or VPS) has an entire OS structure, keeping all software and processes completely isolated. The primary use of this technology has been to provide multiple different OS flavors on the same physical piece of hardware. Multiple OSes are, in particular, a requirement in development and testing scenarios, where engineers are developing software simultaneously on different operating systems.

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OS Virtualization

OS partitioning or virtualization is a newer virtualization technology offered by Virtuozzo, Zones in Sun's Solaris OS, and also open source projects such as VServer. OS virtualization technology leverages a single common OS and creates separate, isolated virtual environments on a single physical server to share hardware, licenses and management effort. Processes and software are kept separate when necessary to distinguish between different virtual environments, but the design intent is to share the same software (when appropriate) across virtual environments. The primary use of this technology is to deploy production application and data on live servers. Each server runs an OS and may run other similar OSes, but since most enterprise applications run on one of a few OSes this tends to not be a barrier for production servers.

Which is the Best Approach for Production Servers?

Most IT organizations are considering virtualization as an answer to controlling runaway IT infrastructure growth. In many cases organizations are considering technology that could create drawbacks in other areas such as server performance or failing to fully capitalize on other benefits of virtualization such as decreased management cost.

Examine the full emulation partitioning technology as a solution for server consolidation, as an example, for existing and/or future applications. The host structure itself provides some initial overhead and typically 20%+ of the RAM is reserved for server overhead to run the virtualization software. Full hardware emulation is designed to mimic hardware to make it flexible enough to run any OS and completely separate virtual environments but making it inflexible from resource control and allocation perspective.

The OS partitioning implementation was designed to solve the issues of production servers so the architecture and design fit the consolidation criteria well. Typically when applications are deployed, they are grouped on the same OS for ease of management, maintenance and performance. The common OS affords a much more efficient server resource design and smaller overhead allowing hundreds of virtual environments (VEs) to run well on a single physical server and share resources effectively. In addition to this flexibility, resource allocations for VEs can be changed in real time without a reboot or service interruption. The low system overhead, dynamic resource management and high performance make VEs nearly as scalable as the host server in its native format.

Other Considerations?

Technology aside, each of these software solutions comes with a different set of features and capabilities. Here are a few additional considerations that may help in making the correct decision on a software virtualization solution:

* What OS do you require? Many of theses solutions have limited OS support, and few have the ability to manage virtual environments across OSes from a single management console.

* How many different OSes will you place on a single server? The first part of this article described the two major virtualization technologies; hardware emulation allows multiple OSes while OS virtualization places a single OS on a server.

* How much resource flexibility is required for server resources? Would applications at peak loads benefit from additional resources? Would the ability to reassign resources without downtime or service interruption be beneficial?

* How virtual are the environments? Can the virtual environment be moved to another server with no or close to no downtime?

* Does the virtualization technology speed or ease provisioning time for both virtual environments and applications?

* Are there tools to help move existing servers into virtual environments?

* Do you have high-powered applications that will require assigning multiple CPUs to a single virtual environment?

* Does the virtualization software help administrators manage software and application patches and upgrades?

* Are there tools for virtual environment owners to have full control and management over the virtual environment without the support of the IT organization?

* Is there resource and accounting reporting information to facilitate charge backs to organizations? As companies attempt to move to a more utility computing model, virtualization and system resource allocation measurement are key enablers.

Conclusion

The two virtualization technologies were each built to meet the needs of a particular operational application. Hardware emulation virtualization was designed to allow multiple OSes to reside and operate independently on a single box, which solves a very important problem particularly for development and testing servers. OS virtualization was developed to allow many production servers to be consolidated onto a single server, its resource management and processing efficiencies are optimized to equal or surpass the performance of standalone servers.

It is likely that an IT organization's virtualization strategy will successfully incorporate both almost "complementary" approaches for different scenarios. Virtualization technology provides real benefits to IT professionals due to the enablement of lower costs, high flexibility, increased server uptime and more. Furthermore, hardware and OS virtualization are true contributing enablers to Utility Computing and Dynamic Datacenter visions which are now becoming planned and implemented more broadly.

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Technology Comparison For Production Servers:

                  Hardware Virtualization         OS Virtualization

Platform Support  Broadest support for operating  Broad support for
                  system versions and types,      Linux operating system
                  including capability to         and forthcoming
                  flexibly mix and match them on  support by Virtuozzo
                  the same physical server.       for the Windows
                                                  platform but with
                                                  inability to mix and
                                                  match operating
                                                  systems such as Linux
                                                  and Windows on the
                                                  same physical server.
Application       Supports applications out of    Supports applications
Support           the box without modification    out of the box without
                  or reconfiguration.             modification or
                                                  reconfiguration.
Hard Drive        Each server has a host OS and   One single OS
                  additional guest OSes, using    environment exists on
                  resources to create multiple    a server, maximizing
                  environments flexibly.          system resources by
                                                  sharing OS and
                                                  application software
                                                  as appropriate.
System Calls      Most system calls or kernel     OS virtualization
                  access operations have          shares a single kernel
                  additional overhead; hardware   for the host system
                  emulation commands and          and all VEs. VEs are
                  exceptions processes are very   isolated by additional
                  CPU-consuming and add load      access control
                  to the memory and CPU.          mechanisms inside the
                                                  kernel which do not
                                                  create overhead for
                                                  the common code path
                                                  of system calls.
Network           Hardware emulation              OS virtualization
Performance       technologies make copies of     makes the same number
                  each packet. Delivery of        of data copies that
                  packets requires additional     are made in the host
                  context switches (TLB           operiang system. VEs
                  flushes), consuming CPU cycles  add nearly no network
                  and reducing the system         overhead and do not
                  performance.                    noticeably impact
                                                  system performance.
Filesystem        All common code and data is     OS virtualization uses
                  duplicated. Every virtual       a copy-on-write file
                  environment has its own copy    system. Common data is
                  of OS, code, and data.          stored only once and
                                                  is available to
                                                  multiple environments.
Filesystem        Each file operation copies      OS virtualization uses
Performance       data at least one additional    a copy-on-write file
                  time between the OS kernel,     system and efficient
                  host OS user space and the      system call design
                  guest OS kernel. The extra      creates very small CPU
                  copies cause even simple        usage overhead. With
                  operations to have higher CPU   no additional data
                  usage and reduced server        copies made, VEs place
                  performance.                    no noticeable overhead
                                                  on CPU resources.
Filesystem        The host and each guest OS      OS virtualization
buffers/cache in  each have a separate buffer     places a single
memory            and cache. The same data is     instance of data and
                  cached in the host system       cache across the
                  kernel and in each guest OS     server, maximizing
                  kernel that accesses the data,  memory efficiency.
                  unnecessarily duplicating
                  data.
Memory            Virtual environments created    OS virtualization has
Management        using hardware emulation        very flexible memory
                  technology are bound to its     management. Memory
                  assigned memory resources.      resources can be
                  Non-dynamic memory assignment   "dedicated" to VEs in
                  makes it very difficult for     real-time to
                  production servers with         accommodate varying
                  varying workloads to use        work loads.
                  system resources well.

Carla Safigan is senior product manager at SWsoft (Herndon, VA)

www.swsoft.com