As virtualization gains momentum, users are becoming increasingly concerned with managing the burden of installing and updating large numbers of operating system instances. For appropriate workloads, operating system virtualization tools such as Solaris Containers and SWsoft Virtuozzo provide many of the same benefits as virtual machines, while avoiding much of the overhead associated with managing each virtual server.

Advocates of virtualization often promise that it will improve server utilization. As the story goes, many organizations have become accustomed to deploying one workload per server. While expedient, this approach leads to relatively low utilization of server resources, with many servers today using only 8% to 15% of their available capacity. As a result of such suboptimal usage, organizations tend to deploy far more servers than necessary to handle their workloads. Virtualization helps to increase server utilization by enabling multiple resource-intensive applications to run simultaneously on a single server, reducing the “white space” of unused resources that might be incurred by standalone servers. Running multiple critical applications on a single machine requires mechanisms to prevent them from interfering with each other’s operation, so that even if one application fails catastrophically or its consumption of resources surges dramatically, the remaining applications continue to deliver their expected levels of performance.

Resource management tools, virtual machines, and partitions all enable multiple workloads to be consolidated onto a single server, and thus help administrators increase the utilization of their servers. However, these options do not necessarily help address another, more fundamental problem, which is the burden of managing the environments that are needed to host the consolidated workloads. By increasing the utilization of servers that are deployed, fewer physical servers may be required to host an organization’s workloads, resulting in lower overall hardware costs. However, the relative impact of hardware cost is gradually becoming less significant as server prices continue to drop. By contrast, the burden of installing and updating large numbers of operating system instances is becoming an increasingly significant component of user costs.

Virtual machines and partitions both require every instance of a virtualized server to have its own instance of an operating system, which must be installed and maintained. This requirement maximizes the ability for the virtualized server to deliver an operating environment that is specifically tailored for the application that it has to host, but it does not fundamentally alter the management costs that would be associated with hosting those applications on standalone servers. With virtual machines and partitions, the absolute number of operating systems that have to be managed remains largely unchanged, even as the number of physical servers that must be managed is reduced.

Resource management software potentially helps address this issue by enabling multiple dominant workloads to coexist within a single operating system instance. For workloads that are suitable for consolidation via resource management software, administrators can implement management procedures with significantly improved economies of scale, since they can increase the number of workloads that can be hosted without increasing management overhead of additional operating systems. However, traditional resource management software has proven unsuitable for consolidating many classes of applications, because the isolation between workloads is not terribly strong. With most existing resource management solutions, workloads share access to the same system resources, such as files, devices,  and network connections. Thus, applications have relatively little protection from each other in terms of security. Further, there may be far more opportunities for applications to interfere with each other than there is conflicting demand on CPU, memory, and I/O resources. Windows applications have proven to be particularly unsuitable for consolidation with resource management software.

They typically do not “play well” with each other, often demanding specific instances of shared resources such as Registry entries and Dynamically Loadable Libraries (DLLs), which causes mutual interference when administrators attempt to host different applications on the same server.

Several solutions for implementing operating system virtualization are now available, each with slightly different design points. Sun’s Solaris 10 operating system, which began shipping at the beginning of this year on both Sun’s SPARC systems and industry-standard X86 hardware, introduced a function called Solaris Containers, which resemble, in concept, the “Jails” mechanism that was originally developed for the FreeBSD operating system. Sun’s approach involves constructing kernel-level “zones” to isolate resources and to contain faults between subsets of resources. Solaris Containers are built on top of zones, preventing any  user-level action from compromising other Containers or the host operating system; Solaris Process Rights Management functions are employed to maintain security privileges between Containers. Each Container appears to users and applications as if it were a private server running Solaris. SWsoft offers an operating system virtualization product called Virtuozzo that supports multiple instances of Virtual Private Servers (VPS) on a single physical server. Each VPS behaves as if it were a standalone server from the standpoint of users and applications. A VPS can be rebooted independently, and has its own root access, users, IP addresses, memory, processes, files, applications, system libraries, and configuration files. Virtuozzo is built on top of standard operating systems; the Linux edition was launched in 2001, and the Windows edition was just recently released. Users first install either Linux or Windows as a host operating system, and then install Virtuozzo to virtualize the host operating system. The Virtuozzo management tools have been evolving since the product launched five years ago, and have become comprehensive and quite sophisticated. In addition to supporting 32-bit Intel X86 platforms, SWsoft recently announced 64-bit support for the Itanium platform, and it will add Opteron support later this year. In a sign that it is gaining traction in the enterprise market, SWsoft recently announced a reseller agreement with the system vendor Bull, which will sell and support Virtuozzo on its NovaScale blade servers, as well as on larger Itanium-based SMP servers and Xeon-based EXPRESS5800 servers.

Sun Solaris Containers Solaris SPARC, Intel X86, AMD-64, EM64T SWsoft Virtuozzo Linux, Windows Intel X86, Itanium, AMD-64 (32-bit now; full 64-bit support in August 2005)

Operating system virtualization functions potentially have several advantages over other server virtualization options such as virtual machines and partitions. These benefits of the operating system virtualization approach include the following:

Table 2 summarizes the tradeoffs between operating system virtualization and

other server virtualization approaches.

Private File Space per Workload No Yes Yes Yes

Private Network Address per Workload No Yes Yes Yes

Private Security Profile and Root Password per Workload No Yes Yes Yes

Distinct OS Kernel Version per Workload No Yes Yes No

Distinct Kernel Patches per Workload No Yes Yes No

Granularity of Resource Allocation Fine Moderate Coarse Fine

Number of Complete Operating Systems to Manage Single Multiple Multiple Single

System Software Licenses Required Single* Multiple* Multiple* Single*

Host OS Is Single Point if Failure Yes Yes No Yes

Startup Time for Virtual Environment Fast Moderate** Slow Fast

Isolation between Workloads Weak Strong Very Strong Strong

* For operating systems that incur commercial licensing fees, i.e. Windows. Solaris 10 no longer has licensing fees, and costs of Linux distributions in virtualized environments vary by product and platform.

** Startup times for virtual machines can be reduced by maintaining and customizing images of running systems.

Based on these tradeoffs, operating system virtualization is most suitable in the following situations:

Perhaps the most obvious candidates for operating system virtualization are the workloads of Internet Service Providers (ISPs) that offer websites for consumers and small businesses. These users typically require only basic services such as email and low-traffic content, which are largely undifferentiated and work well in the kind of environment that virtual operating system instances provide. ISPs are also notoriously cost-sensitive, and would value the ability to exploit the operational efficiency and software licensing benefits of operating system virtualization in order to drive up their margins. But as users in other industry segments are becoming aware of the differentiation that operating system virtualization offers over alternatives such as virtual machines, new kinds of applications are starting to emerge, including some that target more sophisticated enterprise requirements.  For example, one scenario that has emerged for the use of operating system virtualization relates to the deployment of secure remote management consoles.  Server applications increasingly depend on web-based front ends for managing and monitoring their operations. The intention is that these applications should be able to be managed from any location using a standard web browser. However, users that require high levels of security, such as government or financial services organizations, cannot afford the risk of granting remote access to these management interfaces from any workstation or notebook computer on the network. Instead, administrators can set up a virtual browser session running within a virtual operating system for each application that has to be managed remotely (which may run in the hundreds or thousands). Secure access to these virtual sessions can be granted through the remote graphics capabilities and native security mechanisms built into the UNIX and Linux operating systems. This scenario is particularly compelling for Solaris Containers, given the presence that Sun has long established with users in government and financial services environments, and the extensive investments it is making in enhancing the security capabilities in Solaris 10.

Among the tools that are currently available for implementing virtualization, virtual machines clearly retain the lead in terms of deployment maturity. Virtual machines have penetrated the mainstream, and they are now widely used for a variety of applications in both test/development and production environments. However, in spite of their popularity, virtual machines do not necessarily solve a problem that users are becoming increasingly concerned with, which is the burden of installing and updating large numbers of operating system instances. Although virtual machines have the ability to reduce the number of physical servers that users have to manage, they do not generally reduce the absolute number of operating systems that must be managed. By contrast, operating system virtualization tools such as Solaris Containers and SWsoft Virtuozzo provide many of the same benefits as virtual machines, but avoid much of the overhead associated with managing each virtual server. For appropriate workloads, operating system virtualization tools can help reduce the cost and complexity of managing the overall environment in which multiple consolidated workloads are hosted.

1 This year, HP plans to introduce virtual machine technology for its Integrity servers that will allow multiple partitions to be deployed per CPU, increasing the granularity at which resources can be assigned to partitions.