Surmounting storage challenges in a virtual environment

Server virtualization doesn't solve all server utilization concerns. Choose a best shared-storage architecture and use thin provisioning to prevent the overallocation of storage capacity.

Server virtualization is fast becoming a strategic technology to bring new levels of operational flexibility and

efficiency to the data center. Many IT shops now employ virtualization to consolidate multiple application workloads onto a fewer number of physical servers, thereby increasing server utilization while reducing hardware, power and cooling costs. But server virtualization and consolidation introduce some key challenges that IT managers must consider as they plan storage infrastructure. Two of the primary challenges are as follows:

  • selecting the best shared-storage approach for a virtualized server deployment; and
  • preventing poor storage utilization on consolidated servers by preventing overallocation of capacity.
The first challenge is to choose the best kind of networked storage for virtualized servers: Is it iSCSI, NAS or Fibre Channel? Just a few years ago, VMware Inc.'s ESX Server supported only Fibre Channel storage area networks (SANs); but now virtual infrastructure vendors -- including XenSource Inc., Virtual Iron Software Inc. and VMware -- support an array of networked storage options.

While server virtualization is an excellent method for increasing CPU utilization, it can actually drive down storage utilization.
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In fact, many storage vendors, such as 3Par Inc., Compellent Technologies Inc., EqualLogic Inc., Intransa Inc., LeftHand Networks and Network Appliance Inc., have aggressively certified their technologies with server virtualization products. But not all storage options offer an equivalent set of capabilities in virtualized server environments. To narrow down the options, data center and storage managers need to consider not only the potential fit of their existing storage configuration but also various other factors, such as performance, manageability and cost.

Storage managers can streamline the decision-making process by answering a few simple questions:

  • What is the best shared-storage approach for the applications hosted on a physical server? If one consolidates Microsoft SQL Server and Exchange onto a virtualized server, for example, these applications should ideally run on Fibre Channel or iSCSI SANs. File and print workloads, on the other hand, are a better fit for NAS.
  • How well does the choice of storage accommodate requirements for data protection, including backup, recovery and security? The answer to this question, in turn, depends on which factors are driving the server virtualization effort: The backup and recovery needs for noncritical, consolidated applications, for example, will likely be much simpler than those for a disaster recovery deployment.
  • How will shared storage for virtualized servers be managed? Can the organization leverage the same management tools and practices it uses for physical servers? If not, what kinds of additional management tools, interfaces and training are required?
  • What are your virtualization platform vendor's best practices? IT managers should ensure that the vendor fully supports the selected approach.

By addressing these questions, IT managers can quickly converge on the networked storage approach that best meets the requirements of application performance, data protection, manageability and cost.

For IT managers a second challenge is how to contend with reduced storage utilization on servers running multiple virtual workloads. Ironically, while server virtualization is an excellent method for increasing CPU utilization, it can actually drive down storage utilization. Major virtualization platforms such as ESX Server favor large, up-front allocations of capacity to a volume -- up to double the needed capacity per volume per virtual machine (VM) -- in the event that the VM must be suspended and its state written to disk. In a highly consolidated environment, this means that storage administrators will end up allocating six to 10 terabytes of physical capacity for a relatively small number of VMs. The tendency toward overallocation of storage capacity creates both low storage utilization and high costs, both of which drastically reduce the benefits of a virtualized server deployment.

To address this challenge, storage managers should consider thin provisioning, an advanced storage virtualization capability that helps prevent low storage utilization by preventing overallocation of capacity. Though thin provisioning presents a logical volume of any size to VMware, it commits physical storage capacity only when data is written or used on a volume. Thus, physical storage is allocated on demand from a shared pool, but only when needed. By using thin provisioning in combination with server virtualization, an organization can optimize both server and storage utilization rates. While not all storage vendors currently offer this capability, popular virtualization appliances and arrays from such vendors as 3Par, Compellent, DataCore Software Corp. and Network Appliance include thin provisioning.

While server virtualization is a potential boon to the data center, it creates significant challenges for IT managers, who must adapt storage infrastructure to serve both physical and virtual environments. By selecting the right networked storage approach and using thin provisioning to prevent low storage utilization, organizations can more fully exploit the benefits of server virtualization and consolidation.

Steve Norall and Jeff Byrne are senior analysts and consultants at the Taneja Group in Hopkinton, Mass.

This was first published in September 2007

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