Using a storage area network with virtual machines

Using a storage area network with virtual machines

When the physical computer and host operating system are configured appropriately, a virtual machine can access resources located on storage area network drives. A storage area network (SAN) is a set of interconnected devices and servers that are connected to a common communication and data transfer infrastructure such as Fibre Channel. The common communication and data transfer mechanism is commonly known as the storage fabric. The purpose of the SAN is to allow multiple servers access to a pool of storage in which any server can potentially access any storage unit.

Using a SAN to provide storage for virtual machines provides several benefits. By using a SAN, you can:

  • Provide a large amount of storage to your virtual machines
  • Increase data access performance
  • Improve recoverability of the virtual machine's data because of the data redundancy and backup features offered by most SAN implementations
Make sure to place each running virtual machine in a saved state or shut it down before backing up the virtual machine files on the SAN. For general information about backing up Virtual Server and virtual machines, see Backing up and restoring Virtual Server.

When the physical computer is configured to use a SAN, the host operating system treats the SAN as if it were a storage device, such as a disk volume assigned to a local drive letter. Because the host operating system can access the storage device, you can place all files associated with your virtual machines on the SAN. In this scenario the SAN provides all storage and the physical computer that runs Virtual Server provides the memory and CPU resources. The following figure illustrates using a SAN to store virtual machine files.

Storing virtual machine files on a SAN

Typically, data and requests are transmitted using a storage-specific protocol that is tuned for low latency, high-bandwidth data transfers required by storage infrastructure. As a result, accessing SAN data is comparable or faster than accessing data from a local device.

However, if you plan to run a large number of virtual machines simultaneously or run many data-intensive applications, it is possible for a bottleneck to occur on the host bus adapter. You can use a Performance console on the host operating system to determine whether the disk activity being generated by the virtual machines is approaching the throughput capacity of the host bus adapter. If this is the case, one potential solution is to add an additional Fibre Channel card and assign another drive letter to the new volume. Then you can move existing virtual machines or add new machines to the new volume as necessary to balance data transfers.

SAN management tools are available only on the host operating system because Virtual Server does not emulate a host bus adapter. It also is possible to use network-attached storage with Virtual Server. However, if both SAN and network-attached storage are available to you as storage options, we recommend using a SAN. A virtual machine accesses the storage provided by a SAN directly through the storage fabric. Although the storage is external, access to it is isolated from network traffic. This is in contrast to network-attached storage, which provides a network-based means of accessing storage. As a best practice, we recommend storing all files associated with a specific virtual machine in one folder. For more information about this and other recommendations, see Best practices for Virtual Server.