Understanding Cluster Shared Volumes in a Failover Cluster
Applies To: Windows Server 2008 R2
To understand how Cluster Shared Volumes (CSV) works in a failover cluster, it is helpful to review how a cluster works without CSV. Without CSV, a failover cluster allows a given disk (LUN) to be accessed by only one node at a time. Given this constraint, each Hyper-V virtual machine in the failover cluster requires its own set of LUNs in order to be migrated or fail over independently of other virtual machines. In this type of deployment, the number of LUNs must increase with the addition of each virtual machine, which makes management of LUNs and clustered virtual machines more complex.
In contrast, on a failover cluster that uses CSV, multiple virtual machines that are distributed across multiple cluster nodes can all access their Virtual Hard Disk (VHD) files at the same time, even if the VHD files are on a single disk (LUN) in the storage. The clustered virtual machines can all fail over independently of one another.
This topic provides a basic description of Cluster Shared Volumes. For more detailed information, see Using Cluster Shared Volumes in a Failover Cluster in Windows Server 2008 R2 (http://go.microsoft.com/fwlink/?LinkId=182152).
Benefits of using Cluster Shared Volumes in a failover cluster
Cluster Shared Volumes provides the following benefits in a failover cluster:
The configuration of clustered virtual machines is much simpler than before.
You can reduce the number of LUNs (disks) required for your virtual machines, instead of having to manage one LUN per virtual machine, which was previously the recommended configuration (because the LUN was the unit of failover). Many virtual machines can use a single LUN and can fail over without causing the other virtual machines on the same LUN to also fail over.
You can make better use of disk space, because you do not need to place each Virtual Hard Disk (VHD) file on a separate disk with extra free space set aside just for that VHD file. Instead, the free space on a Cluster Shared Volume can be used by any VHD file on that volume.
You can more easily track the paths to VHD files and other files used by virtual machines. You can specify the path names, instead of identifying disks by drive letters (limited to the number of letters in the alphabet) or identifiers called GUIDs (which are hard to use and remember). With Cluster Shared Volumes, the path appears to be on the system drive of the node, under the \ClusterStorage folder. However, this path is the same when viewed from any node in the cluster.
If you use a few Cluster Shared Volumes to create a configuration that supports many clustered virtual machines, you can perform validation more quickly than you could with a configuration that uses many LUNs to support many clustered virtual machines. With fewer LUNs, validation runs more quickly. (You perform validation by running the Validate a Configuration Wizard in the snap-in for failover clusters.)
There are no special hardware requirements beyond what is already required for storage in a failover cluster (although Cluster Shared Volumes require NTFS).
Resiliency is increased, because the cluster can respond correctly even if connectivity between one node and the SAN is interrupted, or part of a network is down. The cluster will re-route the Cluster Shared Volumes communication through an intact part of the SAN or network.
For detailed information about requirements and recommendations for using Cluster Shared Volumes, see Using Cluster Shared Volumes in a Failover Cluster in Windows Server 2008 R2 (http://go.microsoft.com/fwlink/?LinkId=182152).