Hyper-V Cluster Shared Volumes Recommendations

Windows Server 2008 R2 introduced a new feature for Hyper-V called Clustered Shared Volumes (CSV). Cluster Shared Volumes simplifies the configuration and management of clustered virtual machines. With Cluster Shared Volumes, multiple clustered virtual machines can use the same LUN (disk) while still being able to fail over (or move from node to node) independently of one another.

Be careful that you do not use too many CSVs for Virtual Machines in your environment. Aidan Finn points this out in a recent article. Aidan states the following:

Windows Server 2008 and 2008 R2 Failover Clustering use iSCSI3 persistent reservations (PRs) to access storage. Each SAN solution has a limit on how many PRs they can support. You can roughly calculate what you need using:

PRs = Number of Hosts * Number of Storage * Channels per Host Number of CSVs

Let’s do an example. We have 2 hosts, with 2 iSCSI connections each, with 4 CSVs. That works out as:

2 [hosts] * 2 [channels] * 4 [CSVs] = 16 PRs

Your storage solution may actually calculate PRs using a formula with higher demands. But the question is: how many PRs can your storage solution handle? Deploy too many CSVs and/or storage modules and you may find that you have disks disappearing from your cluster. And that leads to very bad circumstances.

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