Starting SQL Server 2014, Failover Cluster Instances supports Cluster Shared Volumes (CSV) in both Windows Server 2008 R2 and Windows Server 2012.
What is Cluster Shared Volumes (CSV)? How Cluster Shared Volumes (CSV) works?
As per Technet below is the definition of CSV.
To understand how Cluster Shared Volumes (CSV) works in a failover cluster, it is important to understand 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.
The above mentioned description is interms of Windows Operating System, which can be better understood by a System Administrator, so let’s try to put it in terms of SQL Server, as CSV is supported by SQL Server 2014 or higher.
Without CSV, a traditional failover cluster which we already know about, allows a given disk (LUN) to be accessed by only one node at a time, meaning SQL Group will own the disk resource and that disk resource cannot be used by any other groups.
Now, with CSV support in SQL Server 2014, we can have multiple groups which are owning different SQL Server Instances, can all use single LUN. Now, we can failover one instance from one node to another node, without affecting or having to failover the disk resources along.
Let’s take an example for better understanding.
– Let’s say we have two nodes Node1 and Node2.
– When SysAdmins add the CSV, we will see the LUN in a path like C:\ClusterStorage\SQLDBFilesDisk\ , where we can rename to VolumeX to more meaningful name.
– We can access this path from both the nodes.
– We then Install one SQL Server 2014 cluster instance, which can failover between two nodes Node1 and Node2. During the SQL Server installation, in that page where we specify the path for “Data Directories” as shown in below image, we need to change the path like C:\ClusterStorage\SQLDBFilesDisk\MicrosoftSQLServer\Instance1\
– We then Install another SQL Server 2014 cluster instance, which can failover between two nodes Node1 and Node2. During the SQL Server installation, in that page where we specify the path for “Data Directories” as shown in above image, we need to change the path like C:\ClusterStorage\SQLDBFilesDisk\MicrosoftSQLServer\Instance2\
– Once the installation completes, we can browse to the database files by going to C:\ClusterStorage\SQLDBFilesDisk\MicrosoftSQLServer\ from any of the nodes. Here we are using the same LUN for both the SQL Server Instances, which was not possible before SQL Server 2014.
– If we open SQL Cluster Manager, and go to SQL Server Group, we will only see SQL Server, SQL Agent, SQL Virtual Network Name and IP Address resoures, but the Disk resources will not be there anymore as there are now independent of any one group.
Hope this clarifies what is Cluster Shared Volumes (CSV) and How it can be used with SQL Server 2014.