Although SRDF/Metro can operate without a Witness (using Bias rules), it is highly recommended to configure one or more Witnesses as arbitrators in case of an unplanned cluster partitioning. If the two sides cannot communicate between themselves, the Witness helps SRDF/Metro determine which side to keep alive to avoid a split-brain situation.
There are two choices for Witness configuration: physical or virtual. More than one Witness can be configured, however, only one can be active at a time. In addition, if both physical and virtual Witnesses are configured, the physical witness takes precedence.
- A physical Witness for SRDF/Metro is simply an SRDF group that doesn’t send any data and merely serves a role of a third site to SRDF/Metro for cluster arbitration purposes only. To create a physical Witness, create an SRDF group from both the ‘local’ and ‘remote’ storage systems to a third VMAX or PowerMax storage system. To do this select “SRDF Metro Witness Group” when creating SRDF group as described in Step 5: Create empty SRDF groups with labels. Repeat SRDF group creation on remote side as well.
- A virtual Witness (or vWitness) for SRDF/Metro is simply a pre-configured VMware ESX virtual appliance (vApp) of Solutions Enabler, Unisphere for PowerMax, or Unisphere for VMAX. Adding a virtual Witness is demonstrated below.
To set up vWitness follow these steps:
- Deploy Solutions Enabler vApp on ESXi server.
- Connect to the vWitness dashboard using a browser using its IP address. As we can see it will not have connectivity to any PowerMax systems until after the deployment.
Figure 4. vWitness Dashboard
- Once SRDF connectivity between local and remote arrays is established as described in Step 2: Create an SRDF connection between the storage systems, configure vWitness using Unisphere for PowerMax.
- To add a virtual Witness select Data Protection (1) > Virtual Witness (2) > Create Virtual Witness (3).
- Enter a name for the virtual witness.
- Enter the IP of the virtual witness (4), check the box to “Add Virtual Witness to remote arrays” (5) and select the arrays that will use the same vWitness (6).
Figure 5. Configuration of vWitness
On Unisphere for PowerMax Data Protection>Virtual Witness, vWitness that is configured, but not yet used by any SRDF/Metro group, will appear as “not in use.”
Figure 6. vWitness configured but not used by any SRDF/Metro groups