Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax and VMAX All Flash: SRDF/Metro Overview and Best Practices > Understanding the array-based witness
As described previously, the optional witness functionality builds upon the base bias feature by adding an external arbitrator to ensure host accessibility in cases where bias alone would restrict access. Configuring a hardware witness functionality will require a third VMAX, VMAX3, VMAX All Flash, or PowerMax system with an applicable ePack installed and SRDF group connectivity to both the primary and secondary SRDF/Metro paired arrays.
Once a VMAX witness system has been configured, it supersedes the previously described bias functionality unless a situation is encountered requiring specific knowledge of the biased system.
The VMAX or VMAX3 code requirements to support witness functionality are:
To configure a VMAX witness system, SRDF groups created with a new witness option must be made visible from the third VMAX, VMAX3, or VMAX All Flash system to both the primary and secondary VMAX3 systems. This requires SRDF remote adapters (RAs) to be configured on the witness system with appropriate network connectivity to both the primary and secondary arrays. Redundant links to the witness system are also recommended as a best practice in a production environment to address possible failures in connectivity.
Once this third system is visible to each of the SRDF/Metro paired VMAX3 or VMAX All Flash systems and the SRDF/Metro groups suspended and reestablished, the configuration enters a “Witness Protected” state. For this reason, it is also a best practice for the witness SRDF groups to be configured prior to establishing the SRDF/Metro device pairs and synchronizing devices.
Multiple VMAX witness systems may be configured in this manner for redundancy purposes. Should either connectivity or the primary witness system fail and no other alternative witness systems may be identified, SRDF/Metro resiliency defaults back to the bias functionality. See Use bias resiliency option and failure scenarios below for use in the event of scheduled maintenance of the witness system. Use of this option prevents dial home events and escalations normally associated with an outage of SRDF/Metro in a witness configuration.
Note: The SRDF personality of devices may also change as a result of a witness action (PowerMax, VMAX, or vWitness based) to better reflect the current availability of the resulting devices to the host. For example, should the witness determine that the current R2 devices remain host accessible and the R1 devices inaccessible, the current R2 devices will change to R1 as a result. Depending on access/availability, the previous R1 device will also change to R2’s as in the case of a bias change.
Using the undesirable bias outcome example described previously, a failure of the biased R1 side with a witness configured would now result in continued host accessibility of the non-biased R2 side:
The SRDF/Metro witness functionality covers a number of single and multiple failure and response scenarios.
Note: To determine the actions necessary to properly recover SRDF/Metro from a specific failure scenario, see the SRDF/Metro Recovery Knowledge Base (KB) article KB516522 (https://www.dell.com/support/kbdoc/en-us/000021764), engage Dell Technologies support directly, or escalate to your local account or support team as the urgency of the situation dictates.
Depicted below are detailed single and multiple failure scenarios and the resulting responses which are covered by SRDF/Metro witness functionality: