Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax and VMAX All Flash: SRDF/Metro Overview and Best Practices > Resiliency features
SRDF/Metro uses the SRDF link between the two sides of the SRDF device pair to ensure consistency of the data. If one or more SRDF device pairs become not ready (NR) on the SRDF link or all link connectivity is lost between VMAX3 or VMAX All Flash systems (suspended or partitioned states), SRDF/Metro selects one side of the SRDF device pair to remain accessible to the hosts, while making the other side of the SRDF device pair inaccessible.
SRDF/Metro supports two resiliency features to accommodate this behavior, bias and witness. While both of these features prevent data inconsistencies and split-brain complications between the two sides of the SRDF device pair, Split-brain complications are data or availability inconsistencies originating from the maintenance of two separate devices (with an overlap in scope) due to a failure caused by these systems not communicating or synchronizing their data.
The first resiliency feature, bias, is a function of the two VMAX3 or VMAX All Flash systems taking part in the SRDF/Metro configuration and is a required and integral component of the configuration. The second feature, witness, builds upon the base bias functionality by adding an optional SRDF/Metro component which allows a third VMAX based (PowerMax, VMAX, VMAX3, or VMAX All Flash) or software based (Virtual Witness) node to act as an external arbitrator to ensure host accessibility in cases where bias alone would restrict access to one side of the SRDF/Metro device pairs. Note that these resiliency features are only applicable to SRDF device pairs within an SRDF/Metro configuration.
Each witness may protect the full number of SRDF/Metro groups available on each array. There is a many to many relationship between SRDF/Metro paired arrays and witnesses for redundancy with each paired array able to be protected by multiple witnesses and each witness being able to protect multiple arrays. The current support for these relationships is outlined in the following table:
| Number of arrays supported | Array pairs supported | Number of SRDF/Metro groups protected per array pair |
Physical Witness | 32 | 16 | 250 – Number of Physical Witness RDF Groups |
Virtual Witness | 32 | 16 | 250 |