Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax and VMAX All Flash: SRDF/Metro Overview and Best Practices > Conclusion
Symmetrix Remote Data Facility (SRDF) solutions provide disaster recovery and data mobility solutions for PowerMax, VMAX, VMAX3, and VMAX All Flash arrays. HYPERMAX OS 5977.691.684 and Solutions Enabler/Unisphere for VMAX 8.1 introduced support for SRDF/Metro for VMAX3 and VMAX All Flash storage arrays. SRDF/Metro significantly changes the traditional behavior of SRDF to better support your critical applications in high availability environments.
SRDF/Metro may be managed through Solutions Enabler SYMCLI or Unisphere for VMAX 8.1 or greater client software and requires a SRDF/Metro license on each PowerMax, VMAX3, or VMAX All Flash paired array.
With SRDF/Metro, the SRDF R2 device is read/write accessible to the host and takes on the external identity of the primary R1 device (geometry, device WWN). By providing this external identity on the R2 device, both R1 and R2 devices may then appear as a single virtual device across the two SRDF paired arrays for presentation to a single host or host cluster.
With both the R1 and R2 devices being accessible, the host or hosts (in the case of a cluster) can read and write to both R1 and R2 devices with SRDF/Metro ensuring that each copy remains current, consistent, and addressing any write conflicts which may occur between the paired SRDF devices. A single VMAX3 or VMAX All Flash may simultaneously support multiple SRDF groups configured for SRDF/Metro operations and multiple SRDF groups configured for non-SRDF/Metro operations.
Performance statistic exchange begins once the SRDF/Metro Active mode and ActiveActive or ActiveBias pair state have been achieved. Each side then incorporates the FAST statistics from the other side to ensure each side represents the workload as a whole (R1+R2 workload). Users may set the required service level objective (SLO) independently on both source and target SRDF/Metro paired arrays. There are currently no restrictions in this area as FAST data movement is transparent from SRDF/Metro.
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 PowerMax, 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 (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.
The following features were introduced with the PowerMaxOS 5978 Q3 2020 Service Release (SR) and Solutions Enabler/Unisphere for PowerMax 9.2:
This release also added support for the 4 port 25 Gb Ethernet I/O module and protocol driver for all SRDF replication and host connectivity (RE/SE). This hardware expands PowerMax support for next generation Ethernet-based SAN fabrics, continuing to provide maximum I/O performance and fabric capabilities to the platform.