PowerMaxOS 5978 and Solutions Enabler/Unisphere for PowerMax 9.1, which were released in Q3 2019, introduced support for SRDF/Metro Online Device Expansion (ODE) and a new Unisphere interface for adding and removing SRDF/Metro devices based on the existing storage group add/remove device workflow. From Unisphere for PowerMax and Solutions Enabler 9.1on, ODE support was expanded to include devices taking part in SRDF/Metro (Active) sessions. This new functionality is based on modifications to our existing Geometry Compatibility Mode (GCM) functionality for host visibility of devices.
Unisphere 9.1 also provides new ease-of-use functionality by automating the addition of devices to a storage group, including corresponding SRDF paired devices for single-hop, concurrent, and cascaded SRDF configurations.
With SRDF/Metro, the SRDF secondary device is read/write accessible to the host and takes on the external identity of the primary device (its geometry, device WWN, and so on). By providing this external identity on the secondary device, both the primary and secondary devices appear as a single virtual device across the two SRDF paired arrays for presentation to a single host or host cluster. With both devices accessible, the host (or hosts in the case of a cluster) can read and write to both primary and secondary devices with SRDF/Metro, ensuring that each copy remains current and consistent and addressing any write conflicts that might occur between the paired SRDF devices. A single PowerMax All Flash array can simultaneously support multiple SRDF groups that are configured for SRDF/Metro operations and multiple SRDF groups that are configured for non- SRDF/Metro operations.
The key differences between SRDF/Metro and standard synchronous and asynchronous SRDF modes are:
If all the SRDF device pairs are not ready (NR) on the link, the user may:
SRDF/Metro has a Virtual Witness resiliency option that is packaged to run as a virtual appliance on a VMware ESX server. If either array detects that the SRDF/Metro session has failed, the array will request a lock from the witness. A session has failed when either the SRDF link or one of the arrays has failed. The side that gains the lock remains available to the host while the other side becomes unavailable. When the failure is resolved, synchronization resumes.