Understanding EMC SRDF/Metro Dual‑Active and VDM MetroSync High‑Availability Storage Solutions

SRDF/Mtreo is EMC's dual‑active feature introduced at WORLD 2015, built on VMAX3. In traditional SRDF, the R1 device is read‑write while R2 is read‑only; only when R1 fails or a manual switchover occurs can R2 become writable.

With SRDF/Metro dual‑active, both R1 and R2 devices are accessible for read and write, presenting a single virtual device to the host. This active‑active mode allows simultaneous writes to both sides, with shared WWN identifiers making the pair appear as one device.

Two application scenarios are supported: a non‑cluster mode where a single host accesses the storage via multipathing, and a cluster mode that requires cluster‑aware software (e.g., Microsoft MSFC, VM HA, Oracle RAC, PureScale). In the cluster mode, I/O can originate from multiple hosts, and SRDF/Metro resolves write conflicts to maintain consistent images.

SRDF/Metro provides two mechanisms to avoid split‑brain situations: a Bias‑preferred configuration and a Witness‑based configuration. The Bias option prefers one array (R1) during a link failure, keeping it accessible while the other (R2) is blocked. The Witness option deploys a third‑site array (or vWitness) to arbitrate which side remains active.

Later, EMC introduced vWitness, a virtual witness that can run on VMware ESXi, supporting up to 32 vWitness instances and offering flexible deployment alongside or independent of hardware witnesses.

VDM MetroSync is an EMC NAS dual‑active solution for VNX2 File, built on Virtual Data Mover (VDM) technology. It uses MirrorView/S replication at the LUN level to create synchronized copies of VDMs, including file systems, checkpoints, CIFS servers, and interfaces, enabling automatic failover or migration between systems.

Configuration steps include establishing control paths between consoles, enabling MirrorView, configuring Write Intent Log (WIL) and Clone‑specific LUNs, enabling the VDM MetroSync service, allocating storage pools, and creating replication sessions and VNX File resources.

Establish control paths between the two system consoles and enable MirrorView connections.

Configure Write Intent Log (WIL) and Clone‑specific LUN (CPL) on both systems.

Enable the VDM MetroSync service.

Allocate storage and configure user‑defined NAS pools for VNX File on both systems.

Create VDM MetroSync replication sessions and VNX File resources.

The VDM MetroSync Manager provides a GUI for monitoring sessions, performing failover or recovery, and automatically initiating failover when issues are detected.

Overall, the article highlights the evolution of dual‑active storage technologies, emphasizing their importance for data protection, business continuity, and the need to keep storage solutions up‑to‑date with advancing IT demands.