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SRDF/Metro is a feature that provides active/active access to the R1 and R2 of an SRDF configuration. In traditional SRDF, R1 devices are Read/Write accessible while R2 devices are Read Only (Write Disabled). In SRDF/Metro configurations, both the R1 and R2 are Read/Write accessible. The R2 takes on the personality of the R1 in terms of geometry and most importantly the WWN. By sharing a WWN, the R1 and R2 appear as a shared virtual device across the two arrays for host presentation. A host or typically multiple hosts in a cluster can read and write to both the R1 and R2. SRDF/Metro ensures that each copy remains current and consistent and addresses any write conflicts which might arise. In VMware environments, the ESXi hosts from two different data centers can be placed in the same vCenter, forming a VMware Metro Storage Cluster (vMSC). A simple diagram of the feature is available in Figure 179.
Figure 180 is a schematic representation of the business continuity solution that integrates a VMware environment and SRDF/Metro. The solution shows two virtual machines accessing devices on PowerMax arrays on VMFS.
In an SRDF/Metro environment with vMSC, all the ESXi hosts see the same VMFS datastore, though some hosts use the R1 and others the R2. The image above is using the R2 only as HA. VMs can be moved through vMotion across data centers, rather than Storage vMotion, despite the use of two separate arrays.
Note: The SRDF SRA supports SRDF/Metro Smart DR configurations with VMware SRM beginning with SRDF SRA 10.1.0.
SRDF/Metro supports 3-site configurations with a leg off either the R1, R2, or both members of a pair. The R1 and R2 are unaware of the existence of an asynchronous or adaptive copy leg off the other. Up until PowerMaxOS 5978 Q3 2020, SRDF/Metro did not support a Star-like configuration where either the R1 or R2 could update a single remote leg. Dell offers SRDF/Metro Smart DR beginning with PowerMaxOS 5978 Q3 2020. It enhances SRDF/Metro and provides Star-like functionality. The SRDF/Metro Smart DR feature extends the high availability (HA) solution of SRDF/Metro to a third array. It supports the ability to have Geo distance DR support for a device in a Metro configuration.
SRDF/Metro Smart DR integrates SRDF/Metro and SRDF/A. It enables HR DR by closely coupling the SRDF/A sessions on each side of the Metro pair to replicate to a single DR device. While only one side of the SRDF/Metro pair (R1) updates the SRDF/A leg at one time, both can do so. The asynchronous replication of data from either side of the SRDF/Metro pair to a tertiary site enables Failover and Failback to the DR site while retaining the Metro environment. SRDF/Metro Smart DR requires a minimum of PowerMaxOS 5978 Q3 2020 and Solutions Enabler 9.2 running on the three PowerMax arrays with Witness configured. Bias is not supported.
Note: In Unisphere for PowerMax, SRDF/Metro Smart DR is also known as MetroDR.
Figure 181 is a schematic representation of the business continuity solution that integrates a VMware environment and SRDF/Metro Smart DR. The solution shows VMware Metro Storage Cluster (vMSC) with a third SRDF/A site where a SnapVX snapshot is taken of the remote R2 and mounted in a test environment. That same test environment could be used for DR if the primary SRDF/Metro became unavailable.
For more detail on SRDF/Metro configurations, see Best Practices for Using Dell SRDF/Metro in a VMware vSphere Metro Storage Cluster.