Home > Workload Solutions > SQL Server > White Papers > Deploying Dell EMC SRDF/Metro Smart DR with Microsoft SQL Server for Physical and Virtual Environments > Step 1: Prerequisites
These are the prerequisites to a successful SRDF/Metro HA setup:
To answer this question, we need to examine the storage devices in a PowerMax system. Each device has two SCSI identities: an internal identity which is the original World Wide Name (WWN) and geometry, and an external identity which can be changed to look like another device. SRDF/Metro changes the external SCSI identity of the remote devices to make them identical to the local devices WWNs. This can only happen once SRDF/Metro is fully in sync, and the paired devices’ data is identical, with a replication state of active/active. Only then can Windows Server failover cluster use these devices on the remote storage system since now they will be in a RW (Read Write) state.
In an SRDF environment a minimum of two storage systems are required. SRDF designates the name R1 to the replication source devices and R2 to the replication target devices. Once SRDF/Metro is in active/active state, both sides are equally replicating writes back and forth, so there are no differences between source or target systems, R1 or R2 devices. They are seen as the same device by the host.
If for any reason SRDF/Metro replication stops (planned or unplanned), SRDF will choose a surviving (winning) side, either by the user’s choice (planned), or by using the Witness rules or Bias rules (unplanned). The devices at the winning side will automatically become or remain R1 devices that are set to RW state and maintain the external SCSI identity used during the active/active replication period. The devices on the side that did not win will become or remain R2, set to WD (Write Disabled state), and their external SCSI identity does not stay the same as it was when they were in the active/active replication period. This is the expected behavior when a cluster is partitioned to avoid any chance of a split-brain situation.