Dell SRDF Introduction

Create the SRDF/Metro configuration

First, the storage administrator determines the storage needs of the application and identifies an appropriate set of devices to meet those needs. The administrator also identifies another, identical set of devices on the remote array. These devices are the other side of the SRDF/Metro device pairs.

To set up the SRDF/Metro configuration, the administrator:

1. Creates two, empty SRDF groups, one on each array
2. Uses the create pair operation to create each device pair and add it to the two groups

   When adding a pair, the administrator indicates that it is a SRDF/Metro device pair which marks the group as an SRDF/Metro group. In response, SRDF/Metro sets each pair to operate in Active mode.

3. Makes the device pairs Ready on the SRDF link between the arrays

   As part of this step, the administrator defines the failure resilience mechanism that each pair is to use (Witness or Device Bias). Since Witness is the default, the administrator specifies a mechanism only if a pair uses Device Bias.

Synchronize each device pair

Next, each device pair synchronizes its data in preparation for providing high availability:

• The pair state becomes SyncInProg.
• Invalid tracks synchronize.
• R2 copies the device personality (for example, geometry and identity) of the R1 device.
• R2 adopts the personality of the R1 device.

While the devices are synchronizing, the application can run. However, only the R1 side can process I/O requests. The R2 side is inaccessible to the application host.

Enter high-availability mode

Once synchronization is complete, the final steps occur to provide high availability. SRDF/Metro:

• Makes the R2 devices accessible to the host using the federated R1 personalities
• Sets the state of each pair to one of:
  • ActiveActive—if a Witness resiliency mechanism is in use.
  • ActiveBias—is the Device Bias resiliency mechanism is in use.

From now on, SRDF/Metro monitors the SRDF link and the devices on that link. If the link fails, or either partner in a device pair becomes Not Ready, SRDF/Metro decides on the winning side according to the resiliency mechanism in use. The winning side remains accessible to the application host and the losing side becomes inaccessible.

The application host now discovers the R2 devices through their federated personalities. Once they are discovered, the configuration is providing high data availability:

• The application host can send write operations to either side. When SRDF/Metro receives a write operation it sends the I/O to the other array in the configuration. SRDF/Metro confirms the I/O to the application host only when that other array acknowledges receipt of the Write operation.
• Reading from either side always returns the same data since both sides have identical content.