Dell EMC Solutions Enabler 9.2 SRDF Family CLI User Guide

Cascaded operations overview

Cascaded SRDF is a three-way data mirroring and recovery solution that consists of:

• A R1 device replicating data to
• An R21 device at a secondary site, which replicates the same data to a
• R2 device located at a tertiary site

Cascaded SRDF reduces recovery time at the tertiary site because replication continues to the tertiary site if the primary site fails.

This enables a faster recovery at the tertiary site, if that is where the data operation is restarted. You can achieve zero data loss up to the point of the primary site failure.

The following image shows a basic cascaded SRDF configuration.

Cascaded SRDF uses a new type of SDRF device: the R21 device. An R21 device is both an R1 mirror and an R2 mirror, and is used only in cascaded SRDF configurations.

An R21 device is both:

• An R2 in relation to the R1 source device at the primary site, and
• An R1 in relation to the R2 target device at the tertiary site.

There are two sets of pair states in a cascaded configuration:

• Pair states between the primary and secondary site (R1 -> R21)
• Pair states between the secondary and tertiary sites (R21 -> R2)

These two pair states are separate from each other.

When performing a control operation on one pair, the state of the other device pair must be known and considered.

TheDell Solutions Enabler SRDF Family State Tables Guide lists the applicable pair states for cascaded operations.