- Notes, cautions, and warnings
- Preface
- Introduction
- Disaster recovery
- High availability
- Data migration
- SRDF I/O operations
- SRDF write operations
- SRDF read operations
- SRDF/A resilience and performance features
- Management tools
- More information
Dell SRDF Introduction
R21 devices
R21 devices have a dual role and are used in cascaded three-site configurations where:
- Data on the R1 site is synchronously mirrored to a secondary (R21) site, and then
- Asynchronously mirrored from the secondary (R21) site to a tertiary (R2) site:
The R21 device acts as a R2 device that receives updates from the R1 device, and as a R1 device that sends updates to the R2 device.
When the R1->R21->R2 SRDF relationship is established, no host has write access to the R21 device.
In arrays that run Enginuity 5978 and earlier, the R21 device can be diskless. That is, it consists solely of cache memory and does not have any associated storage device. It acts purely to relay changes in the R1 device to the R2 device. This capability requires the use of thick devices. Systems that run PowerMaxOS or HYPERMAX OS contain thin devices only, so setting up a diskless R21 device is not possible on arrays running those environments.
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