Cascaded

NOTE: Difference between R21 and DL R21 - The R1 SRDF mirror of the R21 device operates in adaptive copy disk mode while the R1 SRDF mirror of the DL R21 device operates in adaptive copy write pending mode between Symmetrix B and Symmetrix

Cascaded
Cascaded SRDF is a three-site disaster recovery solution where data from a primary (R1) site is synchronously mirrored to a secondary (R21) site, and then asynchronously mirrored from the secondary (R21) site to a tertiary (R2) site.

 

Setting up a cascaded SRDF relationship
Setting up a cascaded SRDF relationship is a two-step process:
1. Create the initial R1 -> R21 pair between Symmetrix A and Symmetrix B for the first hop. SRDF/S, SRDF/A, adaptive copy disk mode, or adaptive copy write-pending mode is allowed over the first hop.
2. Set up the R21 -> R2 pair between Symmetrix B and Symmetrix C for the second hop. SRDF/S, SRDF/A or adaptive copy disk mode is allowed over the second hop. The most common implementation is SRDF/S mode for the first hop and SRDF/A mode for the second hop.

Note: Only one hop (R1 -> R21 or R21 -> R2) can be asynchronous at a time. If R1 -> R21 is in asynchronous mode, R21 -> R2 must be in adaptive copy disk mode.


Benefits
The benefits of cascaded SRDF are:

• Faster recovery times at the tertiary site enabled by the continuous mirroring from the secondary site to the tertiary site in the event of the primary site failure.

Note: In single session mode, SRDF continues mirroring data from the secondary to the tertiary site if the primary site fails. In MSC mode, user intervention is required.

• Tight-integration with TimeFinder product family.
• Management capability by using the current storage management portfolio of software products.
• Geographically dispersed secondary and tertiary sites.