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Conceptually and operationally, SRDF is designed to work in a WAN/Internet/Cloud/SAN environment with multiple Symms involved, while Timefinder is local to a Symm, but performs the same functions.

The difference, SRDF can be performed without Geographic boundaries, while Timefinder is local. The following are various different forms of SRDF that can be used by a customer to perform SRDF operations. 

Synchronous mode

With Synchronous mode, the remote symm must have I/O in cache before the application receives the acknowledgement. Depending on distance where these Symmetrix machines are located, this may have a significant impact on performance. This form of SRDF is suggested to be implemented in a campus environment.

If you want to ensure that the data is replicated real time without dirty tracks from one symmetrix to the other, you might want to enable Domino effect. With Domino effect, your R1 devices will become not ready if the R2 devices cant be reached. 

Semi-synchronous mode

With Semi-synchronous mode, the I/O between the R1 and R2 devices are always out of sync. The application receives the acknowledgement from the first write I/O to the local cache. The second I/O isn’t acknowledged until the first is in the remote cache. This form of SRDF is faster than the previous mentioned Synchronous mode. 

Adaptive Copy-Write Pending

With Adaptive Copy-Write Pending, all the R2 volumes are copied over without the delay of acknowledgement from the application. With this mode, we can setup a skew parameter that will allow max number of dirty tracks. Once that number is reached, the system switches to a preconfigured mode like the semi-synchronous mode until the remote data is all synced. Once this is hit, SRDF is switched back to Adaptive Copy-Write Pending mode.