Fast failback versus difference-based synchronization
In the case of fast failback, the data blocks that changed while the original Primary was unavailable are tracked using the DCM for each volume. Difference-based synchronization computes MD5 checksums for a fixed size data block on the Primary and Secondary data volumes, compares it, and then determines whether this data block needs to be transferred from the Primary data volume to the Secondary data volume. The fast failback feature is recommended over the difference-based synchronization for the following reasons:
For difference-based synchronization, all the blocks on all the Primary and Secondary data volumes are read; in the case of fast failback, only the blocks that changed on the new Primary are read and hence the number of read operations required is smaller.
For difference-based synchronization, the differences are determined by computing and comparing checksum of each of the data chunks on the Secondary and Primary; in the case of fast failback, there is no need to compute checksum because the differences are tracked as they happen, which makes fast failback faster.
The following sections describe each of the above methods for failing back to the original Primary.