Administrator Guide

Data replication primer
11 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
Figure 2 demonstrates the write I/O pattern sequence with respect to asynchronous replication.
1. The application or server sends a write request to the source volume.
2. The write I/O is committed to the source volume.
3. Finally, the write acknowledgement is sent to the application or server.
The process is repeated for each write I/O requested by the application or server.
4. Periodically, a batch of write I/Os that have already been committed to the source volume are
transferred to the destination volume.
5. The write I/Os are committed to the destination volume.
6. A batch acknowledgement is sent to the source.
Asynchronous replication write I/O sequence
2.1.3 Semi-synchronous
With SC Series storage, semi-synchronous replication behaves like synchronous replication in that application
transactions are immediately sent to the replication destination storage (assuming that the replication link and
destination storage have the bandwidth to support the current rate of change). The difference is that the write
I/O is committed at the source volume and an acknowledgement is sent to the storage host and application
without a guarantee that the write I/O was committed at the destination storage. Semi-synchronous replication
is configured in Dell Storage Manager by creating asynchronous replication between two volumes and
checking the box for Replicate Active Snapshot. A snapshot is an SC Series storage term that describes
frozen data. The Active Snapshot refers to newly written or updated data that has not yet been frozen in a
snapshot. Semi-synchronous offers a synchronous-like recovery point objective (RPO) without application
latency, but the RPO and loss of data in an unplanned outage scenario cannot be guaranteed.