Administrator Guide

Data replication primer
10 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
Figure 1 demonstrates the write I/O pattern sequence with synchronous replication:
1. The application or server sends a write request to the source volume.
2. The write I/O is mirrored to the destination volume.
3. The mirrored write I/O is committed to the destination volume.
4. The write commit at the destination is acknowledged back to the source.
5. The write I/O is committed to the source volume.
6. 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.
Synchronous replication write I/O sequence
2.1.2 Asynchronous
Asynchronous replication accomplishes the same data protection goal in that data is replicated from source
storage to destination storage. However, the manner and frequency that the data is replicated differs from
synchronous replication. Instead of committing a write at both replication source and destination
simultaneously, the write is committed only at the source and an acknowledgement is then sent to the storage
host and application. The accumulation of committed writes at the source volume are replicated to the
destination volume in one batch at scheduled intervals and committed to the destination volume.
Aside from replicating the active snapshot (semi-synchronous replication is discussed in section 2.1.3),
Asynchronous replication in SC Series storage is tied to the source volume replication schedule. When a
snapshot is created on the source volume, and that volume is configured for asynchronous replication, the
new snapshot is replicated to the destination volume. Snapshots on a volume may be created automatically
according to a schedule or manually created from a variety of integration tools. Regardless, all snapshots
occur on a per-volume basis. As a result, volumes may adhere to their own independent replication schedule,
or they may share a replication schedule with other volumes leveraging the same snapshot profile. This type
of replication is also referred to as a point-in-time replication, which is a type of asynchronous replication that
specifically leverages volume snapshots. Because asynchronously replicated transactions are not required to
wait for write committals at the replica destination volume, the replication link and/or destination storage will
not contribute to application or transaction latency at the source volume.