Asynchronous replication writes data to the primary storage array first and then, depending on the implementation approach, commits data to be replicated to memory or a disk-based journal. It then copies the data in real-time or at scheduled intervals to replication targets.
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Synchronous vs. asynchronous replication
The primary difference between synchronous replication and asynchronous replication is the way in which data is written to the replica. Most synchronous replication products write data to primary storage and the replica simultaneously. As such, the primary copy and the replica should always remain synchronized.
In contrast, asynchronous replication products write data to the primary storage first and then copy the data to the replica. Although the replication process may occur in near-real-time, it is more common for replication to occur on a scheduled basis. For instance, write operations may be transmitted to the replica in batches on a periodic basis (for example, every five minutes).
The benefits of asynchronous replication
There are two main benefits to asynchronous replication:
- It tends to cost significantly less than synchronous replication. Synchronous replication requires more bandwidth than asynchronous replication and may also require specialized hardware (depending on the implementation).
- It is designed to work over long distances. Since the replication process does not have to occur in real time, asynchronous replication can tolerate some degradation in connectivity.
Synchronous replication is typically used to provide high availability of critical applications. In this scenario, failover from the primary to secondary array is nearly instantaneous, to ensure little to no application downtime. As noted above, it is also expensive.
Asynchronous replication use cases
There are a variety of data replication tools available today, and choosing one will depend on what you want to accomplish. Asynchronous replication is commonly used in data backups. These backups may be local, but the technology is frequently used for offsite backups. Cloud backups also use asynchronous replication. Some enterprise-grade hypervisors include an asynchronous replication feature that allows entire virtual machines (VMs) to be replicated to a remote location so VMs may fail over to that location in the event of a disaster. This is commonly referred to as instant recovery or recovery-in-place, and a number of backup software products support this functionality. Asynchronous replication is also frequently used with storage snapshots for continuous data protection.
In the video to the left, data storage expert Jon Toigo describes the challenges of using synchronous replication and asynchronous replication during a disaster situation.
See also: cloud backup