Synchronous vs asynchronous replication: Order of events during data writes

Synchronous vs asynchronous replication: Order of events during data writes

Synchronous vs asynchronous replication: Order of events during data writes

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Synchronous vs asynchronous replication: How do they differ with regard to the order of events during data writes? 

When comparing synchronous vs asynchronous replication, let’s take a scenario where a host on Site A talks to a primary storage array on Site A with data on the storage array replicated to a secondary storage array on Site B. As the processing takes place at Site A, this will be called the local site, and Site B the remote site. For both synchronous and asynchronous replication, the storage array on Site A will send the transaction acknowledgement to the host on Site A. The difference between the two replication technologies is the order of events that take place after the host sends the transaction to the local storage array.

For asynchronous replication the order of events will be as follows:

1.    Site A host sends a write transaction to Site A storage array.

2.    Site A storage array commits the transaction to cache and sends an acknowledgement to the host.

3.    Site A storage array sends the update to the Site B storage array following a time delay.

4.    Site B storage array sends an acknowledgement to the Site A storage array.

For synchronous replication, the order of events will be as follows:

1.    Site A host sends a write transaction to Site A storage array.

2.    Site A storage array commits the transaction to cache and immediately sends the update to the Site B storage array.

3.    Site B storage array sends an acknowledgement to the Site A storage array.

4.    Site A storage array sends an acknowledgement to the host.

With synchronous replication, both arrays process the transaction before an acknowledgement is sent to the host, meaning the arrays will always be synchronised.

With asynchronous replication, the secondary storage array will almost always be a few transactions behind the primary array. It should be noted that as the distance between the primary and secondary data centres increases, the acknowledgement delay caused by synchronous replication can make it inappropriate for some applications.

This was first published in April 2011

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