The Asynchronous Mirroring feature differs from the Synchronous Mirroring feature in one essential way: it captures the state of the source volume at a particular point in time and copies just the data that has changed since the last image capture.

With synchronous mirroring, the state of the primary volume is not captured at some point in time, but rather reflects all changes that were made on the primary volume to the secondary volume. The secondary volume is identical to the primary volume at every moment because, with this type of mirror, each time a write is done to the primary volume, a write is done to the secondary volume. The host does not receive an acknowledgment that the write was successful until the secondary volume is successfully updated with the changes that were made on the primary volume.

With asynchronous mirroring, the remote storage system is not fully synchronized with the local storage system, so if the application needs to transition to the remote storage system due to a loss of the local storage system, some transactions could be lost.

Comparison between mirroring features:

Asynchronous Mirroring	Synchronous Mirroring
Replication method
Point-in-Time Mirroring is done on demand or automatically according to a user-defined schedule. Schedules can be defined at the granularity of minutes. The minimum time between syncs is 10 minutes.	Continuous Mirroring is automatically executed continuously, copying data from every host write.
Reserved capacity
Multiple A reserved capacity volume is required for each mirrored pair.	Single Single reserved capacity volume is required for all mirrored volumes.
Communication
iSCSI and Fibre Channel Supports iSCSI and Fibre Channel interfaces between storage systems.	Fibre Channel Supports only Fibre Channel interfaces between storage systems.
Distance
Unlimited Support for virtually unlimited distances between the local storage system and the remote storage system, with the distance typically limited only by the capabilities of the network and the channel extension technology.	Restricted Typically must be within about 10 km (6.2 miles), of the local storage system to meet the latency and application performance requirements.

Asynchronous Mirroring

Synchronous Mirroring

Replication method

Point-in-Time

Mirroring is done on demand or automatically according to a user-defined schedule. Schedules can be defined at the granularity of minutes. The minimum time between syncs is 10 minutes.

Continuous

Mirroring is automatically executed continuously, copying data from every host write.

Reserved capacity

Multiple

A reserved capacity volume is required for each mirrored pair.

Single

Single reserved capacity volume is required for all mirrored volumes.

Communication

iSCSI and Fibre Channel

Supports iSCSI and Fibre Channel interfaces between storage systems.

Fibre Channel

Supports only Fibre Channel interfaces between storage systems.

Distance

Unlimited

Support for virtually unlimited distances between the local storage system and the remote storage system, with the distance typically limited only by the capabilities of the network and the channel extension technology.

Restricted

Typically must be within about 10 km (6.2 miles), of the local storage system to meet the latency and application performance requirements.