Synchronous SnapMirror is a SnapMirror feature in which the data on one system is replicated on another system at, or near, the same time it is written to the first system. Synchronous SnapMirror synchronously replicates data between single or clustered storage systems situated at remote sites using either an IP or a Fibre Channel connection.
Note
You can use Synchronous SnapMirror only on volumes, not qtrees.
Before Data ONTAP saves data to disk, it collects written data in NVRAM. Then, at a point in time called a consistency point, it sends the data to disk. When the Synchronous SnapMirror feature is enabled, the source system forwards data to the destination system as it is written in NVRAM. Then, at the consistency point, the source system sends its data to disk and tells the destination system to also send its data to disk. Finally, the source system waits for the destination system to acknowledge that it sent data to disk before continuing with the next write.
If there are problems with your network, your synchronous replication might go into an asynchronous mode. Ordinarily, the source and destination systems periodically communicate with each other. In the event of a network outage, synchronous SnapMirror goes into an asynchronous mode if the periodic communication is disrupted. When in asynchronous mode, the source system tries to communicate with the destination system once every minute until communication is reestablished. Once reestablished, the source system asynchronously replicates data to the destination every minute until a synchronous replication can be reestablished.
You should consider the following criteria before using the synchronous SnapMirror feature:
Synchronous SnapMirror is not supported on the following systems:
The following are configurations that are not supported:
You decide which data you want synchronously replicated. For example, you might synchronously replicate database data and asynchronously replicate home directories. If the home directories contain important log data, you might synchronously replicate it, but adjust the synchonicity of the transfers to maintain performance levels. Synchronous SnapMirror allows a user-defined lag time of a number of operations or milliseconds before transferring data. See snapmirror.conf file entry syntax and the na_snapmirror.conf(5) man page for details.
Synchronous SnapMirror is supported only for configurations of which the source system and destination system are the same type of system and have the same disk geometry.
The type of system and disk geometry of the destination system impacts the perceived performance of the source system. Therefore, the destination system should have the bandwidth for the increased traffic and for message logging. Log files are kept on the root volume; therefore, you should ensure that the root volume spans enough disks to handle the increased traffic. The root volume should span four to six disks.
Synchronous SnapMirror can support traffic over Fibre Channel and IP transports. SnapMirror also allows multipathing, giving you the ability either to balance the load between two paths or to reserve the second path for failover. For optimizing performance, you can use the best route available between the source and destination system and you can restrict the route to traffic between the two systems. See Using SnapMirror over multiple paths for details.
In addition to a standard SnapMirror license, the synchronous SnapMirror feature requires a special license key. You must enter this key and receive important product information prior to enabling the synchronous SnapMirror feature.
To enable the synchronous SnapMirror feature, use the license add command, and enter one of the following codes.
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Last updated: 01-Dec-2005 16:36:03