FusionSync requires the use of a Journal LUN within each data center. The LUN is only used when FusionSync is suspended between data centers. This section describes some of the Journal LUN metrics so that you can correctly size, provision, and maintain the Journal LUN.

Regardless of the number of LUNs in the data center that are being projected by Core and protected by FusionSync, you only need one Journal LUN for each data center. The LUN only needs to keep track of the write operations, so no actual data blocks are stored on the Journal LUN. Because no storage is required, the size of the Journal LUN is quite modest compared to the size of the LUNs on the backend storage array.

You can size the Journal LUN with this formula:

Therefore a product deployment that has 25 LUNs needs a 51 GB Journal LUN.

The size of the Journal LUN is checked against the number of LUNs it is configured to support for FusionSync. If the size is too small, then an alert is raised and the FusionSync replication service does not start.

The Journal LUN can be thin-provisioned and, if required, it can also be dynamically increased in size.

Journal LUN-error handling is fairly extensive and generally tends to include checks for loss of connectivity and offlining, but it can include events such as out of storage, corruption, and shrinking size. In these cases, an alarm is raised on the Core that alerts to various conditions. For more details on error handling, see the Core user guide.