NSS mirroring is a checkpoint based synchronous mirroring solution. Data blocks are written synhronously to multiple storage devices. It is an alternative to SAN array based synchronous replication options.
Hardware configuration and placement for NSS mirroring can vary depending on geography and fault tolerance requirements. For example, the following figure depicts a simple hardware configuration in which one side of the mirrored NSS volume is located in a separate building from the rest of the cluster hardware. Should a disaster occur in one building, data would still be safe on the mirrored NSS volume in the other building.
Figure 1-1 Single Cluster with Mirrored NSS Volumes in Separate Buildings
The following figure depicts a more complex hardware configuration in which two clusters are placed in separate buildings, but each has one side of a mirrored NSS volume in the building with the other cluster. Should a disaster occur in one building, data would still be safe and immediately accessible on the mirrored NSS volume in the other building.
Figure 1-2 Two Clusters with Mirrored NSS Volumes in Separate Buildings
The maximum distance for a single stretch of fibre without optical repeaters to boost distance is about 10 Kilometers. There are various devices available that convert Fibre Channel to IP in order to extend SANs to WAN scale.
NSS mirroring is synchronous, and so mirror I/Os have to complete before the next I/O can be sent. Performance is a function of time to do an I/O over the longest link.