Cross-Site High-Availability Model

In the next model, illustrated in Figure 17.2, the same concepts apply, except that the SQL mirror, witness, and additional SharePoint farm members are physically housed in a separate site. This is an ideal model, but it requires a backup data center that is physically close by because the network latency must be less than 1ms and the bandwidth between sites must be verifiably above 1Gb at all times.

Figure 17.2. Examining the cross-site high-availability SQL mirroring model.

In this model, if the primary data center were to completely fail, the SharePoint environment would continue to run on the synchronously mirrored databases in the backup data center.

As a side note, Microsoft has supported enviroments with latency levels up to 10ms, but it is critical that there not be spikes in the WAN traffic and that there is guaranteed bandwidth and low latency because these spikes can cause transactions to timeout on the production side of the SQL mirror. That said, the official Microsoft stance at the time of this writing is that 1ms of latency is required.