Now that Exchange 2000 Server Service Pack 1 (SP1) has shipped, we're learning the real scoop about Exchange clustering. I covered this topic in the May 25, 2001, Exchange and Outlook UPDATE. I also covered this topic in my Windows 2000 Magazine article "Exchange 2000 SP1 on Datacenter," July 2001. (You can read both articles at their respective URLs below.) However, since I wrote those articles, Microsoft has made some important changes to Exchange 2000 clustering support.
First, under pressure to get more than 1000 connections per active/active cluster node, Microsoft has extended the estimated connection limit from 1000 concurrent connections to 1500 for two-node active/active clusters (two-node and four-node N+1 clusters still have the same limits and guidelines). These connection limits are still theoretical: Your actual mileage will vary depending on variables such as client type and workload profile. However, the main determinant of concurrent connection limits is available virtual memory on the server (out of a maximum of 3GB in Win2K).
To alleviate the limitations that a shortage of virtual memory causes, Microsoft has made changes to Exchange 2000 SP1 that let Store.exe preallocate virtual memory so that large virtual-memory blocks are available in the event of a cluster failover. Microsoft also added some Performance Monitor counters to better monitor virtual-memory parameters that can cause problems during cluster failover. You need to monitor these counters for both active/active and N+1 clusters. The following counters are available in the MSExchangeIS object.
- MSExchangeIS\VM Largest Block Size. This counter displays the size in bytes of the largest free block of virtual memory. When this counter drops below 32MB, Exchange 2000 logs a warning in the event log (Event ID=9582) and logs an error if the counter drops below 16MB. Monitor this counter to ensure it stays above 32MB.
- MSExchangeIS\VM Total 16MB Free Blocks. This counter displays the total number of free virtual-memory blocks that are greater than or equal to 16MB. Monitoring trends on this counter lets a systems administrator predict when the number of 16MB blocks is likely to drop below three, at which point Microsoft recommends you restart or fail over all the services on the node.
- MSExchangeIS\VM Total Free Blocks. This counter displays the total number of free virtual-memory blocks regardless of size. You can use this counter to measure the degree to which the system is fragmenting available virtual memory.
- MSExchangeIS\VM Total Large Free Block Bytes. This counter displays the sum in bytes of all the free virtual-memory blocks that are greater than or equal to 16MB.
Although Microsoft hasn't delivered the ideal clustering solution for Exchange 2000, the changes in Exchange 2000 SP1 are welcome and certainly an improvement over Exchange Server 5.5 clustering. If you pay attention to these details and follow the deployment guidelines that Microsoft provides at the third URL below, you can successfully deploy Exchange 2000 clusters.