I have read up all the information there is about the store.exe (exchange db) utilising tonnes of memory, but I haven't found any reference guide as to how much of it is actually worth using per user.
My question is, is 20GB of store.exe db memory consumption too much for SBS 2011 Exchange 2010 with only 13 users?
It's a normal behavior. Exchange allocate all memory and release it to the OS if another application need it.
By default, the msExchESEParamCacheSizeMax key is not set, which means the store can allocate the memory it needs dynamically. ESE (store.exe) will grow the cache to consume almost all available RAM on the server if there is no other memory pressure on the system For example, if the server contains 16gb physical memory, if there is no other memory pressure, one could expect that the store.exe process will grow to use up to 14gb memory (16gb minus 2gb allocated to Kernel mode). This much larger database cache size results in greatly reduced disk I/O, and is preferred anyways, as reading information from memory is much faster than reading information from disk. If memory pressure occurs, as when other applications request/require memory, ESE will appropriately shrink the size of the database cache. It’s not recommended to modify the msExchESEParamCacheSizeMax attribute of the information store object. Lowering this value may degrade performance, in terms of server performance as well as in terms of end-user experience.
