This difference is the maximum amount of memory you can assign to the current SQL Server instance. Before you set the amount of memory for SQL Server, determine the appropriate memory setting by subtracting, from the total physical memory, the memory required for the OS, memory allocations not controlled by the max_server_memory setting, and any other instances of SQL Server (and other system uses, if the computer is not wholly dedicated to SQL Server). SQL Server can use memory dynamically however, you can set the memory options manually and restrict the amount of memory that SQL Server can access. For more information, see Database Engine Service Startup Options. If you cannot start SQL Server after changing this option, start it using the -f startup option and reset max server memory to its previous value. Setting max server memory to the minimum value can even prevent SQL Server from starting. However, setting this value too low could cause significant memory pressure and performance problems. Setting max server memory value too high can cause a single instance of SQL Server to compete for memory with other SQL Server instances hosted on the same host.
