You can determine the extent to which the performance of the file system cache varies with the size of the cache. Compare the size of the cache over time to the rate at which data sought in the cache is found there.
Use the Memory: Cache Bytes counter as an indicator of the size of the cache. Use Memory: Cache Faults/sec as an indicator of the rate of cache misses, and Cache: MDL Read Hits % as an indicator of the rate of cache hits.
If Memory: Cache Faults/sec rises and Cache: MDL Read Hits % falls when the file system cache is smaller, the cache might be too small to be of much benefit to the server. A less effective file system cache is likely to degrade the performance of an IIS server significantly, especially if the size of the cache is often reduced due to a general memory shortage.
If cache performance is poor when the cache is small, use the data you have collected to infer why the cache size is reduced. Choose a period when the cache is small and note the available memory on the server and the processes and services running on the server, including the number of simultaneous connections supported.
When you add physical memory to your server, the system allocates more space to the file system cache. A larger cache is almost always more efficient. In addition, defragmenting your disks makes it more likely that related pages are copied into the cache together and this improves the hit rate of the cache. Finally, consider reducing the workload on the server by moving some of the load to another server.