Now that I had time to think about it a little more. 80% MDC cache hit ratio isn't that good. How much memory would it take to get the hit ratio up to 95% (and can you afford the memory).
My concern now, is that your application mix may not be a good caching candidate. i.e. if you have large amounts of active disk, (now going off on a tangent), say terabytes in size, in order to have good MDC hit ratios, you may need large (say gigabyte range) MDC. Problem is, every read I/O, first has to search MDC. MDC is effecient, but eventually the CPU cost of searching MDC is greater than the cost of just doing the I/O and let some other processors (channel processors/controllers) user their CPU. And, in this case, if you have a MDC miss, not only you had the CPU overhead of searching MDC, but then you still had to do the I/O and insert the data into MDC. So, it may not be best to scale up MDC to handle everything. But you can still turn off MDC for all volumes but your very active ones, in order to keep the MDC cache size to a reasonable size, and keep very high hit ratios. And, if you use a subset of cached volumes, you can always code an EXEC to change what volumes are subject to MDC prior to your batch processing (if that makes sense for your batch type jobs). Tom Duerbusch THD Consulting
