Brian, you raise an interesting question about tuning MDC on a per user basis vs. a system basis vs. a combination. A single solve all algorithm would be cool, but beyond my imagination to make it perfect. I believe there was a lot of research originally about
While MDC is a write-through cache, it does not automatically insert writes into the cache if the disk location being updated is not already in the cache. This makes it a little more forgiving for workloads that have slightly higher write activity. I wonder if the 80/20 rule could be applied with something such as if my read/write ratio is lower than "n", do not use MDC. The tricky part is determining "n". If the ratio is 1 or lower, the data is probably read once, write once and won't benefit from the cache. Part of what it depends on, is the write channel program some are worse than others. In most cases, MDC can determine which blocks are involved with the write I/O and handle appropriately. There are exceptions where complicated write channel programs confuse MDC and for integrity reasons, it will purge more of the cache then it needs. Those are my few random thoughts on this. Emphasis on random. :-) Bill Bitner - VM Performance Evaluation - IBM Endicott - 607-429-3286
