MDC is a write-through cache so it does not apply to write I/Os It can help read I/Os, but the answer to how/if to use it is "it depends". As Carsten mentioned, it can be great for shared minidisks with significant read I/O. There are also cases where for non-shared minidisks, I might want to use it. While Linux does a lot of caching, that requires memory. In order to get very high hit rates at the Linux level, you need to supply enough virtual memory for its cache. The downside is Linux will use it, and increase pressure on overall system memory requirements. So there are cases where it is better to have a lower virtual machine sizes for the Linux guests and have MDC pick up some of the missed I/Os. This is a trade-off between memory (linux cache) and processor resoures (MDC handling the I/O).
Another case where MDC may be helpful is that it may avoid having to bring a guest page below 2GB for processing if its an MDC hit. This can be helpful if you are constrained by memory below 2GB. When used for this approach, we sometimes suggest forcing MDC to be used from real storage, not expanded storage. I do recommend turning MDC off for write-mostly disks (log files), most Database applications, and back-up processing. Though, there are always exceptions. Other information may be found at http://www.vm.ibm.com/perf/tips/prgmdcar.html Bill Bitner - VM Performance Evaluation - IBM Endicott - 607-429-3286 ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
