Similar story at my former customer: At a certain time, the VM systems got a DASD subsystem that MVS no longer needed as it was replaced by a more modern one. These MVS guys wanted to see our IO responsetime, which was worse than on MVS. Conclusion (from the MVS guys): VM isn't as good as MVS. I had to repeat over and over again that VM's MDC avoided the IO's that cache best. To my surprize: when we started sharing a DS8000 with z/OS, the I/O response times of z/VM and z/OS became close.
2008/10/31 Rob van der Heij <[EMAIL PROTECTED]>: > On Fri, Oct 31, 2008 at 2:30 PM, Mary Anne Matyaz > <[EMAIL PROTECTED]> wrote: > >> As for MDC, I've been curious about that lately. About a week ago, I turned >> off mdc for a highly active volume, and it seemed to me that resp increased >> rapidly and markedly. > > It's good that you try and measure rather than rely on hearsay or guts > feeling. The nasty part is that I/O measurement is complicated (that's > why I have been postponing such research to the point where I have > more time to spend on it). > > When you set MDC OFF for a virtual machine or virtual device, it > avoids further inserts but anything in MDC will remain there and gets > used on a read (unless you also purge it). I recall from some > experiments in the past that system-wide MDC OFF was the only thing > that really made a difference when the workload did not take advantage > of it. > > Further, the actual I/O performed by CP is may be different with MDC > enabled. When Linux wants a series of blocks and one is found in MDC, > the rest is read through 2 I/O operations by CP. The consequence is > that the size of the average I/O goes down, so the I/O response time > (per I/O operation) gets lower. Whether that makes the throughput of > the application higher with the same factor is not obvious. > > Another gotcha is that MDC takes the low-hanging fruit. So when you > enable MDC the remaining I/O to the DASD subsystem are less likely to > cache than when you would do all I/O. > :anecdote type=sad. > Long ago, we replaced some 3390's with a RAID based subsystem. The > vendor had promised a certain cache hit ratio internally in the > subsystem. When that was not met for VM devices, we were told to > disable MDC because it "interfered" with the DASD subsystem. Clearly, > when VM MDC already avoided the ones that were easy to cache, the > remaining work for the DASD was harder. But going to the subsystem > cache is still slower than taking it out of MDC, so the application > throughput got worse... > :eanecdote. > > Rob > -- > Rob van der Heij > Velocity Software > http://velocitysoftware.com/ > -- Kris Buelens, IBM Belgium, VM customer support
