On Thu, Jul 2, 2009 at 10:47 PM, Kris Buelens<[email protected]> wrote: > The simple MW approach is surely wrong, it will not create a PAV > environment: Linux will think it has 3 different devices, accidents will > happen.
You're wrong. A very simple experiment could have shown you that: lxrob1:~ # head -n 1 /sys/bus/ccw/devices/0.0.04*2/uid ==> /sys/bus/ccw/devices/0.0.0402/uid <== IBM.68000000033025.0120.28.000008bb00000bda0000000000000000 ==> /sys/bus/ccw/devices/0.0.0412/uid <== IBM.68000000033025.0120.28.000008bb00000bda0000000000000000 lxrob1:~ # vmcp q md 402 loc TargetID Tdev OwnerID Odev Dtype Vol-ID Rdev StartLoc Size ROBLX1 0402 ROBLX1 0402 3390 VM4V05 0148 2235 800 lxrob1:~ # vmcp q md 412 loc TargetID Tdev OwnerID Odev Dtype Vol-ID Rdev StartLoc Size ROBLX1 0412 ROBLX1 0402 3390 VM4V05 0148 2235 800 Note: The casual observer may spot the beginning and end of the extent in the VM appendix to the device identification. > MDC will avoid the IO; Control Unit cache hit is still IO as concerned for > the z Series, but here PAV would help. AFAIK, PAV will not help if the > concurrent IOs are not satisfied mostly from the control unit cache: the > real disk can only handle one IO anyhow. Not entirely. The current DASD subsystems under the covers also do various magic that may enable the back-end to spread a logical ECKD volume over multiple physical drives and thus have more than one I/O active at the same time on a single logical volume. It is fairly easy to demonstrate whether it works. One reason I did not (yet) react on Bob's questions is that it is rather complicated to determine which workload would benefit from it, if any. I stopped working on the draft response twice because it got way too long to post ;-) In general, I don't put energy in letting a single virtual machine monopolize any subsystem. On z/VM you normally have sufficient others that share the resources. Performance measurements that try to drive a single virtual machine as fast as possible have little value in my world. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/
