Without PAV, having Linux issue 3 I/Os to the same physical volume will NOT help performance. If one of those I/O must be satisified by activity to a real disk, all of the remaining I/O queued on the device will wait, even those with data residing in cache who could be satisfied immediately.
And the fact that VM "orders the I/O by seek address" really makes no difference with today's DASD. It's mostly a waste of cycles. The inability for VM (and linux guests under VM) to efficiently use large disks is the issue with VM not having PAV - installations with SHARK or Symetrix are better off defining many smaller "PHYSICAL" devices. This is how VM gets around not having PAV. Thus when linux issues 3 I/O, they are to 3 different "physical" devices, VM can drive all 3 I/O, and those I/O directly satisfied by the cache do not have to wait for those I/O needing disk access. In addition, and specifically for databases where high bandwidth is needed, not only do you want the LVM'ed volumes to be on different physical volumes, but also having different channel paths, and different internal paths on the storage controller when possible. > >I always thought, and I may be wrong, that one of the main >advantages to having either a PAV or individual minidisks was >so that instead of having 1 queue for the device, you now have >many. If your first I/O in the queue needs to do a physical >I/O, then all the other I/Os wait. If most of the I/Os have >their data already in cache, then those I/Os will be done at the >same time as the physical I/O. I know we get about 95% of all >our I/Os on MVS from cache. > >Eric Bielefeld >Sr. MVS Systems Programmer >P&H Mining Equipment >Milwaukee, WI >414-671-7849 >[EMAIL PROTECTED] > >>>> [EMAIL PROTECTED] 03/26/03 02:33PM >>> >Physically, that's true -- ultimately there is only one physical >I/O in progress. However, by splitting up the disks into a >larger number of small chunks and presenting them to Linux in >the virtual machine configuration, the Linux system sees the >smaller minidisks as separate volumes, and thus schedules >multiple I/Os to what it thinks is multple devices. CP >coordinates all the actual disk I/O and everybody wins. > >If you can, stripe the pieces across multiple physical voluems, >but it's not as important. > >-- db > >David Boyes >Sine Nomine Associates "If you can't measure it, I'm Just NOT interested!"(tm) /************************************************************/ Barton Robinson - CBW Internet: [EMAIL PROTECTED] Velocity Software, Inc Mailing Address: 196-D Castro Street P.O. Box 390640 Mountain View, CA 94041 Mountain View, CA 94039-0640 VM Performance Hotline: 650-964-8867 Fax: 650-964-9012 Web Page: WWW.VELOCITY-SOFTWARE.COM /************************************************************/
