So regardless of MDC settings, it doesn't matter where the guest runs when IO is required. Only when moving guests around MDC could be a potential problem. That is not the case, at least not at the moment.
Thanks, Berry. -----Original Message----- From: Linux on 390 Port [mailto:[email protected]] On Behalf Of Rob van der Heij Sent: maandag 23 maart 2009 13:20 To: [email protected] Subject: Re: Share DASD in 4 VM's 2009/3/23 van Sleeuwen, Berry <[email protected]>: > Now let's look at the sharing of DASD volumes and with multiple > (non-CMS) minidisks on them. What, if any, would be the penalty if the > DASD is shared on multiple VM's with linux minidisks? Like I stated > before, VSE can't handle this if DASD is not set to SHARED (either > minidisk or fullpack volumes). Would we get some unexpected IO queuing > or even locks when two minidisks are accessed at the same time from > different VM's? (Or to put it like this: if we start a guest om VM1 > would the guest on VM2 still be able to access his minidisk that lives > on the same pack?) With reserve / release it's different because the protocol is to ensure exclusive access to the real volume while the reserve is held. If you would put other data on the same volume, you risk being locked out too long or even risk a deadly embrace. When a real volume is just shared between two systems in that users on both systems use different mini disks on it, then it's up to the DASD control unit to review the I/O's and see which can be done in parallel. Each guest I/O specifies the range of cylinders that it operates on, worst case the full mini disk. So to the control unit it will be clear that different virtual machines don't interfere and could go in parallel. In your case, spreading the virtual machines over multiple z/VM LPARs may even be an advantage. When the virtual machines run in the same z/VM, then CP will queue the I/O from the users for that volume and it requires work from CP to start the next I/O in the queue once the previous one completed. When you're on different LPARs, the I/O will queue in the DASD control unit and is performed without any CP intervention. Re your idea to have MINIOPT to disable MDC: be aware when you have also overlapping full pack MDISK definitions (for backup for example). Unless those also have the MINIOPT on that MDISK (which you probably should) then that would still get data in MDC and may surprise the guest when he travels. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- 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
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