On Friday, 06/03/2011 at 11:30 EDT, gclo...@br.ibm.com wrote: > A millennium ago (before year 2000) I ear that DEDICATE dasd have better
> performance, maybe due no need to translate CCW. But, I change all my > definitions when DEVNO was introduced: gain in flexibility, enabling dasd > sharing for full packs without need to any Attach (when not in use, dasd > remains FREE, isolated from CMS). And no performance problems. > My question: nowadays, the performance still is a factor to use DEDICATE versus > MDISK DEVNO? Back when we had native mode on the machines (no LPAR), dedicated devices were eligible for I/O Assist in V=R and V=F guests. That's all gone. Today, a dedicated device: - Does not have CCW command word validity checks - Does not use minidisk cache - Can be attached to only one user at a time (subject only to gaming of the IOCDS) - Is not subject to ESM security controls - (I think) does not have a CCW chain broken by CP in some circumstances (working allegiance is maintained by default) As to whether that will affect performance in any meaningful way depends entirely on the I/O workload. But I don't think I'd use performance as my primary criteria. I'd probably start with DEVNO and then see if I have any issues that DEDICATE would solve. Alan Altmark z/VM and Linux on System z Consultant IBM System Lab Services and Training ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 alan_altm...@us.ibm.com IBM Endicott
