One question, is the DASD subsystem you are on shared with z/OS systems (running outside of z/VM). We have used PAVs for several years for our z/OS systems. We have gone from what is called WLM managed PAV to HyperPAV. I have avoided using PAVs for our z/VM lpars because I was not sure how well z/OS and z/VM would play together with the management of the PAV addresses. To avoid a possible headache I have define the DASD for z/VM as just 3390 (not 3390B).
Paul Feller AIT Mainframe Technical Support From: The IBM z/VM Operating System [mailto:[email protected]] On Behalf Of Mike Walter Sent: Monday, December 20, 2010 12:24 PM To: [email protected] Subject: Re: General CMS minidisks and SFS on PAV DASD? Thanks, Kris. I knew about SFS (we dedicate whole volumes to individual SFS filepools and their servers, with no volumes shared by servers), and DB2 (tho we don't use DB2 on z/VM). To answer George's question about "why not?"...' PAV first became available for z/VM CMS mdisk use with z/VM 5.2. We're at z/VM 5.4. I just didn't want to be one of the first folks on the block to actually use PAV for CMS minidisks, and then find my life complicated by more problems caused by a relatively new, lesser-used z/VM feature. Just because something is documented as working and supported does not mean that it may be a good idea to try it at any given time without some research. It *has* been a while since z/VM 5.2 came out, but even that doesn't mean that people have begun using PAV for CMS minidisks. Our z/VM systems have been far too stable to risk trying something without due consideration. And again, until this weekend, all the PAV DASD here was allocated to our many z/OS systems - so I never had to give it's actual use any serious consideration. I just wanted to get hear if real people using PAV for CMS minidisks had experienced wonderful results, or awful headaches. Sort of a "belts AND suspenders" approach. Thanks! Mike Walter Aon Corporation The opinions expressed herein are mine alone, not my employer's. "Kris Buelens" <[email protected]> Sent by: "The IBM z/VM Operating System" <[email protected]> 12/20/2010 11:24 AM Please respond to "The IBM z/VM Operating System" <[email protected]> To [email protected] cc Subject Re: General CMS minidisks and SFS on PAV DASD? CMS IO can indeed profit from PAV: CP knows how to look at PAV aliases when a disk is busy. SFS is another story: the SFS SW knows where its minidisks are located. It will not attempt to launch an IO to a volume to which it has an active IO. So: if you have multiple SFS servers on a given PACK, SFS will profit of PAV, else it won't. (and, I do encounter many installations where people place all MDISKs of a given SFS on as few disks as possible. It is better to spread). DB2 is yet another story: if you use dataspaces, DB2's IOs are performed by CP's paging routines, so no PAV.... Else I think the same rules as for SFS apply. 2010/12/20 George Henke/NYLIC <[email protected]<mailto:[email protected]>> Why would you NOT want PAV for CMS mds? The IO Supervisor has not kept up with the hardware. It still thinks of a disk device as a "spinning platter" when in fact it is a rank of RAID devices striped over numerous HDs and cached in a disk controller from where it is actually being read thereby permitting multiple IOs to the same device number.. The problem is that the IO Supervisor checks the IOB Busy Bit before issuing a SIO and, if it is on, unnecessarilly suspends the SIO until the device is idle. Instead of changing the IO Supervisor, IBM has opted to fake it out by defining alias devices for the same device number in PAV. Since most workloads these days are still IO bound, why would you still want to unnecessarilly "single thread" IO, why would you NOT want PAV on CMS mds, SFS, or whatever? Mike Walter <[email protected]<mailto:[email protected]>> Sent by: The IBM z/VM Operating System <[email protected]<mailto:[email protected]>> 12/20/2010 11:33 AM Please respond to The IBM z/VM Operating System <[email protected]<mailto:[email protected]>> To [email protected]<mailto:[email protected]> cc Subject General CMS minidisks and SFS on PAV DASD? We've always avoided using PAV volumes for general user (non-fullpack) CMS minidisks. Instead we've only used DASD defined as 3390-3 so that I/O queueing is minimized. That choice was in part due to management decisions. Over the past weekend we needed to move some z/VM DASD quickly -- and the target DASD was already defined as PAV DASD. The z/VM CP Planning and Administration manual clearly states that "z/VM Paging and SPOOLing operations do not take advantage of PAV." (no argument here). We'll still plan to keep page and SPOOL volumes on non-PAV 3390-3 DASD. But the same manual also states that "When multiple CMS volumes are defined on a real PAV volume, I/O operations by CMS can be concurrently scheduled on any real PAV base or alias subchannel by z/VM. The CMS user does not need to take any action for this to occur." Well, that's "book larn'in". Can anyone provide real-life results of using PAV volumes for general-purpose CMS user minidisks, and... for SFS filespaces? Do you see real I/O improvement for those apps? If so, the next time we're asked we might recommend larger 3390 volumes, mod 9's or 27's (depending on the number of available paths) to permit larger minidisks without SFS overhead, and improved SFS performance. Thanks! Mike Walter Aon Corporation The opinions expressed herein are mine alone, not my employer's. The information contained in this e-mail and any accompanying documents may contain information that is confidential or otherwise protected from disclosure. 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