> I'll shuddup now ;-) I won't shuddup now! I suspect that some "Blanket statements" can turn out to be "Wet Blanket statements", ruining the party at some customer sites, especially at new z/VM sites running POC's on hand-me-down machines without a lot of real memory. As always: "**It depends**!!" {probably (c) Bill Bitner}
Saying that all Linux swap disks should be defined as VDISKs does not take into account different customer situations. We happen to be blessed with lots of cheap(er), hand-me-down DASD from the 500lb Gorilla. OTOH, we _could_ be short of real memory (three large z/OS guests in a virtual sysplex on that system). Memory can be relatively expensive compared to relatively cheap DASD (especially when one gets "hand-me-downs"). Those secondary 'warning flag' VDISKs eat away at real, shared memory. If you have a lot of Linux guests and not a lot of spare memory, aside from making a call to delight your IBM hardware rep, you might want to consider/total the memory cost of hundreds of small 'warning flag' swap disks vs cheap disk. After all, they those swap disks are not supposed to be used regularly. They are a warning that something else needs to be evaluated and improved - maybe even making that servers swap VDISK larger! More likely: trimming the default virtual storage size of the Linux guest to something a lot smaller than the x86 folks say it MUST BE to function properly. But if you let a lot of servers fail over onto the warning flag swap disks without doing anything about it, then someone could complain that all your Linux guest performance is bad and abandon the migration to the mainframe. Of course, a good performance monitor **and the skills to understand how to use it and how to fix the problems** can alleviate that situation. That assumes a lot, as many new z/VM customers (and many old ones, too!) don't have those skills. Summary: if you do use small failover swap space (a very good idea), ensure that the warning is to pop up properly when they are used (as Barton mentioned), test that warning by causing swap to the failover disk (VDISK -or- real disk, regardless), and then react promptly when something sets off the alert in production. Mike Walter Hewitt Associates Disclaimer: What I -don't- know about performance fills lots of books... literally! "Scott Rohling" <scott.rohl...@gmail.com> Sent by: "The IBM z/VM Operating System" <IBMVM@LISTSERV.UARK.EDU> 04/29/2009 08:51 PM Please respond to "The IBM z/VM Operating System" <IBMVM@LISTSERV.UARK.EDU> To IBMVM@LISTSERV.UARK.EDU cc Subject Re: SWAPGEN Right - I got that from Barton's post. Consider that there are times when using a real disk might make sense .. I'm just not big on blanket statements... Dedicating a resource is not always a big waste and does not always benefit one to the detriment of the rest. It's a balancing act and it takes all the tricks in the bag to keep it from falling over. A real disk might make real sense with an ill-behaved, unpredictable guest that is causing you swapping headaches. That's what I was trying to point out.. Maybe I'd stay quiet if I saw some qualifiers like 'in general' when people talk about BIG/REAL WASTE. Guidelines are nice, but I object to them being presented as 'rules'. I'll shuddup now ;-) Scott On Wed, Apr 29, 2009 at 6:16 PM, Rich Smrcina <rsmrc...@wi.rr.com> wrote: The biggest benefit of z/VM is to virtualize resources as much as possible. Dedicating a resource to a specific guest is a big waste and benefits one to the detriment of all the rest. It would be much better to allocate all virtual disk swap for the Linux guests, then allocate the disk that would have been dedicated to swap as page space. That way everyone wins. Scott Rohling wrote: Maybe -- but having a real disk (and setting an alert for that) helps isolate the issue to a single guest rather than affecting critical shared resources (in this case memory/paging) when a guest starts swapping more than normal or than it 'should'. I like the idea of having a 'failover' swap area on real disk -- whether you have a single VDISK or 2 prioritized ahead of it. I guess whether it's waste of resource depends on your POV.. But your point is taken.. Scott On Wed, Apr 29, 2009 at 4:42 PM, Barton Robinson < bar...@vm1.velocity-software.com> wrote: giving real disks to swap is a real waste of resource. It is much better to take the "extra disk resource" that you allocate but never want to use, and assign it to z/VM paging to enhance your paging subsystem. Then define two vdisks for swap, prioritize them, and set an alert when the 2nd disk is being used. Martin, Terry R. (LOCKHEED MARTIN Performance Engineering/CTR) (CTR) wrote: Hi I am using SWAPGEN to define by z/Linux VDISKS I also want to define a real disk for swap. My question is can I use SWAPGEN to define a swap on real DASD? If you have an example of the control card syntax to accomplish this that would be great? //Thank You,// //Terry Martin// //Lockheed Martin - Information Technology// //z/OS & z/VM Systems - Performance and Tuning// //Cell - 443 632-4191// //Work - 410 786-0386// //terry.ma...@cms.hhs.gov <mailto:terry.ma...@cms.hhs.gov>// -- Rich Smrcina The information contained in this e-mail and any accompanying documents may contain information that is confidential or otherwise protected from disclosure. If you are not the intended recipient of this message, or if this message has been addressed to you in error, please immediately alert the sender by reply e-mail and then delete this message, including any attachments. Any dissemination, distribution or other use of the contents of this message by anyone other than the intended recipient is strictly prohibited. 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