I checked to make sure that Barton hasn't answered before I started writing this... :)
Both ESALPS and the Performance Toolkit will show that an unused Vdisk will use very little storage. Even if you do manage to start using it, after a certain amount of time it is likely to get paged out if unreferenced long enough. Barton has been presenting these topics at SHARE and WAVV for several years now, they should be well burned in... Although there are probably some cases where a real swap disk may benefit, it is very much the exception not the rule. Mike: Also consider the other costs of your hand-me-down DASD... computer room space, electricity, heating, maintenance.... -- Rich Smrcina ---- Mike Walter <[email protected]> wrote: > > 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! >
