I'm using a slightly modified version of the SWAPGEN EXEC on v-disk for swapping in V-disk.
I have a user who needs LOTS of swap space. I'd like to give him two swap areas, one in v-disk and one on a real disk (with different priorities), but the real disk formatted with CMS FORMAT/RESERVE so I can use the minidisk cacheing option in SLES8. Can I use SWAPGEN EXEC to build a swap file on real 3390's? I'll have to format it with 512-byte blocks, right?
Or can I create a FORMAT/RESERVE disk and just use mkswap on it?
The problem with VDISK was that the formatting goes away when you logoff the virtual machine. With disk that is not the case and you can simply mkswap it once and be done with it.
When you swap to real disk, you definitely don't want to make it 512-byte blocks. You want 4K blocks, whether you use the Diag driver or eckd driver. The only reason for using 512 byte blocks with VDISK is a restriction (imho a bug) in the diag driver.
I think not all numbers are in the redbook, but I believe that swapping with the diagnose driver is more expensive than with the eckd driver (mainly due to MDC, but when you disable MDC it is not faster anymore). I would love to have time to dig into this a bit deeper and understand what is happening. When you already topped off the first bit of swapping, then there may not be much need to use the diag driver for the rest.
Recommondation is not to use huge swap disks, so you follow that recommendation by doing it like this. I do encourage you to see whether the user really needs that much swap space, but one can certainly configure applications to need that.
Rob
