On Dec 3, 2007 2:43 PM, Romanowski, John (OFT) <[EMAIL PROTECTED]> wrote:
> It seems hasty to say that "Because of the Linux algorithm for using > swap, a VDISK used for swap even a little will eventually be used > completely". > That's the same as saying a linux swap area used even a little will > eventually be used completely. Why would linux do that? That's not > what my SLES9 guests do. Maybe our idea of "eventually" is different. ;-) But yes, in order to optimize the Linux I/O (reduce seek times, allow I/O's to be merged, etc) Linux prefers to pick a "virgin" pages in the VDISK rather than ones that have been freed by swap-in. In the view of z/VM, the freed pages are still "used" because there is something in them and Linux has not told VM can forget it. So with some amount of swapping going on, eventually all pages of the VDISK have been used and VM views them as in-use, even though Linux still has only a small amount of pages swapped out. If your performance monitor shows use - linux number of swapped pages - vdisk number of resident pages - vdisk paging rates then it becomes very clear that this is happening. > Now that the swap topic's open again: > > What is the basis for advising z/VM VDISK users to have a hierarchy of > multiple linux swap areas of increasing sizes? Are there feature(s) of > the swapping algorithm that make that hierarchy principle optimal? Exactly the thing above. When you have one big VDISK and the oldest frames get paged out by VM, every page that Linux selects for swap-out will first require a page-in by z/VM (useless, because Linux does not need that data). Ideally you want your top swap disk to be large enough that it does not overflow even when Linux needs most memory. And small enough that it remains resident on z/VM. If there's different levels of utilization in Linux during the day, you may need multiple levels of VDISK to fit those requirements. At the beginning of such a level of high resource requirements you will find z/VM page in the VDISK, but then it remains resident during the period of high usage. The idea with the stack of VDISKs in different size (and with different swap priority) is to get started when you have no clue about the requirements. When you have measured, you can probably come up with something smarter. Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/
