On Thu, 24 Mar 2005 08:00:31 -0600, Alan Schilla <[EMAIL PROTECTED]> wrote:
> HI everyone. This post has created some confusion within my VM group and I > wanted to see if I understand XSTORE and MDC. Must be my fault... I hear that a lot ;-) > Linux does not utilize XSTORE for MiniDisk Cache and you are better off using > XSTORE for VM paging. The Linux I/O is eligeable for z/VM MDC just like other I/O, but it is not always very effective because of some restrictions in z/VM that I would like to see fixed. The reason to give up some main memory and get xstore back for that is to help paging. If you were to use that for MDC you would be better off doing that in main memory rather than xstore (whether for Linux or other users). > Especially if you over commit VDISK swap space. We tend to overcommit swap > just by the nature of how we clone. You mean by overcommit VDISK that you give it a VDISK for swap that is way bigger than it will ever use? Now we get into more ugly details... With SLES 8 we see that Linux typically uses fresh pages when finding a new slot in the swap disk. So when you have a little bit of paging going on, ultimately you will have used each page in the VDISK and have z/VM do page-out of that. You can avoid that by using multiple VDISKs for swap with different priority (and force Linux to re-use pages rather than go after free ones each time). I have been told that SLES9 changes this into re-use automatically, which would mean a big VDISK will not hurt at all (but I have not tried that myself yet). > VM's default setting assumes MDC is meant to improve disk performance for VM > guests. Is this pretty close or am I way off base? The defaults for VM tend to favor MDC too much, and thus force virtual machine pages to be paged out more than you want. This is why we limit the amount of MDC or change the bias. Indeed. Rob -- Rob van der Heij rvdheij @ gmail.com ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
