First of all, the mainframe doesn¹t take the I/O performance hit that the X86 world does, so there is only minimal need to cache I/O in (Linux) memory. Let z/VM and the caching controllers take care of this. Do not allocate memory to Linux that it will only use to cache disk.
Second, do not allocate a huge swap for the virtual machine: If you¹re using swap, you¹re doing it wrong. You should have a small swap (maybe 1Gb at most) allocated on vdisk. And, if you find yourself pushing into significantly, then allocate the VM additional memory. Let Linux think it has ³real² memory, and let z/VM do the paging. Z/VM has a much more mature paging algorithm, and can handle much higher paging rates than Linux can. Plus it knows what is going on in the rest of the real system, where Linux does not. Third, z/VM can efficiently handle a memory overcommitment of 2:1, without really breaking a sweat. (I know, because we did it for an extended period of time.) There¹s no reason to match real memory to the sum of the sizes of the virtual machines. At some point above the 2:1 level, though, you will begin to see a performance hit, so try to stay near or below that mark. Let z/VM worry about the memory and I/O. It¹s good at it, and Linux will not out-guess its decisions. And, rather than caving to the developer¹s demands, ask them to let you show them what you can do. Size the virtual machine so that it just barely pushes into the swap. If swap is 0, then the machine has (possibly way) too much memory. If it is using 50% of the swap, then the machine needs more virtual memory. It NEVER needs more swap; letting Linux swap under z/VM is just plain stupid. -- Robert P. Nix | Sr IT Systems Engineer | Data Center Infrastructure Services Mayo Clinic | 200 First Street SW | Rochester, MN 55905 507-284-0844 | [email protected] "quando omni flunkus moritati" On 4/6/16, 9:41 AM, "Linux on 390 Port on behalf of Levy, Alan" <[email protected] on behalf of [email protected]> wrote: >Hopefully someone can resolve an argument that I'm having with a >colleague. > >We are competing with the distributed side which using VMware to create >sles linux servers. They create servers with 8G of memory and 8G of swap >for EVERY server. My colleague wants to follow this architecture for our >zvm servers (giving them 8G of memory and 8G of vdisk). > >My opinion is to give them a default of 2G of memory and 2G of vdisk and >increase the main memory as needed. > >My colleague is concerned that if we give them less, they will always go >to the distributed side since they give more. I am concerned about giving >so much memory might negatively impact our zvm systems. > > > > >________________________________ > >This e-mail, including any attachments, may be confidential, privileged >or otherwise legally protected. It is intended only for the addressee. If >you received this e-mail in error or from someone who was not authorized >to send it to you, do not disseminate, copy or otherwise use this e-mail >or its attachments. Please notify the sender immediately by replying to >this e-mail and delete the e-mail from your system. > >---------------------------------------------------------------------- >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 >---------------------------------------------------------------------- >For more information on Linux on System z, visit >http://wiki.linuxvm.org/ > ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
