Skip to main content

IBM Systems  >   System z  >   z/VM  >  

Performance Considerations for OS/390 Guests

Updated 30 March 1999

When running OS/390 as a guest of VM/ESA when VM/ESA is running in an LPAR itself, the following functions or performance advantages are lost.

  • ADMF - Asynchronous Data Mover Facility is not available to the OS/390 guest because VM/ESA depends on PR/SM function to make that available and the PR/SM function is already in use by LPAR.
  • DAT-on MVPG used by MVS or OS/390 is lost for the guest when VM/ESA is run on an LPAR. Note, that DAT-off MVPG such as that used by DB2 for VM and VSE is available, except on some older machines.
  • I/O Assist and the ability to run multiple preferred guests is lost in VM/ESA when running in an LPAR. Only a single V=R guest without benefit of I/O Assist is permitted when VM/ESA is running in an LPAR.

There are other features that are unavailable to MVS and OS/390 when running as a guest of VM/ESA, regardless of whether VM itself is running in basic mode or in an LPAR. Storage reconfiguration is one such function.

An MVS or OS/390 guest cannot use dynamic-I/O configuration to reconfigure the real I/O hardware. However, the guest's virtual-I/O configuration can be dynamically changed. The MVS or OS/390 HCD component can be used in the normal manner to change the software view of the MVS or OS/390 I/O configuration. VM's dynamic-I/O-configuration support can be used to perform any necessary hardware I/O configuration. VM operator commands and directory facilities can be used to assign or remove I/O resources to or from a guest's virtual-machine configuration.

The VM/ESA guest coupling support is available to OS/390 guests even when VM/ESA is running in a logical partition.

The following links may have some additional information of interest.

Thanks to Damian Osisek and Frank Brice for the bulk of this material. Bill Bitner owns any errors.
Return to the Performance Tips Page