Performance Considerations for OS/390 Guests

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.

• VSE/ESA performance when running under VM/ESA