Specifications

Chapter 15. Virtualization 165

򐂰 Each virtual machine (for zPDT) had a base Linux. As usual, the Linux disk cache is a

critical performance factor. Using the vCenter monitoring facilities we noted that the

complete virtual machine memory was heavily used. We suggest that giving more

memory to each virtual machine than you might provide in a non-virtual environment

(without overcommitting memory) may improve performance. In effect, this allows more

buffering of I/O to the real disks on the server.

15.4.1 Open notes

We strongly suggest disabling hiperthreading for “normal” zPDT usage, as stated several

times in this series of documents. The basic problem is that if a z/OS CP is spinning and

another System z CP that is attempting to resolve the spin loop are both dispatched on the

two “halves” of a hiperthreaded core, then there can be a deadlock for those two System z

CPs until some external action causes the core to be dispatched in a different way. In a small

system (perhaps two cores, hiperthreaded into four apparent cores) this deadlocking can be

very apparent. In a larger server, such as might be used in a virtualized environment, the

problem is much less apparent but may still lead to decreased performance for z/OS. The

frequent redispatching of cores in a virtual environment tends to hide such temporary

deadlock problems.

zBX has hiperthreading enabled; we could not disable it. We did normally disable it on the

servers used for VMWare, but experimented with it enabled. We did not observe any obvious

problems with hiperthreading enabled, but we did not make detailed performance

experiments to quantify degredation (if any) due to hiperthreading.

The zBX environment incorporates a VLAN function that, at first level, is invisible to the guest

systems. VMWare can do the same, and this is probably the typical configuration.

We did

not experiment with multiple levels of VLANs or usage of z/OS VLANs. Simple LAN usage

worked well with both zBX and VMWare. Complex LAN configurations may require more

thought.

VMWare can also assign physical network adapters to individual guests, bypassing any VMWare internal VLAN

and routing operations.