z/VSE Parallel Access Volume White Paper 12/08/2009
©Andreas Vischer
IBM Germany Development & Research GmbH
z/VSE General Concepts / Considerations / Restrictions
•
Even if the hardware allows the definition of more than seven alias devices, z/VSE will only
support one base and up to seven alias devices. This takes into account that a PAV enabled control
unit can have a maximum of eight physically CHPID’s configured.
•
An alias device number must not be used in any z/VSE IPL context. If an ADD statement for a
later – during PAV activation - identified alias device was specified the ADD’ed device will be
boxed and taken out of any z/VSE processing.
•
Alias device numbers are also subject to the z/VSE three digit CUU limitation.
•
PAV activation requires to have one ‘spare’ I/O device eligible, thus restricting the z/VSE device
limit to 1023. If 1024 devices are added MSG1YS2I is issued.
•
During PAV activation all available alias devices for a base will be considered useable. There is
no support to exclude or only use some of the aliases available.
•
PAV activation on a native system or under LPAR may take some time, depending on your I/O
configuration (
ALIAS recognition ).
•
A dynamic add of alias devices is not supported. Thus devices becoming active are just ignored.
However, if a base device becomes ready while having PAV support active all corresponding
aliases will be activated, too.
•
Alias devices becoming not operational are taken out of PAV processing. As indication the alias
cuu is put in brackets on the VOLUME cuu,DETAIL output.
•
Even though an alias is considered somewhat virtual from a z/VSE administrator and operator
point of view, it will make use of copy blocks, channel queue entries and other I/O related
resources like a ‘real’ device. Therefore the allocation of such resources must consider alias
devices.
•
For former processing, transparency and due the nature that all alias devices correspond physically
to the same base device any system service and message will make use of the base device (cuu)
only.
•
Inactivation of the PAV support via the SYSDEF command can lead to delays as the support is set
inactive only after all I/O’s on all alias devices have been completed.
•
PAV processing will not improve performance on a ‘single application’ base.
Start I/O – WAIT – Start I/O ….
Obviously having more tasks in parallel issuing I/O’s against a single device will benefit from
PAV.
•
Subsystem Monitoring Facility (SMF) is enabled to collect base device information only.
