Alan wrote:
"The primary reason PAVs exist is to provide relief to
fully-populated z/OS systems with 256 chpids and 64K
devices."
That assumes an even access density on 256 chpids and
64k devices. Real world experience suggests that 80%
of your devices have low access densities and you
don't care about PAV's, but 20% of devices have a real
impact on your overall performance with higher access
densities, generally getting higher queing.
It also glosses over the fact that each PAV requires a
subchannel address, so they do very little to relieve
the 64K device limit.
Access density is also why the cache on dasd
subsystems is effective - even though they have to
grow as more dasd is put behind the cache. If the
access density was even throughout a shop, cache would
not be effective.
With the current DASD implementations where 80% or
more of active data comes directly from cache, even
over FICON channels, the bottleneck becomes the
software queueing causing by the subchannel
contention. One might even benefit from PAV's on the
"smaller" 3390-3 if the access density is high enough
- 2.3 GB index that doesn't fit in memory, for
example.
In a VM environment, one probably doesn't get much
queuing on minidisk. What about Page Devices? Its
going to be interesting to see who VM pages on a 15288
cylinder paging device. What about full pack minidisks
needed to hold large data bases (Oracle, DB2 UDB) or
other high density data?
"we can solve it administratively instead of with
technology." hrmph! At some point, moving large
amounts of data becomes impractical. And the VM
systems programmers ranks are getting thinner.
There are two bottom lines to this discussion. 1) The
VM people assume that the guest OS will handle the
PAV's, and 2) the real answer is to measure and
determine where your bottlenecks are.
Jim Sibley
"Computers are useless.They can only give answers." Pablo Picasso
(The NSHO's expressed here represents no-one but myself).
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