Disk arms and faulting: (was - We've Added more memory...but I can't remember!)

Charly,

No, I'm afraid I don't understand.  I understand the impact of disk arms
on overall performance.  I understand the impact of paging/faulting on
overall performance.  I do not understand the impact of too few disk
arms on faulting.

If you have 17 disk arms servicing thousands of interactive users, I can
appreciate that they could be overburdened.  If this is the case, then
your performance reports or WRKDSKSTS display should indicate a level of
activity which would justify purchasing additional arms.

So while I can see the effect that faulting would have on disk activity,
I don't see the effect of disk activity on faulting.  Other than
tinkering with expert cache, adjusting your workload, or changing your
activity levels, what can you do about faulting/paging other than
increase memory?

If your disk activity is within acceptable limits, then the extra disk
accesses resulting from faulting/paging will increase the response time
for some users by the duration of those accesses.  If I interpret your
status display correctly, this is less than one fault per interactive
transaction.  That one fault could add about 10 milliseconds to the
overall response time of the transaction.  This doesn't strike me as
extreme.

I don't have the answers, but I was responding to your paragraph below,
which seemed to imply that a shortage of disk arms leads to faulting:

"Most systems I have seen recently have lots and lots of memory and it
is being mostly wasted.  I can tell because they have an automatic tuner
moving memory around like crazy - the faulting is still high - the
bottleneck is usually the disk resources (don't get me started on that
topic) - the CPU is not being fully utilized - and the solution to any
performance problem is to buy more CPU or more memory."

I do not see that adding more disk arms to the system you describe would
significantly lessen the level of paging/faulting.  Nor to I think that
the term 'thrashing' is appropriate for a system with non-database
faults of 109/second.  Thrashing usually describes a system which is
spending more processing power moving memory than performing work, this
doesn't apply in your situation.

Regards,
Andy Nolen-Parkhouse