SEQPRES=SEQ or SEQPRES=BYPASS what should be the best option?

HARBRY ARIZA

SEQPRES=SEQ or SEQPRES=BYPASS what should be the best option?
Hi Folks:


I'm wondering if setting the SEQPRES=SEQ in the DSNZPARM , I
would gain better performance allowing Sequential prefetch read data from
cache. Actually , all our devices are Shark (ESS) , and the ESS white papers
said that you would gain better performance without change any db2
parameter but with this devices supporting more than 16 Gig of cache, some
improvements must comes with it even sequential prefetch. I would like to
hear your comments about this issue. Thanks and regards,

harbry




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Max Scarpa

Re: SEQPRES=SEQ or SEQPRES=BYPASS what should be the best option?
(in response to HARBRY ARIZA)
In my opinion if you have a disk cache > 1 Gb (it's common now) it's better
to set SEQCACH=SEQ, this signals to disk

controller to use caching for seq. prefetch and dyn. prefetch, which is
different from controller 'normal' logic:

Cache hit ratio < 10% inhibit cache
" " 10 to 55% cache track
" " > 55% cache cyl

At object level. If I remember well with old 3390 control unit, with small
cache and slow disk speed, it was faster bypassing cache during seq
prefetch activity. For this reason there was a 'hint' to disk controller to
bypass cache during a prefetch

Take a look at Chuck Hoover's paper: Tuning external I/O in Db2 for OS/390.

HTH

Max Scarpa
All disclaimerz in my HDS 7700E apply