I think what everyone is trying to point out in this thread is the
utilization of the write I/O", means if 32 pages can be written in
then why write 3 or 5 or less than that.
Please correct me if i misunderstood anything.
> -----Original Message-----
> From: Joel Goldstein [SMTP:[login to unmask email]
> Sent: Wednesday, December 13, 2000 6:26 PM
> To: [login to unmask email]
> Subject: Re: meaning of VDWQT=0 & CASTOUT CLASS
> Using VDWQT of 0 will rarely have any effect on the overall
> of I/Os, or the CPU time.
> Most systems and pools have a low number of avg pages/write
> and when this is he case there will be no increase of CPU.
> I look at avg <3 pages/write. So when write gets triggered,
the number of
> I/Os does not change with a low vdwqt..... it just smoothes
> or causes a "trickle" write.
> Lowering vdwqt to 0 avoids flooding the system with I/Os when
> takes a checkpoint - and this flooding usually causes large
> in I/O response times for the online transactions.
> See my presentation from IDUG this year for a graph showing
> Message text written by DB2 Data Base Discussion List
> >VDWQT at 0 means you will be Dequeuing 32 pages when 40
pages that belong
> to the same dataset fills a virtual bpool. The effect of it
depends on the
> system configuration especially I/O sub system. Certainly it
> time. Recently by mistake our production system was set up
with 0 which
> resulted in a substantial CPU time with spikes of upto 60%. It
> means more frequent little write I/O rather than less frequent
> lot of people have different opinion on this. My take is, if
the number of
> pages written per I/O is not drastically less, do not try to
> The key here is to avoid a large burst of I/O at anytime, but
> specifically at checkpoint.
> Castout threshold applies to the coupling facility. In each
> pool there is a fixed queue of cast out class queue. when a
page is to be
> written to a queue, it must also tell which queue the page
> pageset pages are always grouped to the same class.
> If the cast out class threshold is exceeded, the system which
is the owner
> of the cast out class will initiate the cast out of Pageset.
> 10%. This is checked everytime a coupling facility write is
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