Re: buff write cache to low
>From: David Henseler <henslerd@bscc.bls.com>
>Date: Mon, 24 Feb 1997 13:45:57 -0500
>X-Informix-List-Id: <news.34348>
>
>I'm in the process of performance tuning an application running and I'm
>finding the buff write cache falling well below 50% while the read cache
>stays above 97%. The specifics of the application are three main
>applications each one horizontally partitioned ( multiple iterations of
>the same application running different subsets of data feeds).
These applications sound as if they read the database more than they write
to it.
>The box is a 16 way SGI with 2gb of mem and a 4 shelve MTI disk array.
>The meaningful config parms are as follows.
>
>BUFFERS 300000 # dlh020597
>[...]
>LRU_MAX_DIRTY 6 # dlh020597
>LRU_MIN_DIRTY 3 # dlh020597
I've trimmed down the meaningful parameters to the 3 above. I suspect that
you have a read-mainly database; most operations on the database are read
operations, and the ratio of disk reads to disk writes is more than 100:1.
Your LRU_{MIN|MAX}_DIRTY parameters also suggest that you have found that
you don't fill up much disk space between checkpoints.
Under these circumstances, IM(not so very)HO, the write cache ratio doesn't
matter in the slightest; there isn't enough write activity for there to be
much room for improvement in performance, even if you managed to get the
write cache ratio magically to 100%.
On big systems, the read cache is critical, and the write cache is often
(but not always) non-critical, especially in applications which are not
intensively OLTP.
Yours,
Jonathan Leffler (johnl@informix.com) #include <disclaimer.h>