Files in Cache - A worthwhile feature? My opinion

Discussion created by ca.portal.admin on Mar 10, 2005
Latest reply on Mar 10, 2005 by ca.portal.admin

One of the IDMS 16.0 features that seemed very promising was Files In
Cache. As a shop with a fair amount of batch processing it could reduce
some of our disk accesses for database areas that are way too big to map
to DMCL buffers. Most of our batch is run local mode to reduce the
overhead of journaling.

I have performed a number of tests and asked questions of CA on how this
technique works. I have also had jobs monitored to see what action takes
place 'above the 2Gb bar'.

It seems that this was mainly made available for online work. This makes
sense as the access is perfectly random and could put some 'hot' data
into cache when it could be swapped out of the DMCL buffer. In a local
mode batch job it seems less likely that a previously accessed database
page is going to be accessed again though it could happen. One other
problem is the reporting of cached hits and I have requested a DAR on
this issue. If you do not have enough 'above the 2Gb bar' memory defined
to the IDMS job you will get messages but there are no online tools to
see the if cache was actually in effect, used or not, and BUFFERSTAT,
for local mode will show 'Cache Hits' if they happen but again you have
no idea if you are actually using the feature.

The information I have gained from CA and z/OS manuals is that this
feature involves the use of the IARV64 macro that can create MEMORY
OBJECTS of up to 1Mb 'above the 2Gb bar' to place the database pages
into. Quite rightly IDMS only loads database pages into the MEMORY
OBJECTS as and when they are required. It does not make sense to load
over 1Gb of database pages into MEMORY OBJECTS at the start of a job
when it may not need to access them all.

If the database pages could be 'pre-loaded', similar to how PREFETCH
works, then Files In Cache could have benefits in local mode batch jobs
as well. It does not have to load more than a tracks worth of database
pages at a time but if access to memory is much better than access to
the actual disk this could work.

I have requested a DAR on this with the use of either a SYSIDMS entry or
an optional apar for local mode use only.

Just my opinion.

Thanks for listening

Chris Wood

Alberta Department of Energy


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Re: Files in Cache - A worthwhile feature? My opinion

That sounds like a good idea for the online system.

I am interested in local mode batch in fast loading data into cache
memory Objects.