Ken,

My TSM 4.1 system has 128 MB of DB Buffer Pool for a 27 GB DB (88% used). 
We normally see sustained cache hits of about 98% which is plenty good 
enough.

I reset the statistics just before launching weekly maintenance 
(expiration, reclamation, etc.).  Those activities hit the DB constantly 
and I usually get 99% or a little better.

When sessions start the cache hit percentage drops a little because NEW 
FILES are not in the database and therefore cause a cache miss.

After the weekly maintenance followed by a week of backups and archives, I 
usually have a cache hit of 97.5 to 98.0 percent.  At this moment, the hit 
rate is 98.21%.  Maintenance started 9:00 am Monday and finished at 2:00 
pm Tuesday so that's maintenance plus two nights of backup sessions.  By 
Sunday the hit rate will be about 97.5%.

Consider two things:
- DB Buffer Pool memory is TSM process memory.  When my server was tight 
on RAM, I set the pool too large and pushed the server just short of VM 
thrashing.  So the size of the pool isn't the only consideration.
- You  can never exceed 100% cache hit.  No matter how much memory you 
allocate, your hit rate will go asymptotic (sp?) to 100% until you put the 
entire DB in RAM.

Tab Trepagnier
TSM Administrator
Laitram Corporation









KEN HORACEK <KHORACEK AT INCSYSTEM DOT COM>
Sent by: "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
08/21/2002 11:18 AM
Please respond to "ADSM: Dist Stor Manager"

 
        To:     ADSM-L AT VM.MARIST DOT EDU
        cc: 
        Subject:        Re: cache hit rate


No experience.  I fact I'm learning from your experience.  While you say

"from my training at IBM that a cache hit rate of 99% and above is
recommended"

I found the following statement in the web interface.

The server database performs best with a cache hit ratio above 97% in the 
database bufferpool. To tune the bufferpool size, (a) reset the bufferpool 
statistics, (b) execute server operations that use the database, and (c) 
view the database details to see if the cache hit ratio is above 97%. If 
the cache hit ration is NOT above this percentage, increase the size of 
the BUFPOOLSIZE specification in the server options file.

Conflicting advice?  Perhaps your 97.7% is good and no problem exists. 
Let's hear from others.  Please......


Ken
khoracek AT incsystem DOT com


>>> Sascha.Braeuning AT SPARKASSEN-INFORMATIK DOT DE 08/21/2002 12:46:04 AM >>>
Hi all,

first I've to thank all participants for the amount of answers I got on my
latest questions. Now I've got a new one which relates on the
bufferpoolsize and the cache hit rate on OS/390 TSM-Servers.

we use the following bufpoolsize:  32768
our chache hit rate:            97,7 %

I know from my training at IBM that a cache hit rate of 99 % and above is
recommended. Is it possible, that a lower cache hit rate can cause
performance-problems? What is your experience?


MfG
Sascha Bräuning


Sparkassen Informatik, Fellbach

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