How to determine optimal buffer sizing in IDMS

Products

• IDMS
• IDMS - Database
• IDMS - ADS

Issue/Introduction

How can I determine if my IDMS CV buffers are optimally sized for best performance?

 

Environment

IDMS - All Supported Releases

Resolution

Start by doing the display
DCMT D STAT BUFF

This display shows the number of page requests that were Found In buffer and the number that required a Physical Read I/O

Optimally sized buffers should show a much higher number for "Fnd-in-Buf"  than  "Phy-Reads"

We like to see "Fnd-in-Buf" at least 10 times greater than "Phy-Reads"  or even better if possible.
For some buffers this may not be possible but "Fnd-in-Buf" significantly higher than "Phy-Reads" should be the goal.

Example:

here is DCMT D STAT BUFF for a hypothetical CV

     DCMT D STAT BUFF                                                                                      
 ----- Buffer -----      Waits  Fnd-in-Buf   Phy-Reads Fnd-in-Cache Phy-Writes
 INDEX-BUF1                  0       28397          53           0          23
 MAIN01-BUF                  0      709980      287498           0       91035
 INDEX-BUF2                  0        2219         585           0         539
 DB-BUFF1                    0        1814         818           0           1
 DB-BUFF2                    0    28310680    14156758           0         153
 DICT-BUF1                   0         237          63           0           5
 INDEX-BUF3                  0       20446        1949           0        7368
 INDEX-BUF4                  0      324419      124414           0       46981
 DB-BUFF3                    0       72962     1057849           0           1
 SECD-BUFF                   0       80986          47           0           1
 DRUN-BUFF                   0        8890        2573           0         261
 DMSG-BUFF                   0       26374         129           0           0
 DICT-BUF2                   0        6039         571           0           0
 DB-BUFF5                    0     9864984      297017           0           6
 DB-BUFF6                    0        1798         187           0          24
 DB-BUFF7                    0       11201        3533           0          66
 DB-BUFF8                    0    23677222      297596           0      115166
 DB-BUFF9                    0    15470176     4026268           0      445249
 DB-BUFF10                   0    81382145     9275712           0      492777
 DB-BUFF11                   0        1087       12010           0           1
 DB-BUFF12                   0    12646805      477167           0      138263
 DICT-BUF3                   0     3162661     9654107           0           1
 DB-BUFF13                   0    69089988     5759636           0      388375
 DB-BUFF14                   0     2221437      483306           0      201486
 V1  ENTER NEXT TASK CODE:      CA IDMS release 19.0 tape GJJ04I node SYST0001     

 In most cases the Fnd-in-Buf is greater than Phy-Reads but three of these buffers stand out with much higher Phy-Reads
than Fnd-in-Buf.

----- Buffer -----      Waits  Fnd-in-Buf   Phy-Reads
DB-BUFF3                    0       72962     1057849   
DB-BUFF11                   0        1087       12010  
DICT-BUF3                   0     3162661     9654107 

  DCMT D BUFF  or LOOK DMCL  will show the page size and number of pages in the buffers.
  In this case we found that two of these buffers were defined with only 5 pages and the other with 10 

  The buffers with several hundred pages or higher are the ones getting the best ratio of Fnd-in-Buf to Phy-Reads.

  So the recommendation for the 3 worst buffers is to significantly increase the number of pages and then continue to
monitor to see if further increase may be needed.