Database buffer sizing for IDMS

Products

• IDMS
• IDMS - Database

Issue/Introduction

This article describes how to determine if IDMS CV buffers are optimally sized for best performance.

Environment

Release: 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"

The "Fnd-in-Buf" figure should be 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

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, it was 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 three worst buffers is to significantly increase the number of pages and then continue to monitor to see if further increase may be needed.

 

With large increases in buffer sizes, be mindful of the 31-bit z/OS storage requirements of the CV.