VSAM buffering tips

Tuning Sequential Access


Default data buffers for KSDS is 2

• one reserved for splits
• #of I/O equal number of data CI’s that contain data 

Determining Optimal Data Buffers for Input Data Set

Best performance is achieved when data component is read in full tracks

CI/Track=(PHYRECS/TRK) x (PHYREC-SIZE)

                     CISZD

PHYRECS/TRK = physical records per track  - from LISTCAT  

PHYREC-SIZE = physical record size  - from LISTCAT  

CISZD= data control interval size - from LISTCAT  

Optimum value of BUFND is =CIs per track +1

Example:

From IDCAMS LISTCAT..

CISIZE--------------4096

PHYRECS/TRK-----------12

PHYREC-SIZE---------4096

CI/Track=12 x 4096 = 12

              12

BUFND (for input dataset)=CI/TRK+1     = 13

BUFND (for output dataset)=CI/TRK+2    = 14

Random Access - Batch

Default index buffers for KSDS is 1

Best performance is achieved when enough buffers are allocated to keep all index CI’s in memory, plus one for the sequence set reads.

Index Buffers for Input Data Set

BUFNI=HURBAI -      HURBAD      + 1

      CISZI    CISZD X (CI/CA)

HURBAI = High Used RBA of index component   - from LISTCAT 

HURBAD = High Used RBA of data component   - from LISTCAT  

CI/CA = number of data CI in each data CA  - from LISTCAT  

CISZI= index control interval size - from LISTCAT  

CISZD= data control interval size - from LISTCAT  

Example:

From IDCAMS LISTCAT..

HI-U-RBA (index)-----------12288 

CISIZE (data)--------------4096

HI-U-RBA (data)----------540672

CI/CA--------------------12

BUFNI = 12288 - 540672  + 1

        1024   4096 X 12

BUFNI = 12 - 11 + 1 = 2