Calculating the size of OSAM and VSAM structures

For sysplex data sharing, the size of the structure required by each access method depends on the number of buffers defined to the access method by each IMS in the data-sharing group. The buffers for all IMS control regions and batch jobs that are data sharing and registered to IRLM must be counted.

The following formula is for calculating the count of OSAM buffers:

OSAM buffer count  =  #osambfrs/IMS1 + #osambfrs/IMS2 + ...  #  osambfrs/IMSn

The following formula is for calculating the count of VSAM buffers:

VSAM buffer count  =  #vsambfrs/IMS1+ #vsambfrs/IMS2 + ... #vsambfrs/IMSn

Here is an example of sizing a VSAM structure with 2 IMS systems (IMS1 and IMS2):

IMS1 has a total of 700 buffers (400 VSAM buffers plus 300 Hiperspace buffers) based on the following specifications:

VSRBF=512,30  
VSRBF=1024,20,I  
VSRBF=1024,10,D 
VSRBF=2048,40    
VSRBF=4096,100,HS100,HSR  
VSRBF=8192,100,HS100,HSR   
VSRBF=16384,50,HS50,HSR   
VSRBF=32768,50,HS50,HSR

IMS2 has a total of 400 buffers based on the following specifications:

VSRBF=512,30      
VSRBF=1024,20,I   
VSRBF=1024,10,D   
VSRBF=2048,40    
VSRBF=4096,100    
VSRBF=8192,100   
VSRBF=16384,50   
VSRBF=32768,50

By adding the total number of buffers for both IMS systems (700 + 400), you get the number to be used for sizing the VSAM structure; 1100.

Recommendation: Review the size of the OSAM and VSAM Structures any time OSAM or VSAM buffer pool definitions are changed. You might encounter poor data sharing performance if these structures are not large enough.

An OSAM structure may optionally be used for caching data. If data caching is not used, the structure contains only directory entries. If data caching is used, the structure contains directory entries and data elements. In this case the size of the structure must allow for data elements. OSAM data is stored in the structure as multiples of 2 KB data elements. Using data caching requires a directory-to-element ratio to be specified. This ratio causes the structure to be subdivided with directory entries and data elements. The ratio is specified with the CFOSAM= keyword on the CFNAMES parameter statement.

For more information on the ratio is specified with the CFOSAM= keyword on the CFNAMES parameter statement, see IMS Version 15.2 System Definition.

Requirement: The OSAM buffer count must include any sequential-buffering buffers that are defined. The VSAM buffer count must include any hiperspace buffers that are defined.

z/OS formula

After you have calculated the OSAM buffer count, put the result in the TDEC field of the z/OS® cache structure size formula. Then do the same for the VSAM buffer count. (The calculation is done separately for each access method.) TDEC is the total directory entry count, or the maximum number of concurrently executing parallel units of work. For more information on TDEC, See Defining the coupling facility. For more information on the z/OS cache structure size formula, see Determining CF cache structure size.

For information on OSAM database coupling facility caching, see IMS Version 15.2 System Definition.