To avoid database I/O, RACF® buffers database blocks in resident storage. In the data set name table (ICHRDSNT), you can specify the number of resident data blocks for each primary RACF data set. This keeps any type of data block (profile and BAM blocks, including index blocks) resident. An installation can specify from 0 to 255 resident data blocks. If RACF is not enabled for sysplex communication, the default value is 10 resident data blocks if you do not provide ICHRDSNT. If enabled for sysplex communication, RACF enforces a minimum of 50 resident data blocks for a primary data set and 10 (20% of 50) for a backup data set. For best performance, specify as large a number of buffers as you can afford, preferably 255.

Resident data blocks reduce the amount of I/O that is required to service the RACF database. While the number of blocks remains the same during an IPL, the function is dynamic because, at any time, the most frequently used blocks are kept in storage.

The number of bytes (per primary data set) of ECSA storage used by the data blocks is 3248 + (4144 x the number of blocks). If RACF is enabled for sysplex communication, an additional 3248 + (4144 × .2 × the number of blocks) is used for the backup data blocks. During IPL, RACF obtains the storage for the number of buffers specified in the data set name table. The RACF manager then keeps track of when each buffer was used last. The RACF manager does different processing for shared and non-shared databases.