tableBASE space requirements

VSAM tableBASE libraries

For VSAM libraries, the default blocksize of 3120 (RECSZ in IDCAMS DEFINE) results in a CISZ of 3584, 13 blocks per track and 71.6% utilization. Increasing the RECSZ to 3577 (the maximum in a 3584 CISZ) results in a 82.1% utilization. Increasing it to 4091 (the maximum in a 4096 CISZ) results in 86.6% utilization. Reducing it to 2041 (2048 CISZ) results in 75.6% utilization; reducing it to 1017 (1024 CISZ) yields 59.2% utilization.

BDAM/QSAM tableBASE libraries

For BDAM/QSAM libraries, the default block size of 3120 results in 15 blocks per track and 82.6% utilization of the space. Increasing block size to 3174 results in 84% utilization (still at 15 blocks per track). Increasing it to 27998 (2 blocks per track) results in 98.8% utilization. Reducing it to 820 (39 blocks per track) results in 56.4% utilization.

tableBASE library space usage

tableBASE attempts to use library space efficiently. If the BLKSIZE (RECLEN) is larger, it fits more directory entries into each block. But every table definition requires 1 full block for the definition.

The actual data for each table is stored in an integral number of full blocks, with the minimum being one. A very small table (e.g., 10 rows with a row length of 12 bytes) still requires a full block, which results in a very low utilization, even if a blocksize of 800 (the minimum allowed) is used. On the other hand, a very large table (e.g., 50000 rows with a row length of 200 bytes) will have over 99% utilization of its allocated space no matter what the blocksize.