MAXSTGIO = maximum storage used for sequential data set transfers for system-determined NCP, maximum I/O storage for user-specified NCP)
This parameter specifies the maximum amount of storage used for BSAM sequential data set transfers. MAXSTGIO has two positional parameters that limit the total I/O buffer size in different circumstances. The first is used to limit it when the system determines the number of channel programs (NCP). The second is used to limit it when you specify the NCP in the COPY statement. (The larger the value, the better the I/O performance for sequential file transfers. However, the larger the value, the larger the REGION size that may be required for the DTF.) The valid value range for both parameters is 60000–8,388,608 (60K–8M). The default value is 1048576 (1M).
- IBM® Connect:Direct® uses these parameters to limit the number of buffers/channel programs used for sequential I/O and calculates the NCP by dividing the MAXSTGIO value by the block size of the data set being transferred. The number of channel programs specified can range from 0 (to have the system determine the value) to 255. For more information on how IBM Connect:Direct processes sequential data sets using BSAM, see How to Improve BSAM Data Transfer Rates.
For example, if you specify the default of 1 MB for MAXSTGIO, the following number of channel programs/buffers are allocated for data sets with the block sizes listed in the following table. Also listed is the amount of storage required for buffers for this transfer, which is a product of the block size and NCP.
| BLKSIZE | Number of Channel
Programs/Buffers |
Storage Used for
Transfer |
|---|---|---|
| 80 | 255 | 20,400 |
| 4,080 | 192 | 783,360 |
| 6,400 | 128 | 819,000 |
| 27,998 | 32 | 895,936 |
- The data sets in the above table did not take advantage of striping or Large Block Interface (LBI) support, which affect BSAM sequential data set transfer rates. In addition, the method for determining NCP did not vary—the NCP values listed above were all system-determined. You can also specify the NCP by using the second positional parameter of the MAXSTGIO initialization parameter.
- If you specify a large value for MAXSTGIO, be sure to review the REGION size specified for the DTF. The region size must be large enough to accommodate the maximum number of sequential transfers that could take place at any one time, multiplied by the value coded for MAXSTGIO, plus the normal amount of region that the DTF requires.
Modifiable through MODIFY INITPARMS command: YES