Use the A_SORT_FILE_MEMORY configuration variable to specify an amount of memory to be used for buffering file operations on the temporary SORT files. The amount of memory is specified in increments of 64KB (as it is for the SORT_MEMORY variable). The minimum value is 0, which disables the file buffer, but this is only recommended for troubleshooting purposes. The maximum value is 65535. The default value is 16.
The recommended setting for this variable will vary depending on your system. In general, more memory used for file buffering will result in less system calls and better performance. If your system has very good file system buffering already, you may get better or as-good performance by setting this variable to a small amount of memory.
If the SORT_MEMORY variable is set to a value larger than A_SORT_FILE_MEMORY (and A_SORT_FILE_MEMORY is set to a non-zero value), the memory previously used for SORT_MEMORY is used to improve file buffering.
The memory allocated by A_SORT_FILE_MEMORY only comes in effect when A_SORT_REGIONS=0, or A_SORT_REGIONS_FINAL is greater than 1.