[buffers]
max_pending=64
This value is only used on operating systems for which we support asynchronous file I/O (currently Windows NT,Windows 2000, and Windows XP).
The number is the maximum number of disk writes that may be simultaneously in progress.
This value is used on all operating systems.
DP4 puts all write operations in a "pending" internal queue, and submits writes to a "final" queue to the operating system on a least recently written basis. However the final queue of writes is re-ordered to sequentialise output as much as possible. The max_pending setting is used to determine the maximum amount of re-ordering in this final queue.
If the value is too small DP4 will be performing unnecessarily non-sequential reads and writes. If the value is too large, delays may occur because the result of a read or write is needed but the operation is still in the final queue, and because extra CPU cycles are used keeping this longer queue sorted.