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MAXBUF is a system parameter that sets the maximum allowable size for any single buffered I/O packet. Buffered I/O packets are allocated from the permanently resident nonpaged dynamic pool. The terminal, mailbox, and printer device drivers are examples of device drivers that perform buffered I/O.

The number of bytes specified in the I/O request plus the size of a driver-dependent and function-dependent header area determine the required buffered I/O packet size. The size of the header area is a minimum of 16 bytes; there is no absolute upper limit. However, this header area is usually a few hundred bytes in size.

The default value on Alpha and Integrity servers continues to be 8192.