can_queue

can_queue holds the number of outstanding commands the host adapter can process. Unless RESELECTION is supported by the driver and the driver is interrupt-driven, (some of the early Linux drivers were not interrupt driven and, consequently, had very poor performance) this variable should be set to 1.

cmd_per_lun

The SCSI standard supports the notion of ``linked commands.'' Linked commands allow several commands to be queued consecutively to a single SCSI device. The cmd_per_lun variable specifies the number of linked commands allowed.

This variable should be set to 1 if command linking is not supported.

The difference between the two parameters

Linked commands are fundamentally different from multiple outstanding commands (as described by the can_queue variable above). Linked commands always go to the same SCSI target and do not necessarily involve a RESELECTION phase. Further, linked commands eliminate the ARBITRATION, SELECTION, and MESSAGE OUT phases on all commands after the first one in the set. In contrast, multiple outstanding commands may be sent to an arbitrary SCSI target, and require the ARBITRATION, SELECTION, MESSAGE OUT, and RESELECTION phases

More Information

http://tldp.org/LDP/khg/HyperNews/get/devices/scsi.html