Here is interesting discussion about the topic ... bellow are the most valuable statements from the thread:
By default a QLogic HBA Execution Throttleis set to 16. This setting specifies the maximum number of outstanding (SCSI / Fiber Channel) commands that can execute on any single Target port(WWPN). When a Target port’sExecution Throttleis reached, the host computer will not issue any new commands until one of the current commands finishes executing.
To increase the number of outstanding (SCSI / Fiber Channel) commands to the Target port,increase the Execution Throttle. However, increasing the Execution Throttlebeyond what the Target portmay handle will cause SCSI timeouts to occur.
According to QLogic it is considered a Best Practiceto optimize performance for all devices that may communicate with the Target port. To do this QLogic recommends that you divide the Target port'sMaximum Outstanding Commands (or Queue) by the number of host ports (Initiators) that access the Target port. This type of computation is used to determine the Target portfan-in ratio as well.
(Maximum Storage Port Command Queue) / (Host Ports) = Execution Throttle
Ideally Storage team has to tell what is the QD that needs to be set on the HBA based on how many LUNS are published on a particular port
Queue depth is the number of I/O operations that can run in parallel on a device
Calculating the queue depth\Execution Throttle value
To prevent flooding the target port queue depth, the result of the combination of number of host paths + execution throttle value + number of presented LUNs through the host port must be less than the target port queue depth. In short T => P * q * L
T = Target Port Queue Depth
P = Paths connected to the target port
Q = Queue depth
L = number of LUN presented to the host through this port
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