What is QLogic execution throttle and how does it relate to queue depth?

Environment

• Red Hat Enterprise Linux
• QLogic HBA / qla2xxx driver

Issue

• SAN is overloaded by I/O and need to adjust queue depth and execution throttle

Aug 17 01:14:13 host kernel: qla2xxx [0000:06:00.0]-3840:0: QUEUE FULL detected.
Aug 17 01:14:13 host kernel: qla2xxx [0000:06:00.0]-3822:0: FCP command status: 0x15-0x828 (0x28) nexus=0:1:372 portid=01b7c0 oxid=0x100 cdb=2a0005782d2000001000 len=0x2000 rsp_info=0x0 resid=0x2000 fw_resid=0x2000 sp=ffff885d990b2800 cp=ffff885d698f57c0.
Aug 17 01:14:13 host kernel: qla2xxx [0000:06:00.0]-3840:0: QUEUE FULL detected.
Aug 17 01:14:13 host kernel: qla2xxx [0000:06:00.0]-3822:0: FCP command status: 0x15-0x828 (0x28) nexus=0:1:352 portid=01b7c0 oxid=0x102 cdb=2a0009653fe000001000 len=0x2000 rsp_info=0x0 resid=0x2000 fw_resid=0x2000 sp=ffff885d68e2b440 cp=ffff885da69f0ac0.

Resolution

• Queue depth is the maximum number of outstanding I/O requests a device will allow. There are two different queue depths.

  • "Target port queue depth" - maximum number of requests that can be queued by the target port. These requests are waiting to be serviced by the backend logical unit.
  • "HBA queue depth" - maximum number of I/O that the HBA will allow to be 'in-flight' at any time. More information on this can be found at What is the HBA queue depth, how to check the current queue depth value and how to change the value ?

• Execution throttle is a QLogic HBA BIOS parameter that limits the maximum number of scsi commands allowed per target port. This includes inquiry commands such as TUR or INQ as well as standard scsi commands and I/O requests. This configurable must be set from within the HBA card's configuration utility. Note that changing the execution throttle may have an effect on the HBA queue depth. Also note that both execution throttle and queue depth should be set at the level recommended by your SAN and hardware vendors.

• A generalized suggestion by QLogic in regards to HBA queue depth setting can be calculated by the following.

TQ / AP = QD

TQ = Target Port Queue Depth
AP = Number of HBA adapter ports that will be accessing that port
QD = Recommended HBA queue depth

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