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Controlling LUN queue depth throttling in VMware ESX/ESXi
Symptoms
If the ESX host detects a queue full condition, it may abort the SCSI commands
Queue Full may show up as QFULL or Task Set Full state
If QFULL conditions exist, the ESX VMkernel log may contain entries similar to:
status = 40/0 0x## 0x## 0x##" (ESX 3.x) H:0x0 D:0x28 P:0x0 Valid sense data: 0x## 0x## 0x##" (ESX 4.0)
Note: The hexadecimal 28 or decimal 40 in the above error is the SCSI Status Code for the queue full state.
Resolution
VMware ESX 3.5 Update 4 and ESX 4.0 introduces an adaptive queue depth algorithm that adjusts the LUN queue depth in the VMkernel I/O stack. This algorithm is activated when the storage array indicates I/O congestion by returning a BUSY or QUEUE FULL status. These status codes may indicate congestion at the LUN level or at the port (or ports) on the array. When congestion is detected, VMkernel throttles the LUN queue depth. The VMkernel attempts to gradually restore the queue depth when congestion conditions subside.
This algorithm can be activated by changing the values of the QFullSampleSize and QFullThreshold parameters. When the number of QUEUE FULL or BUSY conditions reaches the QFullSampleSize value, the LUN queue depth reduces to half of the original value. When the number of good status conditions received reaches the QFullThreshold value, the LUN queue depth increases one at a time.
Note: The QFullSampleSize and QFullThreshold are system-wide configuration parameters. If the system has different vendor storage arrays connected, all the arrays experience the throttling effect if one LUN/port from any array returns the QUEUE FULL or BUSY errors.
To enable the algorithm:
Using the VMware Infrastructure Client or vSphere Client, navigate to the Configuration tab of the VMware ESX host you want to modify.
Click Advanced Settings under the Software section.
Click Disk in the left side pane.
Set QFullSampleSize to a value greater than zero. The usable range is 0 to 64.
Note: For a 3PAR, NetApp and IBM XIV storage arrays, set the QFullSampleSize value to 32. Note: For other storage arrays, please contact your storage vendor.
Set QFullThreshold to a value lesser than or equal to QFullSampleSize. The usable range is 1 to 16.
Note: For a 3PAR storage arrays, set the QFullThreshold value to 4. Note: For a NetApp and IBM XIV storage arrays, set the QFullThreshold value to 8. Note: For other storage arrays, please contact your storage vendor.
The QFullSampleSize and QFullThreshold are system-wide configuration parameters. If the system has different vendor storage arrays and if a LUN or port from any of the arrays returns the qfull or busy error, all the arrays experience the throttling effect.
These settings take effect immediately. You do not need to reboot the ESX host.
Note: Careful consideration is needed if multiple hosts access the same LUN or array ports. For the adaptive queue depth algorithm to be effective, all hosts accessing the LUN/port need to have some form of adaptive queue depth algorithm. In the case where some hosts run the adaptive queue depth algorithm while others do not, the hosts not running the algorithm may consume the resources/slots on the array that are freed up by the adaptive hosts. This causes the hosts running the algorithm to exhibit lower disk I/O throughput. This may also increase the I/O congestion that initially triggered the adaptive algorithm.
If hosts running operating systems other than ESX are connected to array ports that are being accessed by ESX hosts, and the latter are configured to use the adaptive algorithm, either make sure the operating systems use an adaptive queue depth algorithm or isolate them on different ports on the storage array.
Tags
queue-depth
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