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:
• In ESX 3.x
  
  status = 40/0 0x## 0x## 0x##
  
• In ESX/ESXi 4.0 and later:
  
• H:0x0 D:0x28 P:0x0 Valid sense data: 0x## 0x## 0x##
• H:0x0 D:0x08 P:0x0 Valid sense data: 0x## 0x## 0x##

The hexadecimal 28 or decimal 40 in the error is the SCSI status code for the queue full state. The value 0x08 in the above error is the SCSI Status code indicating a device busy state.

• In ESX/ESXi 4.0 and later, you also see a device busy error

Resolution

VMware ESX 3.5 Update 4 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: 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 must have some form of adaptive queue depth algorithm. If some hosts run the adaptive queue depth algorithm while other hosts do not, the hosts that are 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/ESXi are connected to array ports that are being accessed by ESX/ESXi hosts, and the ESX/ESXi hosts are configured to use the adaptive algorithm, either make sure the operating systems use an adaptive queue depth algorithm or isolate those hosts on different ports on the storage array.

VMware ESX versions 3.5-5.0

In ESX/ESXi versions 3.5-5.0,  QFullSampleSize and QFullThreshold are system-wide configuration parameters. Even 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:

1. Use the vSphere Client or vSphere Web Client to navigate to the Configuration tab of the VMware ESX host you want to modify.
   
2. Click Advanced Settings under the Software section.
   
3. Click Disk in the left side pane.
   
4. Set QFullSampleSize to a value greater than zero. The usable range is 0 to 64.
   
   • For 3PAR, NetApp and IBM XIV storage arrays, set the QFullSampleSize value to 32.
   • For other storage arrays, contact your storage vendor.
     
5. Set QFullThreshold to a value lesser than or equal to QFullSampleSize. The usable range is 1 to 16.
   
   • For 3PAR storage arrays, set the QFullThreshold value to 4.
   • ForNetApp and IBM XIV storage arrays, set the QFullThreshold value to 8.
   • For other storage arrays, contact your storage vendor.

The settings take effect immediately. You do not need to reboot the ESX/ESXi host.

VMware ESXi 5.1

In ESXi releases earlier than ESXi 5.1,  QFullSampleSize and QFullThreshold are set globally, that is, on all devices seen by the ESXi host. In VMware ESXi 5.1, these parameter are not set globally because different vendors have different optimal values for their arrays. You set QFullSampleSize and QFullThreshold on a per-device level.

Run the following ESXCLI command:. 
esxcli storage core device set --device  device_name --queue-full-threshold  
Q --queue-full-sample-size S 

Settings are persistent across reboots. 
You can retrieve the values for a device by using the corresponding list command. 

esxcli storage core device list

The command supports an optional --device parameter. 

esxcli storage core device list --device device
The recommended values are the same as in earlier releases.
QFullSampleSize:

• For 3PAR, NetApp and IBM XIV storage arrays, set the QFullSampleSize value to 32.
• For other storage arrays, contact your storage vendor.

QFullThreshold:

• For 3PAR storage arrays, set the QFullThreshold value to 4.
• ForNetApp and IBM XIV storage arrays, set the QFullThreshold value to 8.
• For other storage arrays, please contact your storage vendor.

Source:-

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1008113