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Setting the Maximum Outstanding Disk Requests per Virtual Machine

Details

You can adjust the maximum number of outstanding disk requests with the Disk.SchedNumReqOutstanding parameter. When two or more virtual machines are accessing the same LUN (logical unit number), this parameter controls the number of outstanding requests each virtual machine can issue to the LUN.
 
Note that this limit is inapplicable when only one virtual machine is active on a LUN. In that case, the bandwidth is limited by the queue depth of the storage adapter.
 
This change can impact disk bandwidth scheduling, but experiments have shown improvements for disk intensive workloads. If you adjust this value in the VMkernel, you might also want to adjust the queue depth in your storage adapter. For example, knowledge base article 1267 (http://kb.vmware.com/kb/1267) explains how to do this for a QLogic qla2x00 Fibre Channel adapter.

Solution

vSphere 4

For vSphere 4, the procedure to set the VMkernel limit is as follows:
  1. In the vSphere Client, select the host in the inventory panel.
  2. Click the Configuration tab and click Advanced Settings under Software.
  3. Click Disk in the left panel and scroll down to Disk.SchedNumReqOutstanding.
  4. Change the parameter value to the number of your choice and click OK.
VirtualCenter 2.0/2.5 with ESX 3.0/3.5
 
For VirtualCenter 2.0 and later (with ESX Server 3.0 and later) the procedure to set the VMkernel limit is as follows:
  1. Use the VI Client to connect to the system to be configured (or to a VirtualCenter server managing the system).
  2. Select the host to configure.
  3. Select the Configuration tab.
  4. Under Software, click Advanced Settings.
  5. Select Disk.
  6. Change the Disk.SchedNumReqOutstanding parameter to the desired value.
  7. Click OK.
VirtualCenter 1.4 with ESX 2.5

For VirtualCenter 1.4.x and earlier (with ESX Server 2.5 and earlier) the procedure to set the VMkernel limit is as follows:

  1. Use the Management User Interface to connect to the system to be configured.
  2. Click the Options tab.
  3. Click the Advanced Settings link.
  4. Scroll to the Disk.SchedNumReqOutstanding parameter and click the link for its value. The Advanced Settings window appears.
  5. Change the Disk.SchedNumReqOutstanding parameter to the desired value. parameter value to the desired number, then click OK.
  6. Click the Close Window button.
  7. Reboot the server.

Keywords

1268; urlz

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Actions
  • KB Article: 1268
  • Updated: Aug 14, 2009
  • Products:
    VMware ESX
  • Product Versions:
    VMware ESX 2.0.x
    VMware ESX 2.1.x
    VMware ESX 2.5.x
    VMware ESX 3.0.x
    VMware ESX 3.5.x
    VMware ESX 4.0.x