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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 share a LUN (logical unit number), this parameter controls the total number of outstanding commands permitted from "all virtual machines" to that 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 Changing the Queue Depth for QLogic and Emulex HBAs (1267) explains how to do this for a QLogic qla2x00 Fibre Channel adapter.
Solution
vSphere 4 and vSphere 5
To set the VMkernel limit for vSphere 4 and 5:
In the vSphere Client, select the host in the inventory panel.
Click the Configuration tab and click Advanced Settings under Software.
Click Disk in the left panel and scroll down to Disk.SchedNumReqOutstanding.
Change the parameter value to the number of your choice and click OK.
VirtualCenter 2.x with ESX 3.0 and ESX/ESXi 3.5
To set the VMkernel limit for VirtualCenter 2.x with ESX 3.0 and ESX/ESXi 3.5:
Use the VI Client to connect to the system to be configured (or to a VirtualCenter server managing the system).
Select the host to configure.
Select the Configuration tab.
Under Software, click Advanced Settings.
Select Disk.
Change the Disk.SchedNumReqOutstanding parameter to the desired value.
Click OK.
VirtualCenter 1.4 with ESX 2.5
To set the VMkernel limit for For VirtualCenter 1.4.x and earlier with ESX Server 2.5 and earlier:
Use the Management User Interface to connect to the system to be configured.
Click the Options tab.
Click the Advanced Settings link.
Scroll to the Disk.SchedNumReqOutstanding parameter and click the link for its value. The Advanced Settings window appears.
Change the Disk.SchedNumReqOutstanding parameter to the desired value. parameter value to the desired number, then click OK.
Click the Close Window button.
Reboot the server.
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