This is a great KB article from VMware worth reposting. I’m going to start analyzing storage I/O more today on all the arrays I have and LUNs. Article Source: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1019687
Setting the congestion Threshold Value for Storage I/O Control
Details
The congestion threshold value for a datastore is the upper limit of latency that is allowed for a datastore before Storage I/O Control begins to assign importance to the virtual machine workloads according to their shares. You do not need to adjust the congestion threshold setting in most environments.
CAUTION Storage I/O Control will not function correctly unless all datatores that share the same spindles on the array have the same congestion threshold.
Solution
If you do change the threshold setting, set the value based on the following considerations.
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A higher value typically results in higher aggregate throughput and weaker isolation. Throttling will not occur unless the overall average latency is higher than the threshold
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If throughput is more critical than latency, do not set the value too low. For example, for Fibre Channel disks, a value below 20 ms could lower peak disk throughput. On the other hand, a very high value (above 50 ms) might allow very high latency without any significant gain in overall throughput.
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A lower value will result in lower device latency and stronger virtual machine I/O performance isolation. Stronger isolation means that the shares controls are enforced more often. Lower device latency translates into lower I/O latency for the virtual machines with the highest shares, at the cost of higher I/O latency experienced by the virtual machines with fewer shares.
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If latency is more important, a very low value (lower than 20 ms) will result in lower device latency and better isolation among IOs at the cost of a decrease in aggregate datastore throughput.
Procedure
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Select a datastore in the vSphere Client inventory and click the Configuration tab.
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Click Properties.
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Under Storage I/O Control, select the Enabled check box.
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Click Advanced to edit the congestion threshold value for the datastore. The value must be between 10 and 100. You can click Reset to restore the congestion threshold setting to the default value (30 ms).
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Click Close.
For more information see the section “Managing Storage I/O Resources” on the “vSphere Resource Management Guide” (pdf) http://www.vmware.com/support/pubs/.