At a customer site we received several notifications of performance issues with a VMware VI3 environment. After having checked the configuration of the VMs and the Hosts we decided to dive into esxtop. At first sight we did not see any abnormalities. Low %RDY, which is usually the first thing I check, some swapping but not enough to cause any major delays. The weird thing about this one is that it seemed that only when IP was sent/received the VM felt sluggish. As we could not reproduce the issue we decided to start esxtop in batchmode and use esxplot and perfmon to get to the bottom of it. Soon we found what the issue was, receive packets were being dropped at the vSwitch level.

In other words, at times an enormous amount of received packets were dropped. After some research I found an article which actually describes this behavior. ( We tried increasing the buffer size for the E1000 virtual network adapter this VM was configured with but it did not resolve the issue. As there were other drivers mentioned in the post we decided to “upgrade” the NIC to a “vmxnet” NIC and this actually resolved the issue. Although performance is not where we expected it would be yet we are not seeing any dropped packets anymore and can focus on the next possible cause.