Monitoring Memory Usage
Monitoring memory usage, whether on a host or a virtual machine, can be challenging. The monitoring itself is not difficult; it's the availability of the physical resource that can be a challenge. Of the four resources, memory can be oversubscribed without much effort. Depending on the physical form factor chosen to host VMware ESX/ESXi, running out of physical RAM is easy to do. Although the blade form factor creates a very dense consolidation effort, the blades are sometimes constrained by the amount of physical memory and network adapters that can be installed. But even with other regular form factors, having enough memory installed comes down to how much the physical server can accommodate and your budget.
If you suspect that memory usage is a performance issue, the first step is to isolate whether this is a memory shortage affecting the host (you've oversubscribed physical memory and need to add more memory) or whether this is a memory limit affecting only that virtual machine (meaning you need to allocate more memory to this virtual machine or change resource allocation policies). Normally, if the ESX/ESXi host is suffering from high memory utilization, the predefined vCenter Server alarm will trigger and alert the vSphere administrator. However, the alarm doesn't allow you to delve deeper into the specifics of how the host is using memory. For that, you'll need a performance graph.
Perform the following steps to create a real-time graph for a host's memory usage:
1. Connect to a vCenter Server instance with the vSphere Client.
2. Navigate to the Hosts And Clusters inventory view.
3. In the inventory tree, click an ESX/ESXi host. This shows you the Summary tab.
4. Click the Performance tab, and switch to Advanced view.
5. Click the Chart Options link.
6. In the Customize Performance Chart dialog box, select the Memory resource type and the Real-Time display interval.
7. Select Line Graph as the chart type. The host will be selected as the only available object.
8. In the Counters area, select the Memory Usage (Average), Memory Overhead (Average), Memory Active (Average), Memory Consumed (Average), Memory Used by VMkernel, and Memory Swap Used (Average). This should give you a fairly clear picture of how memory is being used by the ESX/ESXi host.
Counters, Counters, and More Counters
As with virtual machines, a plethora of counters can be utilized with a host for monitoring memory usage. Which ones you select will depend on what you're looking for. Straight memory usage monitoring is common, but don't forget that there are other counters that could be helpful, such as Ballooning, Unreserved, VMkernel Swap, and Shared, just to name a few. The ability to assemble the appropriate counters for finding the right information comes with experience and depends on what is being monitored.
9. Click OK to apply the chart options and return to the Performance tab. These counters, in particular the Memory Swap Used (Average) counter, will give you an idea of whether the ESX/ESXi host is under memory pressure. If the ESX/ESXi host is not suffering from memory pressure and you still suspect a memory problem, then the issue likely lies with the virtual machine.
Perform the following steps to create a real-time graph for a virtual machine's memory usage:
1. Use the vSphere client to connect to a vCenter Server instance.
2. Navigate to either the Hosts And Clusters or the VMs And Templates inventory view.
3. In the inventory tree, click a virtual machine. This shows you the Summary tab.
4. Click the Performance tab, and switch to the Advanced view.
5. Click the Chart Options link.
6. In the Customize Performance Chart dialog box, select the Memory resource type and the Real-Time display interval.
7. Select Line Graph as the chart type.
8. In the list of counters, select to show the Memory Usage (Average), Memory Overhead (Average), Memory Consumed (Average), and Memory Granted (Average) counters. This shows memory usage, including usage relative to the amount of memory configured for the virtual machine.
9. Click OK to apply the chart options and return to the Performance tab.
From this performance graph, you will be able to tell how much of the memory configured for the virtual machine is actually being used. This might reveal to you that the applications running inside that virtual machine need more memory than the virtual machine has been assigned and that adding more memory to the virtual machine -- assuming that there is sufficient memory at the host level -- might improve performance.
Memory, like CPU, is just one of several different factors that can impact virtual machine performance. Network usage is another area that can impact performance, especially perceived performance.