Understanding VMware Capacity - Why OS monitoring can be misleading (1 of 10)

Dangers with OS Metrics

Almost every time we discuss data capture for VMware, we’ll be asked by someone if we can capture the utilization of specific VMs, by monitoring the OS. The simple answer is no.

The more complex answer is that we can capture the data from the OS, but it may not be reliable. So here’s an example of why.

We have 2 VMs. Within the 1 second interval we are looking at, one of the VMs was only allocated the CPU for ½ a second. In that ½ second the VM used 50% of it’s possible CPU time. So from the OS perspective it was running at 50% CPU utilization. If we look at data from VMware, we’ll see that VMware knows the VM only used ½ the CPU available in ½ a second.  Or 25%.

The 2nd VM was running on CPU for the entire second. And again it used 50% of it’s possible CPU. So, to the OS, it appears it was running at 50% CPU utilization, and VMware has the same result.

The more contention there is for CPU time, the more time VMs will spend Dormant/Idle, and the further apart the values will be. This effect means that any metrics which have an element of time in their calculation cannot be relied upon to be accurate.

Here is data from a real VM

The (top) dark blue line is the data captured from the OS, and the (Bottom) light blue line is the data from VMware. There clearly is some correlation between the two. At the start of the chart there is about a 1.5% CPU difference. Given we’re only running at about 4.5% CPU that is an overestimation by the OS of about 35%. But at about 09:00 the difference is ~0.5% so the difference doesn’t remain stable either.

Historically it’s not been unusual to see situations where the OS metric is reporting 70% CPU utilization and VMware is reporting 30%.

More on Wednesday, in the meantime don't forget to register for our next webinar 'Top 5 VMware tips for performance and capacity' 

https://www.metron-athene.com/services/webinars/capacity-management-webinars.html

Phil Bell

Consultant

5 Top Performance and Capacity Concerns for VMware

I'll be hosting our webinar VMware and Hyper-V Virtualization Over-Subscription (What's so scary?) on October 12 http://www.metron-athene.com/services/webinars/index.html so I thought it would be pertinent to take a look at the Top 5 Performance and Capacity Concerns for VMware in my blog series.

I’ll begin with Dangers with OS Metrics.

Almost every time we discuss data capture for VMware, we’ll be asked by someone if we can capture the utilization of specific VMs, by monitoring the OS.  The simple answer is no.

In the example below the operating system sees that VM1 is busy 50% of the time but VMware sees is that it was only there for half of half the time and accordingly reports that it is 25% busy.

Looking at the second VM running, VM2, both the operating systems and VMware are in accordance that it is in full use and report that it is 50% busy.

This is a good example of the disparity that can sometimes occur.

OS vs VMware data

Here is data from a real VM. 

The (top) dark blue line is the data captured from the OS, and the (Bottom) light blue line is the data from VMware.  While there clearly is some correlation between the two, at the start of the chart there is about 1.5% CPU difference.  Given we’re only running at about 4.5% CPU that is an overestimation by the OS of about 35%.  While at about 09:00 the difference is ~0.5% so the difference doesn’t remain stable either.  This is a small system but if you scaled this up it would not be unusual to see the OS reporting 70% CPU utilisation and VMware reporting 30%.

This large difference between what the OS thinks is happening and what is really happening all comes down to time slicing.

I'll be looking at time slicing on Monday.

Phil Bell

Consultant

VMware Capacity Management

VMware is the go-to option for virtualization for many organizations, and has been for some time.
The longer it's been around, the more focus there is on making efficiency savings for the organization. This is where the Capacity Manager really needs to understand the technology, how to monitor it, and how to decide what headroom exists.

I'm running a VMware Capacity Management webinar this Wednesday May 25 (8am PDT,  9am MDT,  10am CDT,  11am EDT, 4pm UK,  5pm CEST) where I'll be taking a look at some of the key topics in understanding VMware Capacity.

Topics will include:

• Why OS monitoring can be misleading
• 5 Key Metrics
• Measuring Processor Capacity
• Measuring Memory Capacity
• Calculating Headroom in VMs

Register for your place now http://www.metron-athene.com/services/webinars/index.html

Look forward to seeing you there.

Dale Feiste

Principal Consultant