Let’s go step by step into the title of this post and what is to say about VMware vSphere and VMs having CPU ready numbers.

What means VM CPU ready?

Of course the most obvious start is to explain what VM CPU ready means at all. Long story short, a virtual machine having high ready percentage is never a good thing. Ready could suppose the Virtual Machine is ready to go and all is well. In reality some VM processes are ready to go, but they can’t as no physical CPU resources are available. The virtual machine and the applications inside need to wait for a physical CPU to get free resources and process the data.

Nobody likes to wait, so doesn’t a VM and the database running inside.

The result is a virtual machine that has been slowed down either by a lack of resources or a misconfiguration.

You can read more about –

a deeper explanation from Anthony

how to detect ready values and make it visible using ESXtop at yellow-bricks.com here.

and how it can affect your MS SQL

The good…

Honestly I don’t think there is anything good to say about it except the percentages are close to 0%. Otherwise you’ll have some performance affect, althought it might not be very bad. You should always watch for CPU ready within your VMware vSphere environment and check if there are easy ways to fix them.

CPU ready values that can be assumed OK are below 5% per vCPU. Important – per vCPU. That means 10% total ready on a 4 vCPU virtual machine is about 2,5% per vCPU.

The bad…

Ready values of 5% or more per vCPU should already be seen as a warning and you can already measure performance degradation, especially running high performance applications within the guest.

An Oracle or Microsoft SQL database is going to hate the System administrator responsible for that situation. Actually the application vendor support is also going to hate the poor guy calling, as these situations are hard to pin down, when not seeing the full picture.

You always need to take into account, that the guest OS has no idea about the situation, just the triggered process is slow.

A misconfiguration can be extremly easy to fix, as most of the time you have some systems using too many vCPUs (very often by mistake or without better knowledge) or there is a vCPU limitation hurting the VM performance.

When there is no misconfiguration at all, you can find yourself in a 10% or more CPU ready situation due to extremly large VMs running on a few physical cores or a similar misproportion of vCPU vs. pCPU. That should be checked using ESXtop to get details in realtime.

The ugly…

Ready values of 10% or more per vCPU should already be seen as an error and the performance situation can get really nasty. Very high Ready times for the vCPU can even trigger application errors and crashes. Again, very hard to find when just looking inside of the VM guest. It’s very important to watch the outside VMkernel metrics either using ESXtop or tools like Opvizor Performance Analyzer.