usage and effective demand

[buffalix]

Sorry if this has been discussed

I have hard time understand this

Demand:     Amount of a physical resource that the object might consume without any existing constraints. An object becomes constrained because of underprovisioning or contention with other consumers of the

resource. A virtual machine might require 10GB of memory but can only get 5GB because the virtual machine must share resources with other virtual machines on the host. When the demand amount is less than the usage amount, the environment might have wasted resources. When the demand amount is greater than the usage amount, the environment might incur latency and exhibit decreased performance.

How could demand amount is less than the usage amount - if resource is not demand how could it be used?

Can anyone shed a light on this?

Thanks

[jddias]

Here's an example - I have a DB server which has 4GB vRAM allocated.  When I look at the memory usage from the OS, it's around 70-80% utilization.

However, from the hypervisor's perspective I'm not really ACTIVELY using that memory (same server as above, but the below shows the vC Ops Details view of memory usage).  That's what demand measures - so basically for memory, if demand is BELOW usage then the machine is OK, but if it is consistently below usage you may have an oversized machine.  You would want to reference the Planning tab at that point (Oversized VM report, for example) to validate this.  The Operations > Details view gives you the current 5 minute picture.

[buffalix]

Great. So from the capacity planning prospective, usage is more important than demand?

[jddias]

Actually I'd say that demand is more important if you want to drive better optimization.  Understanding demand as it relates to usage is important.