Memory Pressure

Question

Can someone please explain memory pressure? I have three nodes in a cluster. Each node has 10 ubuntu VMs. Each VM has 3GB of Ram. When the machines had 2GB the memory pressure average around 135% for each. Now that I increased the ram the reading show 86% at idle.

What is the recommended threshold for memory pressure and memory demand? Should I adjust the vm's memory to meet the memory demand? The Ram is set to static. The total amount of available ram between the three nodes is 576GB. I don't know how many machines I will be adding in the future, so this is why I deterred from dynamic memory. Should I be using dynamic memory?

Answer 1

If your VMs run any applications that depend on an external entity for support, always make sure that you satisfy their requirements.
Dynamic Memory is usually a good thing because it follows the way most application use memory: they demand it when they need it and release it when they're done. Dynamic Memory allows you to satisfy the periodic spikes without permanently locking away memory that will sit mostly unused. Some applications request memory differently and those need different treatment.
When I have memory contention, then I aim for a high demand percentage. Since the VM will page to disk when demand exceeds available memory, and the VM gets to decide what memory is safest to page, I often run at or above 100% demand. If something shows performance problems, I add memory. If I have absolutely no idea what a VM will do, then I set a startup of 2GB, a minimum of 512MB, and a maximum of 4GB. Yes, I pulled those numbers from nowhere, but they still seem to work well.
But, since you currently have no contention, then it seems that you've already found a formula that works for you. You know that you can get at least another 10 VMs with that formula and keep 100% availability in an n+1 cluster. You know that you can add 16 or so past that if you want to stuff the cluster full and allow some to go offline during node maintenance or outages. You also know that your 100% utilization line lies somewhere below the 3GB allocation, so you have some room to play.
I would say that today, you can do what you're doing. If you start to fill up, then Dynamic Memory would probably help you a lot.