VM Basics…
With Virtual Servers, they all sit on a Host server, this is a physical server. These physical servers are usually massive beasts, we are talking anywhere between 128gb ram+ with 32cores etc etc… specing out these massive servers allows you to run multiple “Guest” Servers on the host. this gives you the ability to scale out the resources easier to each guest operating system.
VMware Memory Ballooning
Well… infrastructure engineers like to get value out of their resources. and when a server is allocated 10gb ram, they assume that server isn’t going to be using that allocation constantly. so they allow the VM’s to share the resources, allowing the host to reclaim resources back when they are not used & are required elsewhere.
Okay so What Is Memory Over Subscription?
It’s the process of subscribing more memory out to your guest operating systems than you physically have on the server. Think if we had a 100gb memory server. with 10guests each with 10gb of ram allocated, we have used our 100gb. but that memory wont be used at the same time, so at host level, your infrastructure engineers might see that although 100% of the ram has been allocated, only 50% of the ram is actually being used.
Value for money
This is where infrastructure guys like to make their money back, they will see it’s safe to add a couple more Guests on the host, so our 100gb server, might go up to 12guests, with 10gb each, that’s a 120gb allocation, but it’s fine because still only 60% of the ram is actually being used right?…
How does this effect SQL Server?
SQL Server, it’s pretty selfish. it doesn’t like to give memory back, but it will, and it will trim down it’s memory allocation to the minimum working set to satisfy the request of the host. it’s got no choice in the matter after all…. and this is where you will start to see performance issues, plans will start spilling to tempdb, the working set with page to disk (if you haven’t configured LPIM) it all gets rather horrible.
So how do we avoid it?
This is completely infrastructure, avoid over subscribing resources!
Luke Salter 