Guidelines and Requirements

Voice of the DBA

I saw a post from Brent that Microsoft had changed the default memory guidance. At first glance I read this as they’d changed the default values, which would be interesting. However, this is a guideline, set to 75%. I also saw a few thoughts from Randolph West on LinkedIn, and quite a few comments. The comments were interesting in a few ways.

It is easy to look at 75% and say that won’t work for this server that’s on my mind right now because I keep getting woken up. That might be true. However, the 75% number isn’t a hard requirement. It’s a guideline, a recommendation to ensure you have enough memory for the OS, but you’re trying to use most for SQL Server. Feel free to adjust it if you feel the need.

There are certainly people who will also look at that number and then go to a DBA and say, “you’ve set this to 70% (or 85% or whatever) and that’s not what Microsoft says.” Which isn’t true. What the text says is this under the recommended column: “75% of available system memory not consumed by other processes, including other instances. For more detailed recommendations, see max server memory”

If you go to the “max server memory” section, you see something else. It asks you to monitor before you set this, then do some calculations. Then it says: “This is a generic approximation, and your mileage might vary.”

That’s a great statement. What they’ve written might not work for you. That’s true. Maybe you have little RAM and some other stuff on your server, so 75% might be way too high. Maybe you have 4TB of RAM, in which case, if you blindly set 75% you should be asked to work elsewhere. Anyone managing systems with 4TB of RAM should know how to monitor, measure, and then choose something different, which might be 85% of RAM.

While there might be some requirements for managing database systems, there really are a lot of guidelines. You have to make decisions, which means you need some knowledge on which to make good decisions. If you don’t have that knowledge, or are unsure, ask others, ask the GenAI’s, conduct experiments, test things. That’s the job. Learn what you need to make things run better.

Better being what your clients need, want, and desire.

Steve Jones

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