SQL 2019 on Linux Max Memory

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SQL 2019 on Linux Max Memory

Evan King 21

We have VM instances installed with Red Hat Linux as the OS and SQL Server 2019. I am trying to understand how the max memory works. I have read that SQL will use the maximum of 80% of the available memory for SQL, and that can be limited in the mssql.conf file. There is also another limit that you can change in SSMS for a maximum memory limit.

What I don't understand is which one takes precedents over the other. Here are the specs:

Total memory configured at the VM level: 512 GB
Total memory seen by SSMS in the Server Properties/General option under Memory: 412586 MB
Max memory seen by SSMS on the Server Properties/Memory option under Maximum server memory : 434360 MB (the same value as we have in out Windows SQL 2012 machines with the same 512 GB.

Thanks, Evan

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Answer 1

Hi EvanKing-7297,
Welcome to Microsoft Q&A.
Please refer to the following recommendation from this article which might help:

There is no need to change any setup options for a Linux install from the defaults, because the memory.memorylimitmb setting will limit SQL Server to 80% of the available physical RAM by default.

Best Regards,
Amelia

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Answer 2

Erland Sommarskog 120.2K MVP

What you see on the General tab is how much memory SQL Server thinks is available in the server in total. When I look at this on instances that runs on Windows, I see the actual amount of memory in Windows. On my Linux instance, I see a lower number, which rather seems to be 75% of the RAM for the VM in this case. Why I don't know. In any case, the only way you can change this value is to add more RAM to the machine.

What you see on the Memory page is the configuration setting "max server memory", which sets a limit for how much SQL Server can use for the buffer cache. In this case, it appears to exceed the size of available memory, and therefore it is the latter value that sets the limit in practice.

Evan King 21 Reputation points

2021-06-22T21:53:59.607+00:00

So it would be a good idea to change the configuration setting to be less than the amount of memory showing as available?
Erland Sommarskog 120.2K Reputation points MVP

2021-06-22T21:59:27.087+00:00

The default setting for the configuration parameter is 2147483647, which essentially means "let SQL Server handle the situation". It will grab as much memory as it needs, but yield when the OS signals that there is memory pressure.

However, on Windows, we often recommend people to set value below the available memory to the OS space to breathe. This is also important if there are other applications running on the machine, for instance multiple instances of SQL Server.

I will have to admit that when it comes to Linux, I don't know. Since the exposed amount of memory is lower, breathing-space for the OS may already have been accounted for.

Let's see if someone else jumps in and have anything to add.

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SQL 2019 on Linux Max Memory

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