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I have a problem tying to figure out, how the resource management in systemd works.

To make clear where I am and how I got there, I should start with explaining my situation:

After migrating a mysql-db from ubuntu 14.04 to 16.04 I have to deal with systemd. Not my first time and no problem till this day. On 16.04 I noticed the log entry during mysql-startup:

Changed limits: max_open_files: 1024 (requested 5000)

I found out, that the mysql-variable max_open_files (max_open_files is not set in my current config!) is dynamic, depending on 4 different scenarios (see: https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_open_files_limit) and in my case, obviously scenario no. 4 is used.

So my point is: The mysql-config on 14.04 and 16.04 is the same, but it looks like the resource management acts different with systemd as it did on 14.04 (with ulimit & /etc/security.d/limits.conf) The condition that mysql uses scenario no 4 is, that 'if operating system limit is Infinity' is true ... why is the operating system limit Infinity?

Another thing that I am struggling with: If I define max_open_files in my mysql-config as lets say '1000' and have a look into /proc/[myslq-pid]/limits I can see that the soft-limit is set to 1000 and the hard-limit is 4096.

Question 1: Why is the limit 4096, as condition 4 from above hits true on 'if operating system limit is Infinity' ...

And I can set max_open_files to any value between 1 and 4096, and it defines limit - which is the expected behavior. But if it get set higher than 4096, it falls back to softlimit=1024 & hardlimit=4096. I do not understand this behavior, because again, condition 4 from above hits true on if operating system limit is Infinity ...

That leads to my Question 2: Where does this 'default value' come from?? And why I am not able to set the limit higher than 4096?

Note: As soon as I define LimitNOFILE=number in the mysql-service-unit of systemd, 'number' gets set as soft AND hardlimit and I am able to define values higher than 4096. Confirmed by looking into /proc/[myslq-pid]/limits again.

I had a look into the docs of systemd, but for some reason the only thing that got mentioned in terms of 'defaults' are the /etc/systemd/system.conf and /etc/systemd/user.conf files, where I can define DefaultLimitNOFILE=, which is unset by default - so is there a default Default??

I seriously do not get behind this behavior... hope someone has a Tip on how to get this sorted.

I take any advice or hint you have!

I even would be happy to read more documentations, that explains this - but I just have not found the right ones ... everything I found leads me back to systemd.resource-control, setrlimit, systemd.exec and systemd/ControlGroupInterface which does not took me further than endig up here :)

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