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It's not possible to change the number of file descriptors available to squid at runtime with either the "max_filedescriptors" option or by specifying the open files limit in systemd squid unit file.
Attempting to use max_filedescriptors option gets an error:
WARNING: max_filedescriptors disabled. Operating System setrlimit(RLIMIT_NOFILE) is missing.
Rebuilding squid without the "--with-filedescriptors=16384" option in the spec file causes squid to inherit the build environments open files limit as its default (i.e. 1024 as a user), but the squid unit file option "LimitNOFILE=16384" is now obeyed and set if systemd is used to start squid. The max_filedescriptor option also starts working at this point.
Can we get squid built without the "--with-filedescriptors=16384" build option. Thank you.
It would make sense to compile squid without the file descriptors limit and treat systemd LimitNOFILE as a boundary for config value, but there is an issue when squid.conf value is higher than LimitNOFILE. In this case squid logs and probably also uses max_filedescriptor value instead of current fd limit.
I will initiate discussion with upstream on the theme of systemd and squid.
Yes. From my testing (with a squid built without the --with-filedescriptors option), if max_filedescriptors is not set, squid will just inherit the current rlimit setting of nofile (I made an error in my comment 0, it does not come from the build environment). If max_filedescriptors is set, then that is what squid will use, regardless of the nofile rlimit.
Is that necessarily an issue? That seems "ok" to me and the effect I'd want (being able to "override" the rlimit in the squid config).
The only kind of maybe unexpected thing here is if one doesn't use systemd to start squid, the default squid configuration is probably going to inherit a filedescriptor limit much smaller (1024) than the systemd LimitNOFILE specified 16384.
You can find the newest Squid build without "--with-filedescriptors" option in rawhide.