The ext4 filesystem has had the “casefold” option for a while now <https://lwn.net/Articles/784041/>. This allows you to specify, when you initialize a new filesystem, that you want file/directory names to be treated as case-insensitive. At the same time, you have to specify that the filenames are to be interpreted as UTF-8 encoding; normally the kernel doesn’t care, it only singles out ASCII slash and NUL bytes for special interpretation, and simply passes through any other byte values in the names. I was able to create such a filesystem, but I wasn’t able to mount it in my last-upgraded-just-a-few-days-ago Debian Unstable setup. When I try, it fails, and in the dmesg log I see Filesystem with casefold feature cannot be mounted without CONFIG_UNICODE Turns out the Debian 5.10 kernel has not been built with this CONFIG_UNICODE option enabled. However, this situation looks set to change shortly, according to <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=985689>.