What does sdcardfs do exactly?

A lot of Android devices out there use sdcardfs to implement external storage. The actual storage (in the case where primary external storage is actually internal) is on the data partition in /data/media/0, which is then mounted as sdcardfs to (on my device) a directory under /mnt/runtime. I've not been able to find the basic documentation on what sdcardfs does exactly. There are higher level descriptions in the Android docs, but I would like to know for example what mount options sdcardfs supports and what they do exactly. The kind of info you would find in a unix man page . The android code is open source, so I have been able to find the source code for sdcardfs, but that too doesn't seem to include an overview of what the filesystem does. There is a list of mount options it supports, which are: - fsuid=%u - fsgid=%u - gid=%u - debug - mask=%u - userid=%d - multiuser - derive_gid - default_normal - reserved_mb=%u But there is no explanation for them. For some of them I can guess what they probably do, but not all of them. And also, the higher level Android storage documentation says sdcardfs also does case insensitivity mapping and additional tracking, which do not appear to correspond to any mount options. So, is there any man page-level documentation out there? Or can someone describe what exactly the mount options do and what other transformations sdcardfs applies to its underlying filesystem?