once upon a time, many versions ago, there used to be a file system type that ran on top of a more limited file system and emulated a POSIX and/or Linux file system. this was intended to share space an MS-DOS file system so one did not have to partition a hard boundary of space between the 2 systems on a dual-boot machine. it worked by storing POSIX/Linux file system metadata in extra files it stored on the backing file system. it's gone today. in theory, there is no longer a need for it, or much of a use for it.

i have a potentially significant use case for it, or for the way it worked. maybe i could deploy it as a user space file system. maybe someone has done this already. does anyone who was a Linux user back then remember was this was? does anyone know of anything to do this kind of thing, today?

I do not really understand, but probably: you can mount any file and use as a device, including partition(s) and filesystems.

Is umsdos what you're talking about?

http://www.tldp.org/HOWTO/UMSDOS-HOWTO.html

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this umsdos looks like (the predecessor of) cygwin for me.

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yes, that's the one ... UMSDOS.

@pan64: i was just looking for the name. in the process of searching for it, in which i crossed its path but was not sure, yet (the link in post #3 was most helpful), i came across a filesystem named "posixovl" which is implemented for the FUSE interface (which makes it easy to add new filesystems). umsdos and posixovl use a filesystem as their backingstore. they store file contents in files with like names or similar names and metadata like ownership and permissions in similar named files or other files. that means the structure the filesystem uses is a bunch files. encfs is a similar concept but it encrypts or decrypts file contents and file/directory names.

the reason i was looking for umsdos is because i wanted to see if something like that (still) exits today (posixovl might be what i really want) that i could use over top of the s3fs filesystem. s3fs is another similar concept, but stores files in an AWS S3 bucket, without standard POSIX filesystem metadata (since AWS S3 itself does not have that). my goal is to have a filesystem on Linux storing everything in an AWS S3 bucket, plus having full POSIX metadata and semantics like having a full range of file owner uids, group gids, permission/mode bits, attributes, and being able to create block and character special files, named fifo pipes, named sockets and symlinks. this would so an AWS S3 bucket or folder can represent or correctly replicate a system file tree.

Puppy Linux can do this type of thing!!! as a "frugal" install, which is actually the best way to run it as it is more powerful than a traditional "full install"

how does Puppy Linux achieve this sort of thing? does it use umdos or posixovl?