[comex]

> which implement servers

A "server" is an IPC mechanism; this is describing a way for one userspace process to serve filesystems to other userspace processes.

It sounds like the kernel has no built-in notion of a "filesystem", and filesystems just take advantage of the kernel's generic IPC mechanism, which is also used by a lot of other things. That's great – but it's still true that IPC must go through the kernel, and switching from one user process (the client) to another (the server) is a context switch.

It may be that the code also supports locating the client and server within the same process – I have not looked at it. But that's not what the documentation describes, so it's at least not the main intended operating mode.

[zenlibs]

A userspace program can completely avoid kernel IPC if it has no intention to expose the fs to other processes. Client and server code can exist within same "app", without IPC, in the same process

[geofft]

There are plenty of existing libraries that do exactly that. This isn't novel to Fuchsia. A good example is GNOME's GVfs https://en.wikipedia.org/wiki/GVfs , which is basically a plugin architecture to the standard GLib I/O routines. (Although as it happens, it still places the mounts in separate daemon processes.)

Other things that come to mind are SQLite's VFS layer https://www.sqlite.org/vfs.html , Apache Commons VFS for Java https://commons.apache.org/proper/commons-vfs/ , glibc's fopencookie(3) which lets you provide a custom, in-process implementation of a FILE * http://man7.org/linux/man-pages/man3/fopencookie.3.html , libnfs which even comes with an LD_PRELOAD https://github.com/sahlberg/libnfs , etc.

(And as others have pointed out, while client and server code can exist without IPC, as the names "client" and "server" would imply, that isn't the primary intention. The docs you link say, "To open a file, Fuchsia programs (clients) send RPC requests to filesystem servers ...." And even the terminology of a file system as a "server" isn't novel to Fuchsia; that's the approach the HURD and Plan 9 both take for filesystems, for instance.)

[batbomb]

And Parrot VFS

http://ccl.cse.nd.edu/software/parrot/

[Zardoz84]

And, if I remember correctly, Minix 1.0 was talking about filesystem server

[monocasa]

If you have the capabilities to the whole device.