[Avf-fuse-dev] direct_io and on-demand files?

[Valient G. <vg...@po...>]

I've been experimenting with ways to allow clients to query information
about my running filesystem.  My goal was to have a ".proc" directory
underneath my filesystem directory, which would contain files which
could be queried for information.

However, if I don't pre-fill the files then they show up as 0 length,
and the kernel wouldn't issue read requests for a 0 length file.  I've
found that if I add "direct_io" option to the FUSE mount, then I get the
read requests, but that also means that my entire filesystem is mounted
with direct_io enabled.  Is there a down-side to having direct_io
enabled for normal files?

My other attempt was to mount the .proc directory as a second filesystem
(my FUSE interface can handle multiple mounts).  However I didn't have
much luck having a FUSE based filesystem mounted as a subdirectory of
another FUSE based filesystem.  Is this a FUSE kernel module limitation?

Alternatively, I can pre-fill the files as soon as stat is called on
them, and keep the data around in a temporary file until it is either
forgotten or Open+Read are called.  But I don't want to have to generate
the file data until it is actually going to be read..

Has anyone had luck with having proc-style virtual files -- is direct_io
for the entire filesystem the only solution?

regards,
Valient