There is a bit of a performance hit with mhddfs --> everything gets relayed from the kernel to the mhddfs task (userspace) and then back to the kernel and on to the original task.

The CPU usage is noticeable. Not huge, not a problem, but noticeable. I wouldn't put a news/mail server on mhddfs, but it's quite good for a home media server.

One project I might do if I get time/bored, is to write an in-kernel implementation of a simplified version of it, which would get rid of the double copying on reads/writes and make it all pretty much invisible performance-wise.

I don't need to do that, but it just looks like a fun way to play with the Linux VFS layer.

Cheers