I really love the concept of greyhole, and I gave it a good, honest try, but I have discovered that it simply is not the best fit for me. The experience was quite shocking, by the end.
A few SMB shares, 3 drives, greyhole, sabnzbd, transmission. All of this together with some mass file transfers to the greyhole shares and KABOOM!
I don't know if the kernel panicked or what, but the NAS went down hard. I reset the server and watched it boot. Logged into the console and KABOOM!
I repeated this a few times...
Had to boot into recover mode and remove greyhole from the init.d/ dir. Then it booted fine.
Snapraid and aufs is next on the menu, as soon as I rsync as much data off as I can.