I had the same issue. I wanted to extend my /home folder on my server by adding a second drive and chose to use mhddfs. I already had a whole harddrive entirely dedicated to my /home, the system being hosted on a separate drive - this has made things easier.
Here is how I proceeded, after my new harddisk was set up and formated:
I created two new mount points: /mnt/home1 and /mnt/home2
I edited /etc/fstab file to :
- change my older harddisk moint point from /home to /mnt/home1
- Set up my new harddisk mount point on /mnt/home2
- Told mhddfs to merge /mnt/home1 and /mnt/home2 into /home
Here is the result in my etc/fstab:
UUID=f29aa9e5-5988-4603-9ecd-5c24dd804d94 /mnt/home1 ext4 defaults 0 2
UUID=e535c3fc-0842-4557-be85-55277912a058 /mnt/home2 ext4 defaults 0 2
mhddfs#/mnt/home1,/mnt/home2 /home fuse defaults,allow_other 0 0
Of course, you have to follow all these steps without restarting the machine (otherwise you will have no more /home directory).
It works pretty well. My older harddrive is now almost 100% full and my system began to write on the newer one, but practicaly speaking you don't even notice it. Everthing you see is a "normal" /home folder and mhddfs coordinates this in a totally transparent way.
I have tried with forcing fsck disk check on startup to make sure everything was ok - I set up the last parameter for mhddfs on /etc/fstab to "0" to make sure fsck does not create problem. Everything runs well, it seems pretty stable.