What are some alternatives?
When comparing Greyhole and mergerfs, you can also consider the following products
StableBit DrivePool - State of the art, file based drive pooling software.
mhddfs - The driver combines a several mount points into the single one.
Drive Bender - Drive Bender, class leading storage pooling technology
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
btrfs - Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features...
SnapRAID - A backup program for disk arrays. It stores parity information of your data and it recovers from up to six disk failures.