What are some alternatives?
When comparing Greyhole and mhddfs, you can also consider the following products
StableBit DrivePool - State of the art, file based drive pooling software.
Drive Bender - Drive Bender, class leading storage pooling technology
mergerfs - mergerfs is a union filesystem geared towards simplifing storage and management of files across...
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
SnapRAID - A backup program for disk arrays. It stores parity information of your data and it recovers from up to six disk failures.
FlexRaid T-Raid - Unify & organize your data. Protect & recover from disk failures!