What are some alternatives?
When comparing Greyhole and Rockstor, you can also consider the following products
StableBit DrivePool - State of the art, file based drive pooling software.
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
Drive Bender - Drive Bender, class leading storage pooling technology
FreeNAS - FreeNAS is a storage operating system strong and robust enough to meet the needs of enterprise level businesses.
mergerfs - mergerfs is a union filesystem geared towards simplifing storage and management of files across...
Amahi Home Server - Amahi is a media, home and app server software known for its easy-to-use user interface. Amahi has the best media, backup and web apps for small networks.