There are folks that do run Mac Pro and Mac mini systems as servers, and there are ways to configure and connect both to Fibre Channel Storage Area Network (FC SAN) configurations.
There are third-party rackmount options for both Mac Pro and Mac mini, if that's a requirement.
There are changes planned to macOS with Server.app: https://support.apple.com/en-us/HT208312
Network Attached Storage (NAS) might be an alternative to the Xsan and FC SAN configuration, if your network can be structured to run that at a speed that meets your requirements. Storage can be served from macOS or other systems, or from dedicated NAS boxes such as Synology products or configurations running FreeNAS software.
For higger-end performance with FC SAN or similar I/O configurations past what NAS can offer, you're looking at Promise hardware or similar, and at ATTO or other related products. There are Thunderbolt to FC SAN adapters available.