Replicating storage efficiently

You could use lsyncd to have a constant syncing of files between systems. lsyncd installs inotify watches on directories that are synced. Whenever files change in the directories, it will transfer changes to remote server using rsync.

On the block level, the synchronization can be done using Starwind that makes a mirrored disk on both ends. It can run over iSCSI LUNs, making active-active storage. No additional hardware required. https://www.starwindsoftware.com/blog/storage-ha-on-the-cheap-fixing-synology-diskstation-flaky-performance-with-starwind-free-part-3-failover-duration

On the file level, lsyncd and rsync do a mirror synchronizing files between servers. These tools might require tweaking the configuration files in order to ensure the file locking mechanism works as expected and no split-brain would occur. https://linoxide.com/tools/setup-lsyncd-sync-directories/