Not that I want to push ZFS or anything, mdraid/LVM/XFS is a fine setup, but for informational purposes - ZFS can absolutely expand onto larger disks. I wasn't aware of this until recently. If all the disks of an existing pool get replaced with larger disks, the pool can expand onto the newly available space. E.g. a RAIDz1 with 4x 4T disks will have usable space of 12T. Replace all disks with 8T disks (one after another so that it can be done on the fly) and your pool will have 24T of space. Replace those with 16T and you get 48T, and so on. In addition you can expand a pool by adding another redundant topology just like you can with LVM and mdraid. E.g. 4x 4T RAIDz1 + 3x 8T RAIDz2 + 2x 16T mirror for a total of 44T. Finally, expanding existing RAIDz with additional disks has recently landed too.

And now for pushing ZFS - I was doing file based replication on a large dataset for many years. Just going over all the hundreds of thousands of dirs and files took over an hour on my setup. That's then followed by a diff transfer. Think rsync or Syncthing. That's how I did it on my old mdraid/LVM/Ext4 setup, and that's how I continued doing on my newer ZFS setup. Recently I tried using ZFS send/receive which operates within the filesystem. It completely eliminated the dataset file walk and stat phase since the filesystem already knows all of the metadata. The replication was reduced to just the diff file transfer time. What used to take over an hour got reduced to seconds or minutes, depending on the size of the changed data. I can now do multiple replications per hour without significant load on the system. Previously it was only feasible overnight because the system would be robbed of IOPS for over an hour.