Linux raid does that already. You can also add disks to grow an array or change it from raid5 to raid6. The gradual phase-in is what's missing. It makes sense a

Linux raid does that already. You can also add disks to grow an array or change it from raid5 to raid6. The gradual phase-in is what's missing. It makes sense as having what is effectively a raid0 of a 4 drive raid6 and a 6 drive raid6 in the example is a more complex configuration with uneven performance across the array. But for a home NAS meeting the reliability with maximum space ends up being all that counts.