md-raid absolutely supports that and synology has for a long time. They call it "SHR". You simply do raid over disk partitions to enable disks of disparate sizes.
The reason ZFS doesn't support it, and absolutely 0 enterprise storage devices support this is because as the disks fill up, you sacrifice both performance and redundancy. Synology won't even support it on their high-end devices for this very reason. They'll only do it on their devices targeted at home use.
The reason ZFS doesn't support it, and absolutely 0 enterprise storage devices support this is because as the disks fill up, you sacrifice both performance and redundancy. Synology won't even support it on their high-end devices for this very reason. They'll only do it on their devices targeted at home use.