Warspite
DS216j 2-bay basic model
2x 2TB SATA in SHR configuration, ext4 filesystem
At family's place
Iowa
DS916+ 4-bay advanced model with btrfs support
3x 6TB SATA in SHR configuration, btrfs filesystem, 1x SSD read-cache
At home
If I upgraded, I'd probably go for the DS1621+ so that I can add a 10gbps NIC. Alternatively, the DS1817 has onboard 10gig NICs but has no btrfs support or M.2 slots.