I think some devices built the block storage on top of the key-value store. Ie. when you write "hello..." (4k bytes) to address 123, it actually saves key: 12

I think some devices built the block storage on top of the key-value store. Ie. when you write "hello..." (4k bytes) to address 123, it actually saves key: 123, value "hello...". If so, that is probably the reason for a 16 byte key - there is just no way anybody needs a key bigger than 16 bytes for an address anytime soon.