A fun back of the napkin game I remember calculating for the past decade around the time affordable (under 10k) IDE-SCSI terabyte sized raids came out was, “How big is a petabyte?”. Around the time these became interesting (2003-4) it looked like ~16 racks of hard drives and 1u controlling servers in 4-6t raid volumes.
The next big upgrade was around a little before the Sun Thumpers arrived and reduced that size down to ~43 servers (500gb drives) and reduced that size down to a little over 4 racks total.
Today, it looks like you can easily get 80 3.5″ drives in a 4u chassis, so that reduces the total size from 12 racks a decade+ ago down to about 16u today. Assuming I run our 10Gbps pipe at full throttle, that’s around 10 days to fully fill. (not counting network, storage, metadata overhead).
Guess its time to start counting racks per exabyte. (304 at today’s density).