> a spinning 8TB disk is probably about as reliable as a 1TB disk, so that means you have a 8x increase in probability of data loss I don't understand that arg

> a spinning 8TB disk is probably about as reliable as a 1TB disk, so that means you have a 8x increase in probability of data loss I don't understand that argument, not if you think at scale. You need to store 8000TB. You can run 8000 x 1TB, or 1000 x 8TB. You loose 10% of the disks per year (to keep the math simple). So after a year, you have either had to redistribute 800 x 1TB or 100 x 1TB, which is the same to a software defined distributed storage. There are operational upsides to fewer disks, power usage, manpower, rack space etc. But of course more disks are faster. Google wants to have more options than just bigger vs faster available. Since they know exactly, and can influence, how their software behaves, they can gain much from using specialized disks that balance things differently.