I am strongly considering B2 as an option for a dropbox-style system. Something where I run 8-16TB of hot tier on my local LAN, with B2 serving as the slower mass storage tier behind it. It seems that the average B2 access latencies would be ~100-200ms, which is very tolerable for a cache miss on such a massive tier of storage. With this amount of space available you could have pre-fetch rules that do things like pull down entire directories as files within them are accessed.