If you're going to build something like this, especially when the data sets are typically large, this is a very strong case for erasure coding. If you split a

If you're going to build something like this, especially when the data sets are typically large, this is a very strong case for erasure coding. If you split a 5GB file into (20) 250MB chunks and then mirror each of them four times, there is a nontrivial probability that one of the chunks loses all four mirrors. Especially when you're using notoriously unreliable volunteer hosting. If you split the same 5GB file into (20) 250MB data chunks and (60) 250MB erasure chunks, you consume the same amount of total storage but have to lose 61 chunks (>75% of all the hosts) before you lose any data.