This is a really cool concept. I'm a bit confused though. Until there are enough people running the software, will some files become inaccessible if someone turn of their computer?
Im still waiting for someone to answer this guy's question.
No it won't, unless you have a network of 4 nodes or something. The network reconfigures and has a mechanism like this (basci explination, more info on wiki papers and code).
live copies min 2 max (generally) six
Off-line copies up to 16 (under debate this is too high after the network is over a few thousand nodes, just for example though this is fine)
If a node is off line and comes back on line then he is live again. (thats why sometimes more than six)
the off-line nodes are held in order of last seen.
If a node is off line for too long it will be pushed off the chain here. This will mean the node lost a data element. This goes against his rank.
In this mechanism you can see we randomly select nodes, as each data element is pushed around the network (if live copies <3 then 4 more live copies made) until it finds a place it does not get lost. This may be a node that is off for many hours/days/weeks and holding data not requested too often. That data will make it's way to archive chunks after a while though. This is another story and too much for me to type as I would miss my pint that will be waiting for me :-)
I hope this makes some sense. I think of it like we drop a chunk on the network, it moves around testing locations till it finds a suitable rest (sort of like a drop of water in a bucket). Like ants who will continually send an ant up a path they are not going, in case there is food there since the last ant looked (could be poison and the die on that route, but ants will randomly go that way, so does our data chunks).