How SNS repair/rebalance work in Motr

[andriytk]

@mssawant, could you please describe in a few paragraphs how SNS repair/rebalance work in Motr?

I know that there are copy machines on each server and that each server is pushing (sending) the data or parity units of the parity groups with the missing units to the servers where the spare units are located, where the missing units are restored and saved. But how each server finds the parity groups with the missing units, how this process is synchronised between the servers? How the overall process knows when to finish and how much data is there to repair? It would be nice if you could describe that here. Thank you!