You could potentially also speed it up a bit by, after the first pass, trying to read first blocks that are unlikely to be written again soon (normally the long

You could potentially also speed it up a bit by, after the first pass, trying to read first blocks that are unlikely to be written again soon (normally the longer a block has been written last, the less likely it is to be written again soon). This would implicitly leave blocks that are being written over and over for last.