Assuming that your log file only has 1-2% of the rows out of order, you could make a single pass through the complete log, outputing two files: one file that is in order and another file containing the 1-2% of rows that are out of order. Then sort the out-of-order rows in memory and perform a single merge of the formerly out-of-order rows with the in-order rows. This will be much faster than a full mergesort which will do many more passes.
Assuming that your log file has no row more than N rows out of place, you could make a single pass through the log with a sorted queue N rows deep. Whenever you encounter a log row that is out of order, just insert it into the proper place in the queue. Since this only requires a single pass through the log, it's going to be as fast as you can get.
