I had a situation where data was being placed using date patterns in the filename and/or the path in HDFS didn't match the contents of the directories. The expectation was that the data in the
2014/12/29 directory path contained all the data for December 29, 2014. However, the reality was that the date was being pulled from the transport. It turns out that the version of
syslog we were using was rewriting the header, including the date portion, causing the data to take on the transport time and not reflect the original time of the record. Usually, the offsets were tiny, just a second or two, so nobody really took notice. However, one day, one of the relay servers died and when the data that had got stuck on upstream servers was finally sent, it had the current time. In this case, it was shifted by a couple of days, causing a significant data cleanup effort.
Be sure this isn't happening to you if you are placing data by date. Check the date edge cases to see...