Github user kl0u commented on the issue: https://github.com/apache/flink/pull/5811 Well it seems like for these tests, the `flush` is not actually flushing. The files are there, the `validPartLength` is correct (=6 as we just write `test1\n`) but the data is not actually on disk. If you call `close()` on the in-progress file when snapshotting, then the tests succeed and the data is there. I would recommend to just remove the check for now, and open a followup JIRA that contains the check that you will remove, and also points on the discussion about HDFS not flushing, and we see how to proceed. I thing that the fact that the end-to-end tests pass point to the direction that sth is wrong with the FS abstraction.