You often need to access to the complete DSS logs. To find them, first, locate your Data Dir (see this article), and then:
The main "backend" log is:
If you are using Hadoop (Hive, Pig...) with DSS, you may find a lot of information in the complete Hadoop logs:
optionally, you may find some info in the nginx logs:
The "backend.log" is the main file to look at when debugging or hunting for errors.