What is directory "/path/_spark_metadata" for?

[Mark Zhao]

 Hey,

 When running Spark on Alluxio-1.8.2, I encounter the following exception:
“alluxio.exception.FileDoseNotExistException: Path
“/test-data/_spark_metadata” does not exist” in Alluxio master.log. What
exactly is the directory "_spark_metadata" used for? And how can I fix this
problem?

Thanks.

Mark