HDFS-10958 introduces instrumentation hooks around DataNode disk IO and HDFS-10959 adds support for profiling hooks to expose latency statistics.
Instead of choosing only one hook using Config parameters, we want to add two separate hooks - one for profiling and one for fault injection. The fault injection hook will be useful for testing purposes.
This jira only introduces support for fault injection hook. The implementation for that will come later on.
Also, now Default and Counting FileIOEvents would not be needed as we can control enabling the profiling and fault injection hooks using config parameters.