ahmarsuhail opened a new pull request, #4458:
URL: https://github.com/apache/hadoop/pull/4458

   ### Description of PR
   
   This PR adds in iostats for the prefetching input stream. This PR is 
dependent on https://github.com/apache/hadoop/pull/4386, which fixes issues 
after the rebase. Once that gets in merged in, this PR can also update 
`ITestS3PrefetchingInputStream` with assertions on more stats.
   
   The following stats are added:
   
   **Counters:**
   
   `STREAM_READ_PREFETCH_OPERATIONS`: Total number of prefetch ops requested
   
   **Duration:**
   
   `STREAM_READ_REMOTE_BLOCK_READ`: Time taken to read a full block of data 
from S3
   `STREAM_READ_BLOCK_ACQUIRE_AND_READ`: Time taken to acquire a buffer from 
the pool and read data into it. This is 
   not for prefetching ops, but when the first block of data is read (on first 
read or after a seek)
   `ACTION_EXECUTOR_ACQUIRED`: Time taken to acquire an executor. Either by the 
prefetching task or the caching task. 
   
   **Gauges:**
   
   `STREAM_READ_BLOCKS_IN_FILE_CACHE`: Blocks currently in file cache
   `STREAM_READ_ACTIVE_PREFETCH_OPERATIONS`: Current active prefetch operations
   `STREAM_READ_ACTIVE_MEMORY_IN_USE`: Current active memory in use
   
   There are still some more stats that can be added, especially around cache 
usage. I'll follow up with another PR for those if we're happy with the 
approach of this PR. 
   
   ### How was this patch tested?
   
   Tested in eu-west-1 by running
   
   `mvn -Dparallel-tests -DtestsThreadCount=16 clean verify`


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