Counters in Hadoop...

1. Task Counters #248
1. MAP_INPUT_RECORDS 
1. No of records read by all Map Tasks
2. MAP_OUTPUT_RECORDS
1. No of records produced by all Map Tasks
3. PHYSICAL_MEMORY_BYTES
1. Amount of Physical memory used on a Particular Task attempt
2. Filesystem counters
1. BYTES_READ
1. No of bytes read by the filesystem by map & reduce tasks
2. BYTES_WRITTEN
1. No of bytes written to filesystem by Map & Reduce tasks
3. FileInputFormat Counters
1. BYTES_READ
1. No of bytes read by Map tasks via FileInputFormat
4. FileOutputFormat Counters
1. BYTES_WRITTEN
1. No of bytes written by Map or Reduce task via the FileOutputFormat
5. Job Counters
1. Are maintained by a Application Master #250
2. Maintains Job statistics
3. TOTAL_LAUNCHED_MAPS
1. No of Map tasks launched (Including the failed one and the one that started speculatively)