code: function usageRead(opts, cb) {
    if (typeof opts === 'function') {
      cb = opts;
      opts = {
        sectorSizeBytes: 512,
        sampleMs: 1000,
        device: 'sda',
        units: 'bytes',
      };
    } else {
      opts.sectorSizeBytes = opts.sectorSizeBytes || 512;
      opts.sampleMs = opts.sampleMs || 1000;
      opts.device =  opts.device || 'sda';
      opts.units =  opts.units || 'bytes';
    }
  
    var time;
    var delta1;
    var delta0 = _parseProcDiskstats()[opts.device].sectorsRead;
    time = process.hrtime();
    setTimeout(function() {
      delta1 = _parseProcDiskstats()[opts.device].sectorsRead;
  
      var diff = process.hrtime(time);
      var diffSeconds = diff[0] + diff[1] * 1e-9;
  
      var totalBytes = (delta1 - delta0) * opts.sectorSizeBytes;
      var totalBytesPerSecond = totalBytes / (diffSeconds * diffSeconds);
      var converted = _bytesTo(totalBytesPerSecond, opts.units);
  
      return cb(converted);
    }, opts.sampleMs);
  }
docs: NOTE: get sector size in bytes: `sudo hdparm -I /dev/sda | grep Physical` NOTE: get sector size in bytes: `cat /sys/block/sda/queue/physical_block_size`