iojs 1.8.1
Linux Mint 17, x64, 3.13.0-24

function formatString(n) {
  var str = '';
  while (str.length < n) {
    str = str + ' ' + str.length;
  }
  return str;
}

var fs = require('fs')
  , str = formatString(100000)
  , buffer = new Buffer(str + '\n', 'utf8');

fs.writeSync(process.stdout.fd, buffer, 0, buffer.length, null);

Running this script produces a truncated output. Here is a tail of it ... 77530 77536 77542 77548 77554 77560 7.

Replacing fs.writeSync(process.stdout.fd, buffer, 0, buffer.length, null) with console.log(str) changes output. Now the full string is output correctly. Tail is ... 9964 99970 99976 99982 99988 99994.