writeTest.js

fs = require('fs')

var bufferA = new Buffer(1e+8)
bufferA.fill(0)

var bufferB = new Buffer(1e+0)
bufferB.fill(0)

var fileA = fs.openSync('fileA', 'w')
var fileB = fs.openSync('fileB', 'w')

console.time('A')
fs.write(fileA, bufferA, 0, bufferA.length, function(err) { console.timeEnd('A')})
console.time('B')
fs.write(fileB, bufferB, 0, bufferB.length, function(err) { console.timeEnd('B')})


// expected output: B << A

// actual output on OS X node v5.0.0:
// A: 314.564ms
// B: 315.864ms

// Output on Linux:
// B: 4ms <-- yes!
// A: 306ms

I got this in Mac OS X and node 5.

A: 115.469ms
B: 116.775ms