code: function (file, cb) {
    fs.stat(file, function (err, stat) {
      if(err) return cb(err)
      fs.open(file, 'r', function (err, fd) {
        if(err) return cb(err)
        stat.length = Math.ceil(stat.size / stat.blksize)
        stat.fd = fd
        cb(null, stat)
      })
    })
  }
docs: TODO: use bloom filters to remember what keys are present to optimize gets.