Storing this in an array will be orders of magnitute faster than saving it to disk and having to re-read whatever file each time you want to access it.
Speed should actually be completely irrelevant for this type of operation, so you can just do somthing like the following:
var updateArray = function(arr, newData) {
if (arr.length > 192) {
arr = arr.slice(1); // Remove first entry
}
arr[arr.length] = newData;
return arr;
};