The "contentstore" is where all course assets are stored. It's really a wrapper of code around a GridFS (MongoDB) backend and it stores binary files which can PDFs, WAVs, JPGs, or other. The contentstore code is mainly here:

https://github.com/edx/edx-platform/tree/master/cms/djangoapps/contentstore

The contentstore has some known technical problems, explained somewhat in this presentation:

http://doctoryes.github.io/mug_talk_modulestore/#1

An effort to move all course assets out of GridFS and into external storage was begun in 2014 and abandoned. The docs from that effort: GridFS Replacement

Since that effort, the performance team has implemented an optimization for non-locked course assets. See Dave Ormsbee or Toby Lawrence (Deactivated) for further details.

Counting Assets in Each Course

If you need to find out how many assets are contained in each course, the JS below will assist you.

/* The original "group" */
db.fs.files.group( {
    key: {"_id.course": 1, "_id.org": 1, "_id.run": 1},
    reduce: function(cur, result) { result.count += 1 },
    initial: {count: 0}
} )

/* ..and with category included. */
db.fs.files.group( {
    key: {"_id.course": 1, "_id.org": 1, "_id.run": 1, "_id.category": 1},
    reduce: function(cur, result) { result.count += 1 },
    initial: {count: 0}
} )

var mapFunction = function() {
    var slicer = function(x) { return x.slice(0, x.lastIndexOf("+")) };
    var split_id = null;
    if (typeof this._id === "string")
        split_id = slicer(this._id);
    var key = [ this._id.course, this._id.org, this._id.run, this._id.category, split_id ];
    emit( key, 1 );
};
var reduceFunction = function(key, values) {
    return Array.sum(values);
}
db.fs.files.mapReduce(
    mapFunction,
    reduceFunction,
    {
        out: {inline: 1}
    }
)

/* For debugging the mapper... */
var emit = function(key, value) {
    print("emit");
    print("key: " + key + " value: " + tojson(value));
}

/* To find all courses which aren't the three specified. */
db.fs.files.find({"_id.course": {$nin : ["DemoX", "import_test", "LargeCourse101"]}})

Contentstore

Counting Assets in Each Course