Mongodb – In MongoDB serverStatus(), what is the meaning of “connection data handles currently active”

mongodbmongodb-3.4wiredtiger

When running db.serverStatus() in the MongoDB shell (3.4, WiredTiger), the results contain the following data:

{
    "connection data handles currently active" : 94862,
    "connection sweep candidate became referenced" : 0,
    "connection sweep dhandles closed" : 91885,
    "connection sweep dhandles removed from hash list" : 648514,
    "connection sweep time-of-death sets" : 1514067,
    "connection sweeps" : 40077,
    "session dhandles swept" : 110175,
    "session sweep attempts" : 710643
}

What is the meaning of "connection data handles currently active"? Does it have an impact on the instance's performance?

Best Answer

Although I can't find any evidence for that in the documentation, it is probably the open file-handles that Mongo holds against the file storage.

I did a test that supports this idea:

Creating new mongodb instance, "connection data handles currently active" minimum value is 11 (sometimes it becomes 12,13 and then back to 11).
db.coll.insert({"foo":"bar"}) - "connection data handles currently active" min value = 13.

I did the same with other operations that cause mongo to create new files on disk, such as index creation, and I have noticed the same behavior. We can conclude that this value, "connection data handles currently active" is related to the number of file-handles that mongo holds.

As for the question if it has an impact on the instance's performance, in some cases it could be true according to the next discussion at Jira: https://jira.mongodb.org/browse/SERVER-27700

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MongoDB: how to get the all the embedded data

If you had multiple documents containing products, you could retrieve all as follows by capturing the query cursor and iterating over the set:

replset:PRIMARY> var cur = db.Category.find( {}, {products:1} );
replset:PRIMARY> while (cur.hasNext()) {
... var doc = cur.next();
... printjson(doc);
... }
{
    "_id" : ObjectId("542450a928db034349fb3bf2"),
    "products" : [
        {
            "_id" : ObjectId("542450a928db034349fb3bf3"),
            "name" : "Producto de categoria",
            "price" : 11.11
        }
    ]
}

If you have multiple products within the "products" array then you would iterate over the array contained in each result document.

Best Way to Reload Data in MongoDB

I'd recommend looking into pre-splitting and/or using a hashed shard key to do the insertion and stick with dropping the collection (with remove you are basically doing a delete for every write, so it will always be slow). The hashed shard key is usually the easiest one to get started with.

If you are looking to measure write throughput then each of those approaches (when done properly) will allow you to utilize all available shards immediately rather than hitting one shard first, waiting for the balancer to move data etc. (also, if you use a monotonically increasing shard key then you will only ever write to a single shard - see the three pitfalls posts for good descriptions of all that).

Best Answer

Related Solutions

MongoDB: how to get the all the embedded data

Best Way to Reload Data in MongoDB

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