Capped Collection Performance Issues - mongodb

Question

I'm doing some tests to see what kind of throughput I can get from Mongodb. The documentation says that capped collections are the fastest option. But I often find that I can write to a normal collection much faster. Depending on the exact test, I can often get twice the throughput with a normal collection.
Am I missing something? How do I troubleshoot this?
I have a very simple C++ program that writes about 64,000 documents to a collection as fast as possible. I record the total time, and the time that I'm waiting for the database. If I change nothing but the collection name, I can see a clear difference between the capped and normal collections.
> use tutorial
switched to db tutorial
> db.system.namespaces.find()
{ "name" : "tutorial.system.indexes" }
{ "name" : "tutorial.persons.$_id_" }
{ "name" : "tutorial.persons" }
{ "name" : "tutorial.persons.$age_1" }
{ "name" : "tutorial.alerts.$_id_" }
{ "name" : "tutorial.alerts" }
{ "name" : "tutorial.capped.$_id_" }
{ "name" : "tutorial.capped", "options" : { "create" : "capped", "capped" : true, "size" : 100000000 } }
> db.alerts.stats()
{
"ns" : "tutorial.alerts",
"count" : 400000,
"size" : 561088000,
"avgObjSize" : 1402.72,
"storageSize" : 629612544,
"numExtents" : 16,
"nindexes" : 1,
"lastExtentSize" : 168730624,
"paddingFactor" : 1,
"systemFlags" : 1,
"userFlags" : 0,
"totalIndexSize" : 12991664,
"indexSizes" : {
"_id_" : 12991664
},
"ok" : 1
}
> db.capped.stats()
{
"ns" : "tutorial.capped",
"count" : 62815,
"size" : 98996440,
"avgObjSize" : 1576,
"storageSize" : 100003840,
"numExtents" : 1,
"nindexes" : 1,
"lastExtentSize" : 100003840,
"paddingFactor" : 1,
"systemFlags" : 1,
"userFlags" : 0,
"totalIndexSize" : 2044000,
"indexSizes" : {
"_id_" : 2044000
},
"capped" : true,
"max" : 2147483647,
"ok" : 1
}
linux version: 3.4.11-1.fc16.x86_64
mongo version: db version v2.2.2, pdfile version 4.5
This is a dedicated machine doing nothing but running the Mongodb server and my test client. The machine is ridiculously overpowered for this test.

Answer 1

I see the problem. The web page I quoted above says that a capped collection "without an index" will offer high performance. But…
http://docs.mongodb.org/manual/core/indexes/ says "Before version 2.2 capped collections did not have an _id field. In 2.2, all capped collections have an _id field, except those in the local database."
I created another version of my test which writes to a capped collection in the local database. Sure enough, this collection did not have any indexes, and my throughput was much higher!
Perhaps the overview of capped collections at http://docs.mongodb.org/manual/core/capped-collections/ should clarify this point.

Answer 2

Capped collections guarantee preservation of the insertion order. As a
result, queries do not need an index to return documents in insertion
order. Without this indexing overhead, they can support higher
insertion throughput.
According to the above definition, if you don't have any index insertion to capped collections does not have to be faster than inserting to a normal collection. So if you don't have any indexes and if you don't have any other reason to go with capped collection such as caching, showing last n elements kind a stuff I would suggest you to go with regular collections.
Capped collections guarantee that insertion order is identical to the
order on disk (natural order) and do so by prohibiting updates that
increase document size. Capped collections only allow updates that fit
the original document size, which ensures a document does not change
its location on disk.