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POCapped Collection Performance Issues
 

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  1. POCapped Collection Performance Issues
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    <p>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.</p> <p>Am I missing something? How do I troubleshoot this?</p> <p>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.</p> <pre><code>&gt; use tutorial switched to db tutorial &gt; 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 } } &gt; 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 } &gt; 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 } </code></pre> <p>linux version: 3.4.11-1.fc16.x86_64</p> <p>mongo version: db version v2.2.2, pdfile version 4.5</p> <p>This is a dedicated machine doing nothing but running the Mongodb server and my test client. The machine is ridiculously overpowered for this test. </p>
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    1. COCan you provide a link to the documentation you've seen that indicates inserting to capped collections is faster?
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    2. COhttp://docs.mongodb.org/manual/core/capped-collections/ "Inserting documents in a capped collection without an index is close to the speed of writing log information directly to a file system."
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