Mongodb Standalone Performs better than Replica Set

Question

I am currently using a standalone mongodb with high read operations. Since replica sets could provide high read performance, I setup a 3 node replica set on 3 vms with read preference as "secondary preferred". However I could not see any performance improvement, in fact the replica set runs a little slower(~2 secs) than the standalone.The configuration is as follows, 4GB of RAM, 7GB of data, 50 GB of hard disk on all 3 vms and for the standalone db and We are using aggregate queries. I would like to know what could be the real reason for the replica sets to function slower compared to the standalone db.

Answer 1

I would like to know what could be the real reason for the replica sets to function slower compared to the standalone db.

Replica sets provide high availability and data redundancy, but these benefits are best realized using separate server resources.

With multiple data-bearing replica set members on a single server you are adding contention for the underlying hardware resources (CPU, RAM, disk):

• Every write has to be replicated and applied by each of the replica set members (which each have their own journal, oplog, and data files), so there will be significantly more I/O as compared to a standalone server.

• A replica set on a single server will be using RAM to store multiple copies of the same data; a standalone can have more of your data & indexes in the same amount of RAM.

If the reason for your performance challenge is a lack of resources, you should be adding dedicated resources (such as more RAM or faster storage) rather than sharing existing resources.

Note that even with replica set members on multiple servers, secondary reads are not a panacea. For some considerations, see: Can I use more replica nodes to scale?.

Before adding additional server resources, I recommend starting with optimising indexes to support your common queries and aggregations. See: Index Strategies in the MongoDB documentation.

Answer 2

Mongodb Standalone Performs better than Replica Set

As per MongoDB BOL here A replica set is a group of mongod instances that maintain the same data set. A replica set contains several data bearing nodes and optionally one arbiter node. Of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.

As mongod is the primary daemon process for the MongoDB system. It handles data requests, manages data access, and performs background management operations.

As per MongoDB Blog documentation here Aggregation pipelines are resource intensive operations – it makes sense to offload aggregations jobs to secondaries of a MongoDB replica set when it is ok to operate on slightly stale data. This is typically true for ‘batch’ operations since they don’t expect to run on the latest data. If the output needs to be written to a collection then the aggregation jobs only run on the primary since only the primary is writable in MongoDB.