[SERVER-26660] MongoDB WT replication network usage Created: 17/Oct/16  Updated: 20/Oct/16  Resolved: 17/Oct/16

Status: Closed
Project: Core Server
Component/s: Replication
Affects Version/s: 3.0.12
Fix Version/s: None

Type: Bug Priority: Major - P3
Reporter: Marc Fletcher Assignee: Unassigned
Resolution: Done Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified
Environment:

EC2 Sharded Replicaset


Attachments: PNG File 1.png     PNG File 2.png     PNG File 3.png    
Operating System: Linux
Participants:

 Description   

I have a mongo replica set where all reads/writes must go via the primary. The majority of the writes to the primary are small UPDATES to relatively large documents (~500kB). Each document may have between 5 and 10 updates one after the other.

Recently I noticed something odd, in that the network IN to the secondaries is MUCH higher than the network IN to the primary. Given that the secondaries should only be handling replication and nothing else I'm at a bit of a loss to explain this effect.

Here is the network IN over the last hour to the PRIMARY:

And here is the network IN over the last hour for the two SECONDARIES:

The network IN on the secondaries appears to average about 200MB (per minute), which is 3 times higher than the 60MB (per minute) of the primary.

What could explain this?



 Comments   
Comment by Marc Fletcher [ 20/Oct/16 ]

Hi @ramon.fernandez,

Thanks for the reply, but I do believe this might be a bug with MongoDB server, or WT (which is the project I originally reported this issue in).

I did post this issue to Stack Overflow, and waited almost 1 month for a response (http://stackoverflow.com/questions/39613389/mongodb-replication-network-usage). The lack of response on SO is what made me believe it's 'unexpected' behavior. I've since also posted on the mongodb-user group as you suggested (also no response there).

If you happen to know that this is NOT a bug a quick acknowledgment of that, alongside a potential solution to ensuring the network IN usage of both primary and secondaries are equivalent that would be greatly appreciated.

Regards,
Marc

Comment by Ramon Fernandez Marina [ 17/Oct/16 ]

Thanks for your report MarcF. Please note that the SERVER project is for reporting bugs or feature suggestions for the MongoDB server. For MongoDB-related support discussion please post on the mongodb-user group or Stack Overflow with the mongodb tag, where your question will reach a larger audience. A question like this involving more discussion would be best posted on the mongodb-user group. See also our Technical Support page for additional support resources.

Regards,
Ramón.

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