[Solved] The reading with secondaryPreferred when chunk migration occurred often fail

umiyosh Asks: The reading with secondaryPreferred when chunk migration occurred often fail
We have some problems regarding balancer configuration. Our application which is set up with secondaryPreferred often fails to read documents from the secondary when balancer( chunk migration ) is running. So I changed the Balancer setting to the appropriate setting as below, but I could observe the unexpected behavior.

use config
   { "_id" : "balancer" },
   { $set : { "_secondaryThrottle" : true ,
              "writeConcern": { "w": 6} } },
   { upsert : true }

->  { "_id" : "balancer",   "stopped" : false,   "_secondaryThrottle" : true,   "writeConcern" : { "w" : 6 } }

1. Environment

Our cluster is as below.

( 1 primary, 5 secondary, one arbiter ) * 6 masters on Sharded cluster on MongoDB 3.2.4 on Google Cloud Platform Mongod instance type is “n1-standard-16 (16 vCPUs, 60 GB memory) with 500GB persistent disk”

2. What did I expect?

I expect that the application with reading as secondaryPreferred can read from the Secondary-mongod without failing. I think that the Balancer can wait for all completion of chunk-migration because I set up “writeConcern:6”. So my expectations are as described above.

3. Test conditions and results

I configured as above settings and restarted balancer and execute the application. The application( secondaryPreferred ) failed with high frequency to read from the secondary-mongod.

Please let me ask a question.

  • Q1. What is the meaning of writeConcern in balancer setting? It seems that writeConcern does not work as shown in the above test.
  • Q2. How can I read without failing when chunk migration occurred?
  • Q3. What is the purpose of _secondaryThrottle? I have read the official document of v3.2. And official document described that _secondaryThrottle is true as the default value and has the same meaning as writeConcern: 2. But we can configure settings as below. If I set up _secondaryThrottle:false, does it makes the writeConcern invalid right? Is “_secondaryThrottle” the flag for enabling writeConcern?
e.g. { “_id” : “balancer”, “stopped” : false, “_secondaryThrottle” : false, “writeConcern” : { “w” : 6 } }


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