How Do I Clear the Alarm that the Dirty Data in WiredTiger Cache Is Too High

Possible Causes

The dirty data in WiredTiger cache is too high in the following scenarios:

1. After the parameter enableMajorityReadConcern of MongoDB is set to true, the service write rate is too high. As a result, the secondary node cannot synchronize oplogs from the primary node in a timely manner.
2. After the parameter enableMajorityReadConcern of MongoDB is set to false, the CPU and memory specifications are too small to bear the write rate of the current service.

Fault Locating

• Check the running parameters of MongoDB. Log in to the console, click the instance name, and choose Parameters to view the value of enableMajorityReadConcern.
  Figure 1 Checking the value of enableMajorityReadConcern
  
• Log in to the management console, choose Cloud Eye > Cloud Service Monitoring > Document Database Service, and view the node metrics of WiredTiger Dirty Data Cache Percentage and Tracked Dirty Bytes in WiredTiger Cache.
  Figure 2 Tracked Dirty Bytes in WiredTiger Cache
  
  Figure 3 WiredTiger Dirty Data Cache Percentage
  
• Common scenarios:

  If the primary/standby replication latency keeps increasing, log in to the management console and choose Cloud Eye > Secondary to view the replication latency.

  Figure 4 Replication Lag
  

Solution