What Can I Do When OOM Occurs During the Migration of MongoDB Databases?

Scenarios

Out of memory (OOM) occurs during the migration of MongoDB databases, causing migration failures.

Possible Cause

The possible causes are as follows:

• If the mongod service of the source database is deployed on a single server, OOM occurs when the migration process consumes large amounts of memory through operations such as creating indexes and sorting queries.
• If the mongod service is deployed on a server with other services and the cacheSizeGB value is not specified, OOM occurs when all available memory has been allocated to other services, so the WiredTiger engine does not have sufficient memory.
  

  By default, the memory used by the WiredTiger engine of mongod is calculated as follows: (Memory in GB-1) x 50% for version 3.2 or (Memory in GB-1) x 50% for version 3.4 and later.

Solution

• If the mongod service is deployed on a single server, do not perform any operations that consume large amounts of memory during the migration.
• If the mongod service and other services are deployed on the same server, set the value of cacheSizeGB to the half of the minimal idle memory to ensure that memory used in peak hours will not be allocated to WiredTiger excessively.