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MMAPv1 storage engine was deprecated in MongoDB 4.0 and removed in MongoDB 4.2, which makes WiredTiger the only available storage engine for MongoDB from version 4.2 and downwards. Even WiredTiger became available in MongoDB version 3.0 most users delayed the migration between the storage engines for various reasons, like bugs or the uncertainty of being an early adopter. As a matter of fact, some users delayed the unavoidable storage engine migration until MongoDB 4.0.

In this presentation, we are going to describe a large-scale migration of an MMAPv1 environment to WiredTiger. For the scope of this presentation, we define as a large-scale environment, as an installation with more than 100 TeraBytes of data, more than 1000 shards involved, and more than 100 different workloads. We are going to focus mainly on the preparation steps, as we think it's the most important piece of this type of migration. At the same time, we are going to analyze the actual migration steps, the rollback procedure, and most importantly the lessons learned