Need suggestions on Mongodb upgrade from 4.0 MMapV1 to Mongodb 4.2 wiredtiger

[Stennie_X (Stennie)]

Hi @Sreedhar_N,

For prerequisites and steps for upgrading a replica set from 4.0 to 4.2, please see Upgrade a Replica Set to 4.2 in the MongoDB server documentation.

@Sreedhar_N

• As mongo 4.2 does not support MMapV1, Before upgrade Am I required to move replicasets from mongo 4.0 MMapV1 to Mongo 4.0 wiredtiger? Is it mandatory?
• Is upgrading from Mongo 4.0 MMapV1 to Mongo 4.2 wiredtiger the correct way?
• Is it really required to change the storage engine from MMapV1 to wiredtiger on Mongo 4.0, then upgrade from 4.0 wiredtiger to 4.2 wiredtiger?

Since support for the deprecated MMAPv1 storage engine was removed in MongoDB 4.2, upgrading to WiredTiger (default storage engine since MongoDB 3.2) is required before you upgrade:

If your 4.0 deployment uses MMAPv1, you must change the 4.0 deployment to WiredTiger Storage Engine before upgrading to MongoDB 4.2. For details, see Change Replica Set to WiredTiger.

@Sreedhar_N

• Tuning of parameters required for changing storage engine from MMapV1 to Wiredtiger.

I would remove any MMAP-specific parameters and start with the defaults for WiredTiger unless you have identified specific performance goals or tuning required. I would also thoroughly test with representative data, workloads, and deployment resources in a staging or QA environment so you can identify any changed performance characteristics.

@Sreedhar_N

The setup is in PSA architecture.

I would strongly consider upgrading to a PSS deployment. For a longer explanation on the consequences of arbiters, please see: Replica set with 3 DB Nodes and 1 Arbiter - #8 by Stennie .

If you prefer to continue with PSA and WiredTiger, please review the PSA 3-member architecture note in the documentation for changing to WiredTiger.

Regards,
Stennie