Recently Upgraded Mongo from 3.X to 4.2.18 but Mongo fails to start

Ops and Admin Installation & Upgrades
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Hi @Blaise,

The secondaryIndexPrefetch configuration was specific to MMAPv1 and was removed (along with the deprecated MMAPv1 storage engine) in MongoDB 4.2. You can remove this line from your configuration file as it is no longer relevant and there is no equivalent needed for WiredTiger.

Note: the default value for secondaryIndexPrefetch was all, so your previous configuration did not need to set this explicitly.

Regards,
Stennie

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