The replication lag observed in SERVER-65054 could recur for any customer who upgrades from MongoDB 7.0.8 or newer to MongoDB 8.0.x and has yet to upgrade the cluster's feature compatibility version to FCV 8.0. SERVER-88162 increased the default value of internalInsertMaxBatchSize from 64 to 500 after it had been lowered to 64 by SERVER-78556. However, the changes from SPM-3381 are only applicable in MongoDB 8.0, FCV 8.0. In a lower FCV value an oplog hole may still be open for an excessive amount of time, thereby stalling replication.

Customers may spend from 1 week to 1 month running their clusters in the lower FCV state. We should lessen the risk of an availability issue by basing the default value of internalInsertMaxBatchSize in the server's logic on the ability to use SPM-3381.