This change makes it illegal to set the planCacheSize parameter on version 6.0. Before this change, the parameter could be set, but had no effect. After this change, setting the parameter errors. This includes failing server startup if the parameter is set in the configuration.

Description of Linked Ticket

planCacheSize which is introduced in mongodb V5.1 as a rapid release is not available for on-prem and hence we tried to upgrade to Mongo V6 and test this option but unfortunately, we couldn't see any difference in the planCacheSize even after adding this to mongo config or via command line.

Scenario:

We are working on adding transaction support in our project. We have a 3 node replica set with PSA topology and since we are using transactions for our "Bot Publish" feature, mongodb retains all the data related to botpublish in cache until transaction is success. this data resides in cache which is also used for plancache, dirty cache that is not yet written to disk (incldues any transaction's data too).

What we observed is mongo's default cache size is (n-1)/2 and in this (n-1)/2 size of cache, 80% is provided to plancache + transaction data and another 20% is for dirty cache that is yet to be written to disk (our assumption). we thought to reduce plancache size and provide more space for transaction data as our transactions are huge running operations, we couldn't do it even after using planCacheSize which is introduced in Mongo V6 on-prem, we added planCacheSize to 5%, 10% etc in config but still see it is taking significantly more RAM (like 50% also).

Please help us on how to set planCacheSize in MongoV6 in Ubuntu 18.04 OS