Many databases/collections, very high memory usage

[John_Moser1 (John Moser)]

Hello

When I am creating basically empty databases (for ex. 20’000) each with 5 empty collections, stay these databases/collections all “open”/cached ? So that they will use more/override the WT cache settings ?

As an ex.

Total RAM = 8GB
WT Cache setting = 1GB
Used RAM by MongoDB ca. 7.5 GB

My expectation would be that due to the 1GB WT Cache the overall RAM usage would be much lower (I know 1GB is not suitable but just wanted to check out what is using RAM).

So could anyone explain how the databases and/or collections are handled regarding caching / memory usage ?

Thanks
John

[John_Moser1 (John Moser)]

Got a reply somwhere else : seems all the files are opened and kept open. Also the metadata gets loaded and this is file system buffer/cache related. So even WT cache is 1GB only the overall cache/buffer can be much higher.