In terms of file system, a DB inside Couch is, basically, several files. Number of files depends on number of the DB shards stored at particular node, and also depends on the number of design documents with indices for the DB.
Opening a file is costly for OS – so having a lot of sharded DBs is bad in terms of opening/closing a lot of files. Concurrent writing into many files is generally bad practice for spindle devices, it may be also not so optimal for some SSDs. Also, since CouchDB uses ‘append only’ strategy inserting/updating data, having consistent inbound feed multiplexed into several DBs may cause excessive storage fragmentation. So – do test. Also please read this at SO: https://bit.ly/2tJIJnN ermouth
