If I get it right if to make mongodump in to directory that already
contains previous dumped data it will still download full collection
data even if is not different from data containing in the dir Though
it could check the metadata. Is there a way to make it not download
collection if it has not "changed" in terms I described?
As per MongoDB documentation here mongodump overwrites output files if they exist in the backup data folder. Before running the mongodump command multiple times, either ensure that you no longer need the files in the output folder (the default is the
dump/ folder) or rename the folders or files.
When you run mongodump without any arguments, the command connects to the MongoDB instance on the local system (e.g. localhost) on port 27017 and creates a database backup named dump/ in the current directory.
To backup data from a mongod or mongos instance running on the same machine and on the default port of 27017, use the following command:
You can also specify the --host and --port of the MongoDB instance that the mongodump should connect to. For example:
mongodump --host mongodb.example.net --port 27017
To specify a different output directory, you can use the
mongodump --out /data/backup/
For further your ref here