I ran into the same issue, I think there is an open ticket for this somewhere if you google it or search these forums. I was able to do a straight file backup without having any memory issues or affecting the running couchbase instance by using this command:
sudo /opt/couchbase/bin/cbbackup couchstore-files:///couchbase/data /db_backup -u <couchbase username> -p <couchbase password>