mongodump is not a recommend way if your data size is huge say >100 gb. It takes a long time to backup and to restore even more. Best way is to take file system snap shot. Refer this guide - docs.mongodb.org/v3.0/tutorial/backup-with-filesy…
For sharded cluster refer this guide - docs.mongodb.org/v3.0/tutorial/backup-sharded-clu…
We use mongodb.com/cloud is a service from MongoDB inc which provides backup, automation and monitoring.