Resync a Member of a Replica Set
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A replica set member becomes “stale” when its replication process falls so far behind that the primary overwrites oplog entries the member has not yet replicated. The member cannot catch up and becomes “stale.” When this occurs, you must completely resynchronize the member by removing its data and performing an initial sync.
This tutorial addresses both resyncing a stale member and creating a new member using seed data from another member. When syncing a member, choose a time when the system has the bandwidth to move a large amount of data. Schedule the synchronization during a time of low usage or during a maintenance window.
MongoDB provides two options for performing an initial sync:
- Restart the mongod with an empty data directory and let MongoDB’s normal initial syncing feature restore the data. This is the more simple option but may take longer to replace the data.
- Restart the machine with a copy of a recent data directory from another member in the replica set. This procedure can replace the data more quickly but requires more manual steps.
This procedure relies on MongoDB’s regular process for Replica Set Syncing. This stores the current data on the member. For an overview of MongoDB initial sync process, see the Replica Set Syncing section.
Initial sync operations can impact the other members of the set and create additional traffic to the primary. The syncing member requires another member of the set that is accessible and up to date.
Delete all data and sub-directories from the member’s dbPath directory. Consider making a backup first.
At this point, the mongod performs an initial sync. The length of the initial sync process depends on the size of the database and the network latency between members of the replica set.
This approach “seeds” a new or stale member using the data files from an existing member of the replica set. The data files must be sufficiently recent to allow the new member to catch up with the oplog. Otherwise the member would need to perform an initial sync.
You can capture the data files as either a snapshot or a direct copy. However, in most cases you cannot copy data files from a running mongod instance to another because the data files will change during the file copy operation.
If copying data files, ensure that your copy includes the content of the
You cannot use a mongodump backup for the data files: only a snapshot backup . For approaches to capturing a consistent snapshot of a running mongod instance, see the MongoDB Backup Methods documentation.
After you have copied the data files from the “seed” source, start the mongod instance and allow it to apply all operations from the oplog until it reflects the current state of the replica set.