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, both of which can be used to restore a replica set 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
mongodwith 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.
Procedures
Automatically Sync a Member
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.
If the instance has no data, you can follow the Add Members to a Replica Set or Replace a Replica Set Member procedure to add a new member to a replica set.
You can also force a mongod that is already a member of the set to
perform an initial sync by restarting the instance without the contents of the
dbPath directory:
Stop the member's
mongodinstance. To ensure a clean shutdown, use thedb.shutdownServer()method from themongoshell or on Linux systems, themongod --shutdownoption.(Optional) Make a backup of all data and sub-directories from the member's
dbPathdirectory. If a full backup is not required, consider backing up just thediagnostic.datadirectory to preserve potentially-useful troubleshooting data in the event of an issue. See Full Time Diagnostic Data Capture for more information.Delete all data and sub-directories from the member's
dbPathdirectory.Start the mongod process, specifying Replication Options as appropriate.
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.
Sync by Copying Data Files from Another Member
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.
Copy the Data Files
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.
Important
If copying data files, ensure that your copy includes
the content of the local database.
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.
Sync the Member
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.