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Downgrade MongoDB from 3.0

Before you attempt any downgrade, familiarize yourself with the content of this document, particularly the Downgrade Recommendations and Checklist and the procedure for downgrading sharded clusters.

Downgrade Recommendations and Checklist

When downgrading, consider the following:

Downgrade Path

Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.8.

Important

If you upgrade to MongoDB 3.0 and have run authSchemaUpgrade, you cannot downgrade to the 2.6 series without disabling --auth.

Procedures

Follow the downgrade procedures:

• To downgrade sharded clusters, see Downgrade a 3.0 Sharded Cluster.
• To downgrade replica sets, see Downgrade a 3.0 Replica Set.
• To downgrade a standalone MongoDB instance, see Downgrade a Standalone mongod Instance.

Note

Optional. Consider compacting collections after downgrading. Otherwise, older versions will not be able to reuse free space regions larger than 2MB created while running 3.0. This can result in wasted space but no data loss following the downgrade.

Downgrade MongoDB Processes

Downgrade a Standalone mongod Instance

If you have changed the storage engine to WiredTiger, change the storage engine to MMAPv1 before downgrading to 2.6.

Change Storage Engine to MMAPv1

To change storage engine to MMAPv1 for a standalone mongod instance, you will need to manually export and upload the data using mongodump and mongorestore.

1

Ensure 3.0 mongod is running with WiredTiger.

2

Export the data using mongodump.

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mongodump --out <exportDataDestination>

Specify additional options as appropriate, such as username and password if running with authorization enabled. See mongodump for available options.

3

Create data directory for MMAPv1.

Create a new data directory for MMAPv1. Ensure that the user account running mongod has read and write permissions for the new directory.

4

Restart the mongod with MMAPv1.

Restart the 3.0 mongod, specifying the newly created data directory for MMAPv1 as the --dbpath. You do not have to specify --storageEngine as MMAPv1 is the default.

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mongod --dbpath <newMMAPv1DBPath>

Specify additional options as appropriate.

5

Upload the exported data using mongorestore.

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mongorestore <exportDataDestination>

Specify additional options as appropriate. See mongorestore for available options.

Downgrade Binaries

The following steps outline the procedure to downgrade a standalone mongod from version 3.0 to 2.6.

Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.8.

1

Download 2.6 binaries.

Download binaries of the latest release in the 2.6 series from the MongoDB Download Page. See Install MongoDB for more information.

2

Replace with 2.6 binaries.

Shut down your mongod instance. Replace the existing binary with the 2.6 mongod binary and restart mongod.

Downgrade a 3.0 Replica Set

If you have changed the storage engine to WiredTiger, change the storage engine to MMAPv1 before downgrading to 2.6.

Change Storage Engine to MMAPv1

You can update members to use the MMAPv1 storage engine in a rolling manner.

Note

When running a replica set with mixed storage engines, performance can vary according to workload.

To change the storage engine to MMAPv1 for an existing secondary replica set member, remove the member’s data and perform an initial sync:

1

Shutdown the secondary member.

Stop the mongod instance for the secondary member.

2

Prepare data directory for MMAPv1.

Prepare --dbpath directory for initial sync.

For the stopped secondary member, either delete the content of the data directory or create a new data directory. If creating a new directory, ensure that the user account running mongod has read and write permissions for the new directory.

3

Restart the secondary member with MMAPv1.

Restart the 3.0 mongod, specifying the MMAPv1 data directory as the --dbpath. Specify additional options as appropriate for the member. You do not have to specify --storageEngine since MMAPv1 is the default.

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mongod --dbpath <preparedMMAPv1DBPath>

Since no data exists in the --dbpath, the mongod will perform an initial sync. The length of the initial sync process depends on the size of the database and network connection between members of the replica set.

Repeat for the remaining the secondary members. Once all the secondary members have switched to MMAPv1, step down the primary, and update the stepped-down member.

Downgrade Binaries

Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.8.

The following steps outline a “rolling” downgrade process for the replica set. The “rolling” downgrade process minimizes downtime by downgrading the members individually while the other members are available:

1

Downgrade secondary members of the replica set.

Downgrade each secondary member of the replica set, one at a time:

1. Shut down the mongod. See Stop mongod Processes for instructions on safely terminating mongod processes.
2. Replace the 3.0 binary with the 2.6 binary and restart.
3. Wait for the member to recover to SECONDARY state before downgrading the next secondary. To check the member’s state, use the rs.status() method in the mongo shell.

2

Step down the primary.

Use rs.stepDown() in the mongo shell to step down the primary and force the normal failover procedure.

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rs.stepDown()

rs.stepDown() expedites the failover procedure and is

preferable to shutting down the primary directly.

3

Replace and restart former primary mongod.

When rs.status() shows that the primary has stepped down and another member has assumed PRIMARY state, shut down the previous primary and replace the mongod binary with the 2.6 binary and start the new instance.

Replica set failover is not instant but will render the set unavailable to writes and interrupt reads until the failover process completes. Typically this takes 10 seconds or more. You may wish to plan the downgrade during a predetermined maintenance window.

Downgrade a 3.0 Sharded Cluster

Requirements

While the downgrade is in progress, you cannot make changes to the collection meta-data. For example, during the downgrade, do not do any of the following:

• sh.enableSharding()
• sh.shardCollection()
• sh.addShard()
• db.createCollection()
• db.collection.drop()
• db.dropDatabase()
• any operation that creates a database
• any other operation that modifies the cluster meta-data in any way. See Sharding Reference for a complete list of sharding commands. Note, however, that not all commands on the Sharding Reference page modifies the cluster meta-data.

Change Storage Engine to MMAPv1

If you have changed the storage engine to WiredTiger, change the storage engine to MMAPv1 before downgrading to 2.6.

Change Shards to Use MMAPv1

To change the storage engine to MMAPv1, refer to the procedure in Change Storage Engine to MMAPv1 for replica set members and Change Storage Engine to MMAPv1 for standalone mongod as appropriate for your shards.

Change Config Servers to Use MMAPv1

Note

During this process, only two config servers will be running at any given time to ensure that the sharded cluster’s metadata is read only.

1

Disable the Balancer.

Turn off the balancer in the sharded cluster, as described in Disable the Balancer.

2

Stop the last config server listed in the mongos’ configDB setting.

3

Export data of the second config server listed in the mongos’ configDB setting.

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mongodump --out <exportDataDestination>

Specify additional options as appropriate, such as username and password if running with authorization enabled. See mongodump for available options.

4

For the second config server, create a new data directory for MMAPv1.

Ensure that the user account running mongod has read and write permissions for the new directory.

5

Restart the second config server with MMAPv1.

Specify the newly created MMAPv1 data directory as the --dbpath as well as any additional options as appropriate.

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mongod --dbpath <newMMAPv1DBPath> --configsvr

6

Upload the exported data using mongorestore to the second config server.

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mongorestore <exportDataDestination>

Specify additional options as appropriate. See mongorestore for available options.

7

Shut down the second config server.

8

Restart the third config server.

9

Export data of the third config server.

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mongodump --out <exportDataDestination>

Specify additional options as appropriate, such as username and password if running with authorization enabled. See mongodump for available options.

10

For the third config server, create a new data directory for MMAPv1.

Ensure that the user account running mongod has read and write permissions for the new directory.

11

Restart the third config server with MMAPv1.

Specify the newly created MMAPv1 data directory as the --dbpath as well as any additional options as appropriate.

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mongod --dbpath <newMMAPv1DBPath> --configsvr

12

Upload the exported data using mongorestore to the third config server.

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mongorestore <exportDataDestination>

Specify additional options as appropriate. See mongorestore for available options

13

Export data of the first config server listed in the mongos’ configDB setting.

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mongodump --out <exportDataDestination>

Specify additional options as appropriate, such as username and password if running with authorization enabled. See mongodump for available options.

14

For the first config server, create data directory for MMAPv1.

Ensure that the user account running mongod has read and write permissions for the new directory.

15

Restart the first config server with MMAPv1.

Specify the newly created MMAPv1 data directory as the --dbpath as well as any additional options as appropriate.

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mongod --dbpath <newMMAPv1DBPath> --configsvr

16

Upload the exported data using mongorestore to the first config server.

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mongorestore <exportDataDestination>

Specify additional options as appropriate. See mongorestore for available options

17

Enable writes to the sharded cluster’s metadata.

Restart the second config server, specifying the newly created MMAPv1 data directory as the --dbpath. Specify additional options as appropriate.

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mongod --dbpath <newMMAPv1DBPath> --configsvr

Once all three config servers are up, the sharded cluster’s metadata is available for writes.

18

Re-enable the balancer.

Once all three config servers are up and running with WiredTiger, Re-enable the balancer.

Downgrade Binaries

Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.8.

The downgrade procedure for a sharded cluster reverses the order of the upgrade procedure. The version v6 config database is backwards compatible with MongoDB 2.6.

1

Disable the Balancer.

Turn off the balancer in the sharded cluster, as described in Disable the Balancer.

2

Downgrade each shard, one at a time.

For each shard:

1. Downgrade the mongod secondaries before downgrading the primary.
2. To downgrade the primary, run replSetStepDown and downgrade.

3

Downgrade the config servers.

Downgrade all 3 mongod config server instances, leaving the first system in the mongos --configdb argument to downgrade last.

4

Downgrade the mongos instances.

Downgrade and restart each mongos, one at a time. The downgrade process is a binary drop-in replacement.

5

Re-enable the balancer.

Once the upgrade of sharded cluster components is complete, re-enable the balancer.

General Downgrade Procedure

Except as described on this page, moving between 2.6 and 3.0 is a drop-in replacement:

1

Stop the existing mongod instance.

For example, on Linux, run 3.0 mongod with the --shutdown option as follows:

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mongod --dbpath /var/mongod/data --shutdown

Replace /var/mongod/data with your MongoDB dbPath. See also the Stop mongod Processes for alternate methods of stopping a mongod instance.

2

Start the new mongod instance.

Ensure you start the 2.6 mongod with the same dbPath:

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mongod --dbpath /var/mongod/data

Replace /var/mongod/data with your MongoDB dbPath.

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