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Change Sharded Cluster to WiredTiger

Note

Starting in version 4.2, MongoDB removes the deprecated MMAPv1 storage engine. Before upgrading to MongoDB 4.2 from a MongoDB 4.0 deployment that uses MMAPv1, you must first upgrade to WiredTiger.

Use this tutorial to update MongoDB 4.0 sharded clusters to use WiredTiger.

For earlier versions of MongoDB:

• To convert a 3.6 sharded cluster that uses MMAPv1, see the MongoDB 3.6 manual.
• To convert a 3.4 sharded cluster that uses MMAPv1, see the MongoDB 3.4 manual.
• To convert a 3.2 sharded cluster that uses MMAPv1, see the MongoDB 3.2 manual.

Considerations

Downtime

If you change the host or port of any shard, you must update the shard configuration as well.

PSA 3-member Architecture

Starting in MongoDB 3.6, "majority" read concern, available for WiredTiger, is enabled by default. However, for MongoDB 4.0.3+, if you have a three-member shard replica set with a primary-secondary-arbiter (PSA) architecture, you can disable "majority" read concern for that shard replica set. Disabling "majority" for a three member PSA architecture avoids possible cache-pressure build up.

The procedure below disables "majority" read concern for MongoDB 4.0.3 PSA architecture by including --enableMajorityReadConcern false. If you are running a MongoDB 4.0.1 or 4.0.2 PSA architecture, first upgrade to the latest 4.0 version in order to disable this read concern.

Note

Disabling "majority" read concern disables support for Change Streams for MongoDB 4.0 and earlier. For MongoDB 4.2+, disabling read concern "majority" has no effect on change streams availability.

Disabling "majority" read concern prevents collMod commands which modify an index from rolling back. If such an operation needs to be rolled back, you must resync the affected nodes with the primary node.

Disabling "majority" read concern affects support for transactions on sharded clusters. Specifically:

• A transaction cannot use read concern "snapshot" if the transaction involves a shard that has disabled read concern “majority”.
• A transaction that writes to multiple shards errors if any of the transaction’s read or write operations involves a shard that has disabled read concern "majority".

However, it does not affect transactions on replica sets. For transactions on replica sets, you can specify read concern "majority" (or "snapshot" or "local" ) for multi-document transactions even if read concern "majority" is disabled.

For more information on PSA architecture and read concern "majority", see Disable Read Concern Majority.

MongoDB 3.0 or Greater

You must be using MongoDB version 3.0 or greater in order to use the WiredTiger storage engine. If using an earlier MongoDB version, you must upgrade your MongoDB version before proceeding to change your storage engine. To upgrade your MongoDB version, refer to the appropriate version of the manual.

Default Bind to Localhost

Starting with MongoDB 3.6, MongoDB binaries, mongod and mongos, bind to localhost by default. From MongoDB versions 2.6 to 3.4, only the binaries from the official MongoDB RPM (Red Hat, CentOS, Fedora Linux, and derivatives) and DEB (Debian, Ubuntu, and derivatives) packages would bind to localhost by default. To learn more about this change, see Localhost Binding Compatibility Changes.

Config Servers

Starting in version 3.4, config servers must be deployed as replica sets (CSRS). As such, version 3.4+ config servers already use the WiredTiger storage engine.

XFS and WiredTiger

With the WiredTiger storage engine, using XFS for data bearing nodes is recommended on Linux. For more information, see Kernel and File Systems.

MMAPv1 Only Restrictions

Once upgraded to WiredTiger, your WiredTiger deployment is not subject to the following MMAPv1-only restrictions:

MMAPv1 Restrictions	Short Description
Number of Namespaces	For MMAPv1, the number of namespaces is limited to the size of the namespace file divided by 628.
Size of Namespace File	For MMAPv1, namespace files can be no larger than 2047 megabytes.
Database Size	The MMAPv1 storage engine limits each database to no more than 16000 data files.
Data Size	For MMAPv1, a single mongod instance cannot manage a data set that exceeds maximum virtual memory address space provided by the underlying operating system.
Number of Collections in a Database	For the MMAPv1 storage engine, the maximum number of collections in a database is a function of the size of the namespace file and the number of indexes of collections in the database.

Procedure

For each replica set shard, to change the storage engine to WiredTiger:

A. Update the secondary members to WiredTiger.

Update the secondary members one at a time:

1

Shut down the secondary member.

In the mongo shell, shut down the secondary.

copy

use admin
db.shutdownServer()

2

Prepare a data directory for the new mongod running with WiredTiger.

Prepare a data directory for the new mongod instance that will run with the WiredTiger storage engine. mongod must have read and write permissions for this directory. You can either delete the contents of the stopped secondary member’s current data directory or create a new directory entirely.

mongod with WiredTiger will not start with data files created with a different storage engine.

3

Update configuration for WiredTiger.

Remove any MMAPv1 Specific Configuration Options from the mongod instance configuration.

4

Start mongod with WiredTiger.

Start mongod, specifying wiredTiger as the --storageEngine and the prepared data directory for WiredTiger as the --dbpath.

Specify additional options as appropriate, such as --bind_ip.

Warning

Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

• General Use (For Most Architectures)
• PSA Architecture

copy

mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)>

Important

If you are running a three-member PSA architecture and MongoDB version 4.0.3+, include --enableMajorityReadConcern false to disable read concern majority. See PSA 3-member Architecture.

copy

mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)> --enableMajorityReadConcern false

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.

You can also specify the options in a configuration file. To specify the storage engine, use the storage.engine setting.

Repeat the steps for the remaining secondary members, updating them one at a time.

B. Step down the primary.

Once all the secondary members have been upgraded to WiredTiger, connect a mongo shell to the primary and use rs.stepDown() to step down the primary and force an election of a new primary.

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rs.stepDown()

C. Update the old primary.

When the primary has stepped down and become a secondary, update the secondary to use WiredTiger as before:

1

Shut down the secondary member.

In the mongo shell, shut down the secondary.

copy

use admin
db.shutdownServer()

2

Prepare a data directory for the new mongod running with WiredTiger.

Prepare a data directory for the new mongod instance that will run with the WiredTiger storage engine. mongod must have read and write permissions for this directory. You can either delete the contents of the stopped secondary member’s current data directory or create a new directory entirely.

mongod with WiredTiger will not start with data files created with a different storage engine.

3

Update configuration for WiredTiger.

Remove any MMAPv1 Specific Configuration Options from the mongod instance configuration.

4

Start mongod with WiredTiger.

Start mongod, specifying wiredTiger as the --storageEngine and the prepared data directory for WiredTiger as the --dbpath.

Specify additional options as appropriate, such as --bind_ip.

Warning

Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

• General Use (For Most Architectures)
• PSA Architecture

copy

mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)>

Important

If you are running a three-member PSA architecture and MongoDB version 4.0.3+, include --enableMajorityReadConcern false to disable read concern majority. See PSA 3-member Architecture.

copy

mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName> --bind_ip localhost,<hostname(s)|ip address(es)> --enableMajorityReadConcern false

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.

You can also specify the options in a configuration file. To specify the storage engine, use the storage.engine setting.

Repeat the procedure for the other shards.

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