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WiredTiger Storage Engine

Change Replica Set to WiredTiger

Note

Starting in MongoDB 4.2

  • The noPadding and usePowerOf2Sizes MMAPv1 options for the collMod command are removed. Do not use those options because upgrading from MongoDB 4.0 to 4.2 causes the 4.2 secondary members to immediately halt.

  • MongoDB removes the deprecated MMAPv1 storage engine. If you're upgrading to MongoDB 4.2 from a MongoDB 4.0 deployment that uses MMAPv1, you must upgrade to WiredTiger.

Use this tutorial to update a replica set to use WiredTiger. The procedure updates the replica set in a rolling fashion to avoid downtime.

Considerations

Replica sets can have members with different storage engines. As such, you can update members to use the WiredTiger storage engine in a rolling fashion.

PSA 3-member Architecture

The "majority" read concern, available for WiredTiger, is enabled by default. However, in three-member replica sets with a primary-secondary-arbiter (PSA) architecture, you can disable the "majority" read concern. Disabling the "majority" read concern for a three-member PSA architecture avoids possible cache-pressure build up.

The procedure below disables "majority" read concern for PSA architecture by including --enableMajorityReadConcern false.

Note

Disabling "majority" read concern has no effect on change streams availability.

For more information on PSA architecture and read concern "majority", see Primary-Secondary-Arbiter Replica Sets.

Default Bind to Localhost

MongoDB binaries, mongod and mongos, bind to localhost by default.

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

The following procedure updates the replica set in a rolling fashion. The procedure updates the secondary members first, then steps down the primary, and updates the stepped-down member.

To update a member to WiredTiger, the procedure removes a member's data, starts mongod with WiredTiger, and performs an initial sync.

A. Update the secondary members to WiredTiger.

Update the secondary members one at a time:

1

Shut down the secondary member.

In mongosh, shut down the secondary.

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.

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, include --enableMajorityReadConcern false to disable read concern majority. See PSA 3-member Architecture.

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.

Important

If updating all members of the replica set to use WiredTiger, ensure that all secondary members have been updated first before updating the primary.

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

rs.stepDown()

C. Update the stepped down 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 mongosh, shut down the secondary.

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.

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, include --enableMajorityReadConcern false to disable read concern majority. See PSA 3-member Architecture.

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.

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Change Standalone to WiredTiger