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- Change Replica Set to WiredTiger
Change Replica Set to WiredTiger¶
Note
Starting in version 4.2, MongoDB removes the deprecated MMAPv1 storage engine. If 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¶
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 replica set with a
primary-secondary-arbiter (PSA) architecture, you can disable
"majority" read concern. 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.
For more information on PSA architecture and read concern
"majority", see Disable Read Concern Majority.
MongoDB 3.0 or Greater¶
Default Bind to Localhost¶
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.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:
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.
Update configuration for WiredTiger.¶
Remove any MMAPv1 Specific Configuration Options from the mongod
instance configuration.
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
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.
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 a mongo shell to the primary and use
rs.stepDown() to step down the primary and force an election
of a new primary.
C. Update the stepped down primary.¶
When the primary has stepped down and become a secondary, update the secondary to use WiredTiger as before:
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.
Update configuration for WiredTiger.¶
Remove any MMAPv1 Specific Configuration Options from the mongod
instance configuration.
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
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.
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.