Step 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. 

mongod --dbpath <newMMAPv1DBPath> 

Specify additional options as appropriate. 

## Step 5: Upload the exported data using mongorestore . 

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 2.6.5 . 

Step 1: Download 2.6 binaries. Download binaries of the latest release in the 2.6 series from the MongoDB Down- load Page$^{281}$. See Install MongoDB (page 5) for more information. 

Step 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 (page 633): 

Step 1: Shutdown the secondary member. Stop the mongod instance for the secondary member. 

Step 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. 

Step 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. 

mongod --dbpath <preparedMMAPv1DBPath> 

