error when downgrading mongodb 3.4 to 2.6.11

Question

I was trying to downgrade my local mongodb version from 3.4 to 2.6.11 in ubuntu 14.04, I'm getting the following error when I'm trying to start mongodb:

2017-01-27T10:24:16.190+0600 [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /var/lib/mongodb created by storage engine 'wiredTiger'. The configured storage engine is 'mmapv1'., terminating

Here is the procedure I've followed

remove existing mongodb version

sudo apt-get --purge remove mongodb-org mongodb-org-*
sudo apt-get --purge autoremove

install 2.6.11 version

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
sudo apt-get install -y mongodb-org
sudo apt-get install -y --force-yes mongodb-org=2.6.11 mongodb-org-server=2.6.11 mongodb-org-shell=2.6.11 mongodb-org-mongos=2.6.11 mongodb-org-tools=2.6.11

pin version

echo "mongodb-org hold" | sudo dpkg --set-selections
echo "mongodb-org-server hold" | sudo dpkg --set-selections
echo "mongodb-org-shell hold" | sudo dpkg --set-selections
echo "mongodb-org-mongos hold" | sudo dpkg --set-selections
echo "mongodb-org-tools hold" | sudo dpkg --set-selections

start mongodb

sudo service mongod start

debugging

cat /var/log/mongodb/mongod.log

2017-01-27T10:24:16.190+0600 [initandlisten] exception in initAndListen: 28574 Cannot start server. Detected data files in /var/lib/mongodb created by storage engine 'wiredTiger'. The configured storage engine is 'mmapv1'., terminating

screenshot

2.6 does not support WirdTiger storage engine, If you want it to work you need to export your data first using mongodump and then restore it into the old version — Sammaye

ares ares · Accepted Answer · 2017-01-27T08:15:50

The default storage engine in 3.4 is wiredTiger whereas it's mmapv1 in 2.6. Hence data from of 3.4 is not compatible with 2.6.

If your data is not important, just delete the content of data directory and you'll be fine:

rm -rf /var/lib/mongodb/*

If you need the data, reinstall 3.4, backup the data using mongodump, downgrade to 2.6 and restore the data using mongorestore.

Since 3.4 supports additional data types, you may run into some compatibility issues.

error when downgrading mongodb 3.4 to 2.6.11

1 Answers