Mongodb – Error while upgrading Mongodb from 3.2 to 3.6

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I needed to upgrade mongodb from 3.2 to 3.6 in my environment. For the process i first migrated from 3.2 to 3.4 as recommended. After successful migration to 3.4, i started migration to 3.6 i am not able to start mongod. When checked log file i found error like:
IMPORTANT: UPGRADE PROBLEM: The data files need to be fully upgraded to version 3.4 before attempting an upgrade to 3.6; see http://dochub.mongodb.org/core/3.6-upgrade-fcv for more details.

MY Mongod.conf

systemLog:
destination: file
logAppend: true
path: /var/log/mongodb/mongod.log

storage:
dbPath: /var/lib/mongo
journal:
enabled: true

processManagement:
fork: true # fork and run in background
pidFilePath: /var/run/mongodb/mongod.pid # location of pidfile

net:
port: 27017
bindIp: 127.0.0.1

Best Answer

I have similar problem, I've upgraded on Ubuntu 16.04 from MongoDB 3.4 to 3.6 but I missed this important step

db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )

Then I must downgrade to 3.4 to do it and then upgrade to 3.6 again. Here is the detail steps:

1. Uninstall 3.6

Backup /etc/mongod.conf
Backup /etc/apt/sources.list.d/mongodb-org-3.6.listed (rename or move it to another folder)

sudo apt-get update
sudo apt-get remove mongodb-org-mongos mongodb-org-server mongodb-org-shell mongodb-org-tools

2. Re-install 3.4
Check folder /etc/apt/sources.list.d/ to see if this file exists or not: mongodb-org-3.4.list. If it does not exist, you can re-create by this command:

echo "deb [ arch=amd64,arm64 ] http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.4.list

then install by apt-get

sudo apt-get update
sudo apt-get install -y mongodb-org
mongod --version
sudo systemctl start mongod

In my case the command systemctl start mongod return error Failed to start mongod.service: Unit mongod.service not found I resolved by these commands:

sudo systemctl enable mongod
sudo service mongod restart
sudo service mongod status

3. Execute very important command
After downgrade to 3.4, run this

mongo
MongoDB shell version v3.4.10
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.10
> db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )
{ "featureCompatibilityVersion" : "3.4", "ok" : 1 }
> exit

4. Upgrade 3.6 again
Restore this file /etc/apt/sources.list.d/mongodb-org-3.6.listed

sudo apt-get update
sudo apt-get install mongodb-org-mongos mongodb-org-server mongodb-org-shell mongodb-org-tools

Restore /etc/mongod.conf. Now, MongoDB 3.6 started without any problem

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