MongoDB从2.6升级至3.4.1后无法连接，寻求技术排查

Hey there, let's break down why you're seeing the "Connection refused" error after upgrading MongoDB from 2.6 to 3.4.1, and walk through how to fix it.

First, let's parse the clues from your logs:

The mongod process was terminated via signal 15 (a graceful shutdown signal), and it’s no longer running—this is directly why your mongo shell can’t connect (the server isn’t active). The root cause here is likely an invalid cross-version upgrade approach, a common pitfall with MongoDB.

Key Problem: You Skipped Required Intermediate Upgrades #

MongoDB does not support jumping directly from 2.6 to 3.4. You must follow the official step-by-step upgrade path:
2.6 → 3.0 → 3.2 → 3.4
Each major version introduces data format changes and compatibility checks that need to be applied incrementally. Skipping versions can corrupt your data or prevent the server from starting entirely.

Step-by-Step Fix Plan #

1. Backup Your Data First (Critical!)
   Never make changes without backing up your data to avoid permanent loss:

   cp -r /var/lib/mongodb /var/lib/mongodb_backup_$(date +%Y%m%d)
   

2. Roll Back to MongoDB 2.6

   • Uninstall the 3.4.1 version
   • Install your original 2.6 version
   • Start the 2.6 mongod service and verify you can connect to your databases normally with the mongo shell.

3. Upgrade Incrementally
   Repeat these steps for each version jump:

   • Upgrade to 3.0:
     1. Install MongoDB 3.0
     2. Start the 3.0 service
     3. Connect to the mongo shell and run compatibility checks:

        db.upgradeCheckAllDBs()
        

     4. Set the feature compatibility version to 3.0:

        db.adminCommand({ setFeatureCompatibilityVersion: "3.0" })
        

   • Upgrade to 3.2:
     Repeat the same process: install 3.2, start the service, run db.upgradeCheckAllDBs(), then set feature compatibility to 3.2.
   • Upgrade to 3.4:
     Install 3.4.1, start the service, run compatibility checks, and set feature compatibility to 3.4.

Additional Troubleshooting Checks (If Issues Persist After Correct Upgrade) #

If you still can't connect after following the proper upgrade path, check these:

• Verify mongod is running:

  ps aux | grep mongod
  # For systemd systems:
  systemctl status mongod
  

• Check Full Startup Logs:
  Your provided logs only show the shutdown sequence—look at the start of /var/log/mongodb/mongodb.log (or use journalctl -u mongod for systemd) for startup errors (e.g., permission issues, invalid config settings).
• Data Directory Permissions:
  Ensure the mongodb user has read/write access to your data directory:

  chown -R mongodb:mongodb /var/lib/mongodb
  

• Config File Compatibility:
  Some 2.6 config options were renamed in 3.4 (e.g., bind_ip → bindIp). Double-check your /etc/mongod.conf for deprecated settings that might cause startup failures.

内容的提问来源于stack exchange，提问作者cnak2