Dovecot mdbox Migration Guide for High-Volume Mailboxes

Migrate from Maildir to Dovecot's mdbox storage to solve inode exhaustion in high-volume mailbox setups. Follow these steps for configuration, verification, and rollback.

Problem: Inode Exhaustion with Traditional Maildir

When managing thousands of user mailboxes, the traditional Maildir format creates one file per message. This quickly exhausts inodes on filesystems, especially with small messages. Dovecot's mdbox storage format solves this by storing multiple messages in a single file, dramatically reducing inode usage.

Prerequisites

Dovecot 2.3+ installed and configured
Root or sudo access to the server
Backup of existing mail data before migration
Available disk space equal to current mail storage size

Configuration Steps

Stop Dovecot service to prevent mail delivery during migration:
```
sudo systemctl stop dovecot
```
Update dovecot.conf to use mdbox format. Edit the configuration file:
```
mail_location = mdbox:~/mail
```
Replace ~/mail with your actual mail storage path if different.

Create the mail directory structure with correct permissions:

sudo mkdir -p /var/mail/vhosts
sudo chown vmail:vmail /var/mail/vhosts

Run the migration tool for each user. Dovecot provides doveadm for this:
```
sudo doveadm migrate -u username mdbox:~/mail
```
Replace username with actual usernames or use a loop for bulk migration.
Restart Dovecot to apply changes:
```
sudo systemctl start dovecot
```

Verification Checks

After migration, verify the setup is working correctly:

Check storage format by examining the mail directory:
```
ls -la ~/mail/
```
Look for maildirfolder or mdbox files instead of individual message files.
Verify mailboxes are accessible:
```
doveadm mailbox list -u username
```
This should list all mailboxes without errors.
Test email delivery by sending a test message and confirming it arrives correctly in your mail client.

Important Considerations

Backup Strategy: Since mdbox aggregates multiple users' mail into fewer files, traditional per-user backups may need adjustment. Consider backing up the entire mail directory or individual mdbox files per user.

Recovery Implications: Filesystem corruption affecting an mdbox file could impact multiple users. Ensure you have recent backups before migration.

Performance Monitoring: Monitor inode usage before and after migration to confirm the expected reduction. Use df -i to check inode consumption.

Rollback Procedure

If migration fails or causes issues, rollback is possible but requires careful steps:

Stop Dovecot again:
```
sudo systemctl stop dovecot
```
Revert mail_location in dovecot.conf to the original Maildir or mbox format:
```
mail_location = maildir:~/Maildir
```
Restore from backup if the migration tool corrupted any data
Restart Dovecot and verify service is operational

Migrating to Dovecot's mdbox Storage for High-Volume Mailboxes