Maildir++ vs. Dict Storage: Which Minimizes Duplicate Writes During Delivery Retries?

Answer

Maildir++ does not provide atomic file‑creation guarantees, so a delivery retry can race with the indexer or another delivery and create a second copy of the same message. The dict interface in Dovecot is a key‑value store used for metadata such as quota, ACLs, and Sieve scripts; it is **not** a mailbox storage backend and therefore does not store message bodies. Consequently, there is no dict‑based mailbox to compare with Maildir++ for duplicate‑write prevention.

1. Maildir++ atomicity guarantees

• Message files are created with open(O_CREAT|O_EXCL) in the tmp/ directory and then renamed to new/.
• The rename step is atomic only on local filesystems that implement POSIX rename semantics (e.g., ext4, XFS). On NFS or other network filesystems the rename may not be atomic, increasing the chance of a race.
• Dovecot does not apply any internal lock or transaction that coordinates with the indexer; the indexer operates independently on the same Maildir layout.

2. Why dict storage cannot be used for message storage

• The dict API is a generic key‑value layer; Dovecot’s built‑in dict backends (sqlite, lmdb, redis, etc.) are used for quota, ACLs, Sieve, and similar metadata.
• Message bodies are always stored by one of the mailbox plugins: maildir, maildir++, mbox, sdbox, or mdbox. No plugin writes messages into a dict database.
• Therefore, any claim that dict storage provides atomic INSERT/UPSERT for messages is incorrect.

3. How to minimize duplicate writes in practice

1. Use a transactional mailbox format that stores messages in a single file with internal locking, such as sdbox or mdbox. Example configuration:

   mail_location = sdbox:~/sdbox
   # or
   mail_location = mdbox:~/mdbox
   

2. If you must stay with Maildir++, reduce overlap between delivery and indexing:
   • Ensure the indexer runs only when no delivery is active (e.g., schedule doveadm index via cron after the typical delivery window).
   • On local filesystems, enable privileged‑group fcntl locks:

     mail_privileged_group = dovecot

   • Disable unnecessary mailbox statistics that can increase I/O overlap (though there is no explicit maildirstat setting, you can turn off mail_statistics if you have enabled it).
3. Verify your active configuration:

   dovecot -n | grep -E 'mail_location|mail_plugins|mail_privileged_group'

Missing diagnostic detail that could change the recommendation: What is your Dovecot version and the underlying filesystem (local ext4/XFS vs. NFS)? Older Dovecot releases (< 2.2) lack some locking improvements, and network filesystems increase the duplicate‑write risk for Maildir++.