Choosing Dovecot’s Mail Storage: Maildir vs. mdbox/sdbox for High‑Volume Servers

Why the storage format matters

In Dovecot, the mail_location setting tells the server where to find each mailbox. The format you choose determines how often the server touches the file system, how many inodes you consume, and how resilient the data is to crashes. For a server that receives and stores thousands of messages per day, these differences can become the difference between smooth operation and a hard‑to‑debug outage.

Three options in a nutshell

• Maildir – one file per message, stored in a directory tree. Very reliable, but can exhaust inodes on filesystems with a low inode limit.
• mdbox – a binary bundle that packs many messages into a single file. Reduces inode usage and can improve read/write throughput, but a single corrupt file can affect many messages.
• sdbox – a hybrid that keeps larger messages in separate files while still packing small ones into a bundle. A compromise between the two extremes.

When to pick each format

Below is a quick decision matrix. The numbers are illustrative; your real thresholds will depend on your filesystem and workload.

Metric	Maildir	mdbox	sdbox
Maximum messages per mailbox	Unlimited (inode‑limited)	~10 000 per file (configurable)	Same as mdbox, but large messages stay separate
Crash resilience	High – atomic file creation	Low – one corrupt file can damage many messages	Medium – large messages safe, small ones bundled
Write throughput	Moderate – many small file ops	High – single file writes	High – most writes go to bundle
Read latency	Low for single messages (no index)	Very low – index jumps directly to message	Very low – same as mdbox for most messages
Inode usage	High – one per message	Low – one per mailbox	Low – one per mailbox + a few large files
Migration effort	None – native format	Requires doveadm convert and temporary space	Same as mdbox

Practical conversion example

Assume you have a mailbox at /var/mail/vhosts/example.com/alice using Maildir, and you want to switch to mdbox for performance. Here’s a step‑by‑step guide you can run on a Linux host with Dovecot 3.4+.

1. Verify current format

   # grep mail_location /etc/dovecot/dovecot.conf
   mail_location = maildir:/var/mail/vhosts/%d/%n
   

   Make sure the path points to a Maildir structure (ends with maildir:).

2. Back up the mailbox

   # tar -czf /tmp/alice-backup.tgz /var/mail/vhosts/example.com/alice
   

   Store the archive outside the mailbox directory to avoid accidental truncation.

3. Stop Dovecot (or use doveadm shutdown for a graceful stop)

   # systemctl stop dovecot
   

4. Configure the new format

   # sed -i 's/maildir:/mdbox:/g' /etc/dovecot/dovecot.conf
   # systemctl start dovecot
   

   Now Dovecot will create an mdbox file in the same directory.

5. Convert the mailbox

   # doveadm -u alice convert mdbox:/var/mail/vhosts/example.com/alice
   

   This command copies messages from the Maildir into a new mdbox file, preserving flags and timestamps. The operation can take minutes on a large mailbox; monitor doveadm log for progress.

6. Verify the conversion

   # doveadm mailbox list
   # doveadm -u alice fetch 1:1
   

   Check that the mailbox shows up as mdbox and that you can fetch a message without errors.

7. Cleanup

   # rm -r /var/mail/vhosts/example.com/alice/*maildir*
   # rm -rf /var/mail/vhosts/example.com/alice/.maildir*
   

   Remove the old Maildir directory to free space.

During the conversion you should monitor disk usage with df -h and inode counts with df -i. If the host runs out of inodes, the conversion will fail; you can pause, free space, and resume.

Limitations and risks

• Hard crashes – if the server or filesystem crashes while writing to an mdbox file, the entire bundle can become unreadable. Use journaling filesystems (ext4 with journaling, XFS) and ensure regular backups.
• Concurrency – a single mailbox file can become a bottleneck under extreme write loads. For a multi‑core server with many simultaneous senders, consider splitting the mailbox into multiple mdbox files or using sdbox to keep large attachments separate.
• Conversion cost – the doveadm convert step requires disk space roughly equal to the size of the mailbox, because it writes a new file while the old one remains. Plan for that when converting large mailboxes.
• Index corruption – the internal index files (index, index.1, etc.) can become corrupted if the filesystem is damaged. In such cases, run doveadm index -f to rebuild.

Actionable next steps

1. Audit inode usage on your current server:

# df -i /var/mail

If you’re close to the inode limit, consider moving to mdbox or sdbox.

2. Benchmark a small test mailbox in each format. Use doveadm fetch or a real IMAP client to measure read/write latency under simulated load.

3. Plan a staged migration. Convert a subset of mailboxes during off‑peak hours, verify performance, then roll out to the rest.

4. Set up monitoring for mailbox size and index health. Dovecot’s doveadm monitor can alert you if a mailbox becomes unreadable.

Choosing the right storage format is a balancing act. Maildir offers the simplest crash safety, mdbox maximizes throughput and reduces inode pressure, and sdbox gives you the best of both worlds for large attachments. By following the steps above, you can evaluate each option in your environment, convert safely, and keep your mail server running smoothly even as traffic grows.