Dovecot doveadm mailbox copy during live traffic – missing behavior on concurrent updates

Answer

doveadm mailbox copy does not capture messages that arrive after the command starts. It takes a snapshot of the mailbox at the moment the command begins and copies only those existing messages. Any new messages delivered while the copy is running are ignored by that invocation and will appear only after a subsequent incremental copy or a synchronization step.

Confirmed facts

• The command obtains no global lock on the source mailbox; it reads the mailbox index and message data directly.
• Dovecot documentation (v2.3+) states that doveadm mailbox copy copies the current set of messages and does not wait for or include concurrent deliveries.

Likely explanation

Because the copy works from a static snapshot, the destination mailbox will be identical to the source as it existed at the start of the command. Updates that occur during the copy are not transferred automatically; they require a follow‑up synchronization.

Steps to achieve a loss‑free live migration

1. Initial copy: Run doveadm mailbox copy -u user source_mailbox dest_mailbox for each mailbox. This transfers all messages present at the start.
2. Catch‑up sync: Immediately after the copy, execute a one‑way dsync mirror: dsync -v -u user mirror. This will transfer any messages that arrived during the copy window and bring the destination up to date.
3. Verification:
   • Compare message counts: doveadm mailbox status -u user mailbox messages on source and destination; the numbers should match.
   • Check the highest UID: doveadm fetch -u user mailbox mailbox uid (or doveadm search -u user mailbox ALL) on both sides; the highest UID must be identical.
   • Run a verification dsync: dsync -v -u user mirror. If it reports “no changes”, the destination is fully synchronized.

Missing diagnostic detail

If the source and destination share the same user namespace, confirm that the UIDVALIDITY value for each mailbox is identical on both sides. A mismatch would cause the destination to treat existing messages as new after the copy, leading to duplicates. Please verify UIDVALIDITY equality before proceeding; if they differ, you must reset the destination mailbox (e.g., doveadm mailbox exec -u user mailbox mailbox uidvalidity set value) or use a full re‑sync.