On 7.5.2012, at 6.22, Daniel L. Miller wrote:

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Given a mailbox with a larger number of older mails - assuming any new mails will have later dates - will the new mail storage files be identical? I'm probably not saying the right - let me try this:

1. "dsync backup mdbox" (with appropriate args) is run for a given user.
2. "dsync mirror mdbox" is done just to catch up.
3. Old mailstore moved off.
4. New mailstore moved to active location.

I'm sure there's a safer way to do the above - but I've got a low-traffic site and I can just shut down mail service altogether for a few minutes if I want during this.

Now - repeat the above four steps. Will mail files m.1 through m.(n-1) be identical to the last run? Is this a valid packing strategy prior to performing an rsync type backup - assuming no changes are being made to the archived mails between pack runs?

If you stop mail delivery and kill any imap/pop3 processes before running 2 (or alternatively run dsync mirror once more as step 5) then they should have identical mails, but the m.* files most likely won't be identical. dsync doesn't sync files, it syncs mails, and it doesn't care how the mails get stored.

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Given the above, and SIS - if the backup is performed in the same spool, i.e. original is /var/mail/domain/user, backup is /var/mail/domain/user-new - will this result in any problems with SIS? If after the pack and rename operation, the old mail tree is simply deleted - will that leave SIS files unreferenced if the user later deletes the messages from their active store?

Yes. Don't just rm -rf the old tree, use doveadm expunge to expunge all mails so they get unreferenced.

And yet another one - dsync does not APPEAR to be copying ACL's in this process.

Yeah, it doesn't currently copy ACLs or Sieve scripts.