[Dovecot] dsync backup questions

[Timo Sirainen]

On Mon, 2013-02-04 at 00:57 +0000, Ben Morrow wrote:
> I can't give authoratitive answers to either of these, but...
> 
> At  6PM -0500 on  3/02/13 you (Joe Beaubien) wrote:
> > 
> > I'm currently trying to setup remote backups of my emails but i'm running
> > into issues (mdbox format, indexes and storage in the same folder
> > hierarchy).
> > 
> > Local backup command: dsync -u "my_user" backup /backups/my_user
> > 
> > (1) Recently, I noticed that the local backup takes up twice the size as
> > the original mail location (8gb vs. 4gb). I purged alot of emails from the
> > original location, so the size shrunk, but the local backup just keeps on
> > getting bigger. I couldn't find any dsync option that would delete extra
> > emails.
> > 
> > - Question: Why isn't the local backup synced properly and remove the extra
> > emails?
> 
> Are you running 'doveadm purge' on the backed-up dbox? It looks to me as
> though dsync doesn't do that. I don't know if there's any (simple) way
> to do that without a running Dovecot instance attached the dbox
> directory: it's not entirely clear to me whether doveadm will run
> locally without contacting a doveadm-server instance running under
> Dovecot, nor how to point 'doveadm purge' at an arbitrary directory.

Right. doveadm -o mail=mdbox:/backups/my_user purge

> It might be easier to dsync to a Maildir instead. This should preserve
> all the Dovecot-visible metadata, and dsyncing back to the dbox for
> restore should put it all back.

Better sdbox than maildir.

> > (2) What is the best why to copy this local backup to a remote location
> > that does NOT have the possibility to run dsync.
> > 
> > - Question 1: is rsync safe to use and will this data work for restore?
> > 
> > - Question 2: Would it be safe to simply rsync the original mail_location
> > to the remote server?
> 
> AFAICT, if Dovecot is stopped on both sides of the transfer it should be
> safe. If either side has a currently running Dovecot instance (or any
> other Dovecot tools, like a concurrent dsync) using that dbox, it's
> likely rsync will copy an inconsistent snapshot of the data, and the
> result will be corrupted.

It won't be badly corrupted though. At worst Dovecot will rebuild the
indexes, which takes a while. And most users probably won't get any
corruption at all.