[Dovecot] Documentation for "altpath" / "altmove" / ?"Alternate storage"

[Timo Sirainen]

On Fri, 2010-09-03 at 17:09 +0100, William Blunn wrote:

> I think the only thing I had further questions on was how alternate 
> storage works with mdbox.
> 
> In http://wiki2.dovecot.org/MailboxFormat/dbox it says we have
> 
>   * dovecot.map.index* files contain the "map index"
>   * m.* files contain the mail data
> 
> I think I am not clear on the "dovecot.map.index* files". Is that really 
> just one file "dovecot.map.index", and the globby asterisk just 
> indicates that there may be may be old versions if the file has been 
> updated by being re-written?

It's the same as dovecot.index files that you see for mailboxes. There
are:

 - dovecot.index updated once in a while (doesn't exist initially)
 - dovecot.index.log always, updated always first
 - dovecot.index.log.2 - the .log is rotated to this for a while

> "dovecot.map.index*" is always stored in the primary storage?

Yes.

> So if we are moving mail to alternate storage, we write them into a new 
> "m.*" file in the alternate storage folder containing the messages we 
> want to move. 

Either a new m.* file or existing m.* file in there.

> So that would mean it only really makes sense for any 
> given numbered "m.*" file to exist in either the primary storage or the 
> alternate storage but not both?

Right, it's a bug if it exists in both.

> So when Dovecot is wanting to find the next unused "m.*" file number, it 
> needs to consider files in both the primary storage and alternate storage?

Yes, but it looks up the next number from index file, not by scanning
what files exist.