dovecot - Feb 2017 - doveadm backup and quota

Hi,

I'm looking to convert all mailboxes from Maildir format to mdbox.

When I try my procedure on a user having ~45GB of messages, its quota is 
growing very fast and is now up to 66GB.

I run this command :

doveadm -v backup -u user at domain.com 
mdbox:/mnt/maildata1/domain.com/user/mdbox

I expected this command to make a copy of user mailbox, it does not make 
sense to count the backup in its quota.

Maybe quota is simply recalculated  ? But running a "du -sh" on its
home
directory is showing a 46G disk usage ! (backup is not stored in home 
dir)

Can someone tell me what happens on my setup ?

I use Dovecot 2.2.24.

Thank you.

Florent

Sorry I didn't search enough before posting.
I found this : 
http://www.dovecot.org/list/dovecot/2012-February/063585.html
I will try the workaround.