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@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