Quota & prefetch uiserDB questions

[Cédric ML]

Hello,

I'm trying to make quota work on my dovecot server.
I'm using prefetch userdb (source : 
http://wiki2.dovecot.org/UserDatabase/Prefetch) with a database located 
on a remote host :

passdb {
   driver = sql
   args = /etc/dovecot/dovecot-sql.conf.ext
}

userdb {
    driver = prefetch
}

userdb {
   driver = sql
   args = /etc/dovecot/dovecot-sql.conf.ext
}


With password_query containing (source : 
http://wiki2.dovecot.org/Quota/Configuration) :
password_query = SELECT \
         username AS user, \
         password, \
         homedir AS userdb_home, \
         maildir AS userdb_mail, \
         uid AS userdb_uid, \
         gid AS userdb_gid, \
         CONCAT('*:bytes=', quota) AS userdb_quota_rule \
     FROM mailbox \
     WHERE username = '%u'



When I change the quota value in DB, it's not reflected to maildirsize 
file of the user.
If I delete maildirsize file, it's re-created but not with the quota 
value which is set in the DB.

Questions : how is this maildirsize file created ? how is it updated ?
is there a way to make maildir++ quota work with dovecot using prefetch 
userDB ? Or do I have to use dict quotas ?

Many thanks for your help.
Regards,
Cédric