Quota: read how much diskspace is available?

Hi,

I've been trying to get domain quotas to work for a while, but to no avail so far.

Is there a way to let Dovecot simply check how much space is still available in a directory? I'm using GlusterFS as my mailstore, so I can configure quotas per directory. All I need is a way to let Dovecot check quota in a way like "df /var/vmail/<domain>".

Kind regards,

Hans