This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

Dictionary quota

The dictionary quota backend supports both storage and messages quota limits. The current quota is kept in a dictionary. Currently the only supported dictionary backend is MySQL.

The plugin parameter format is:

# v1.0:
quota = dict:<quota limits> <dictionary URI>
# v1.1:
quota = dict:<quota root name>:<user name>:<dictionary URI>

If user name is left empty, the logged in username is used (this is probably what you want).

For example:

plugin {
  # v1.0: 10MB and 1000 messages quota limit
  quota = dict:storage=10240:messages=1000 mysql:/etc/dovecot-dict-quota.conf
  # v1.1:
  quota = dict:::mysql:/etc/dovecot-dict-quota.conf
  quota_rule = *:storage=10M:messages=1000

However, the above example won't really work. This is because it would require linking all the binaries with MySQL library, which I didn't really want to do. Currently you'll have to do this via the dictionary proxy (see below). Actually the performance is better that way anyway, and I don't really see a reason not to use it.

Example dovecot-dict-quota.conf:

connect = host=localhost dbname=mails user=sqluser password=sqlpass
table = quota
select_field = current
where_field = path
username_field = username 

Create the table like this:

create table quota (
  username varchar(255) not null,
  path varchar(100) not null,
  current integer,
  primary key (username, path)

v1.0 Inaccuracy problems

With Dovecot v1.1 quota is tracked accurately. With v1.0 you may have a problem:

If two IMAP clients do an expunge at the same time, the quota is reduced twice as much. Maildir++ backend also has the same problem, but it's not that big of a problem with it because it recalculates the quota once in a while anyway. Dict quota is recalculated only if the quota goes below zero (v1.0.rc30+ only).

So either you'll have to trust your users not to abuse this problem, or you could create a nightly cronjob to delete all rows from the SQL quota table to force a daily recalculation. The recalculation will of course slow down the server.

Dictionary proxy server

To avoid each process making a new SQL connection, you can make all dictionary communications go through a dictionary server process which keeps the connections permanently open.

The dictionary server is referenced with URI proxy:<dictionary server socket path>:<dictionary name>. The socket path may be left empty if you haven't changed base_dir setting in dovecot.conf. Otherwise set it to <base_dir>/dict-server. The dictionary names are configured in dovecot.conf. For example:

dict {
  quotadict = mysql:/etc/dovecot-dict-quota.conf

Example quota plugin configuration for this:

plugin {
  # v1.0:
  quota = dict:storage=10240:messages=1000 proxy::quotadict
  # v1.1:
  quota = dict:::proxy::quotadict
  quota_rule = *:storage=10M:messages=1000