...

• is it safe to increment the number of auth processes (the 'count' parameter in the auth block (auth_default { ...} for instance) ?

Should be.

...

• my understanding is that login processes communicate with auth processes through a unix type socket (named 'default') :

  can I create a second auth block in dovecot.conf (for instance 'aut extra {...}') in addition to the existing 'auth default {...}' one ? would it create an second unix socket and thus allowing a heavier login processes load ?

  what would be the difference between that and, say, changing the 'count' parameter to '2' in the 'auth default' section ?

Increasing count allows heavier load, because there are then more processes listening for incoming connections in the same socket.

Actually increasing the count doesn't work. At least not for me on 1.0.0: Jun 14 12:28:39 mail5 dovecot: Dovecot v1.0.0 starting up Jun 14 12:28:39 mail5 dovecot: Auth process died too early - shutting down Jun 14 12:28:39 mail5 dovecot: child 19080 (auth) returned error 89 Jun 14 12:28:39 mail5 dovecot: auth(default): Socket already exists: /var/run/dovecot/auth-client

-- Kenny Dail <kend@amigo.net>