I've just tested the upgrade to zimbra 5.0.13 (from 4.5.9), and there's now a possibility to separate configuration for GAL search and GAL sync.

I've looked at the bug page (bug 21247) which states that it can be used to use ldap frontends for the search and the master for the sync.

Is this because GAL sync is a mechanism that can write in the external ldap ?
If not, what is the purpose of GAL sync ?