fast sync vs. full sync

[Timo Sirainen]

On 05 Mar 2015, at 12:46, Patrick Westenberg <pw at wk-serv.de> wrote:
> 
> Hi everyone,
> 
> can anybody explain the difference between Dovecots fast sync and full sync in replication mode?

There are actually 3 dsync modes, all explained in doveadm-sync man page:

              o      Full  synchronization  (-f  parameter) scans through all the
                     messages in all the mailboxes.  This guarantees that  every-
                     thing  will be synchronized, but it's unnecessarily slow for
                     incremental synchronization.

              o      Fast synchronization (default) first attempts to find  mail-
                     boxes  that  have changed, and synchronize only those.  This
                     is done by checking the  mailboxes'  metadata  (NEXTUID  and
                     HIGHESTMODSEQ).   Usually  this  works fine, especially with
                     one-way synchronization, but if both sides  do  exactly  the
                     same  number  of changes, the metadata may end up containing
                     the same values even if the changes were different.

              o      Stateful synchronization (-s parameter) is  the  most  effi-
                     cient way to synchronize mailboxes.  It relies on having the
                     earlier dsync run's state saved somewhere and  being  passed
                     to  the  next dsync run.  Based on this state dsync can send
                     only the changes that happened after the previous dsync run.
                     As  long as the state or the mailboxes aren't corrupted this
                     algorithm should work  perfectly.   The  replicator  process
                     uses this internally to perform most of the synchronization.