rsync is apparently extremely inefficient in a number of common cases, such as simply renaming a folder (completing synchronizing all the files in the folder again).<p>Also, some IMAP servers store message flags in the filename (when using Maildir). This means that a message that goes from unread to read, it would appear to rsync that one file was deleted and another created (as the filename on disk has changed); the &quot;newly created&quot; file (which could be many MBs in size, depending on its contents&#x2F;attachments) would then have to be completely synchronized again.<p>I didn&#x27;t realize just how inefficient it was until a few days ago when I read an article [0] comparing rsync against ZFS replication (even if you&#x27;re not familiar with ZFS, you&#x27;ll understand).<p>[0]: <a href="http:&#x2F;&#x2F;arstechnica.com&#x2F;information-technology&#x2F;2015&#x2F;12&#x2F;rsync-net-zfs-replication-to-the-cloud-is-finally-here-and-its-fast&#x2F;" rel="nofollow">http:&#x2F;&#x2F;arstechnica.com&#x2F;information-technology&#x2F;2015&#x2F;12&#x2F;rsync-...</a>