There are 2 different synchronizations types : directories/files and file metadata. Directories/files is about synchronizing your directories tree with the album tree in the database. Metadata is about updating photos informations such as filesize, dimensions in pixels, EXIF or IPTC informations.
The first synchronization must be the directories/files one.
Synchronization process may take long (depending on your server load and the quantity of elements to manage) so it is possible to do it album by album.