I had problems to backup because the database search index table had reached an unacceptable size. The table was good for 5.7 GB of volume and prevented a standard backup of the site. Also if you want to move a site you do not need to carry the 5GB.
Re-indexing a site does not help because there remains a kind of revision of this index! She remains as big as before!
You can see this in the database structure.
There is a module (Search index Wipe) which allows to empty this index, but it does not work right now (9/11/18). I get a white screen. You get a button with which you can empty the database, but as I said it is not working.
You must in this case empty the databases manually.
Search_dataset, search_index search_total and must be emptied. Do not remove them! The CP will have a link to it (see above) to empty the table.
Rest then to re-index the site. This is just matter of time ...