On a Drupal 8 website I'm administering the ratio of paged indexed by search_cron stays at a value far below 100%.
New pages are indexed, but it looks as if updated pages are not indexed again. I suspect that there is some problem that keeps nodes from being (re-)indexed. However, I see no error in the log. Search_cron takes very long - several minutes even when indexing only 20 pages per cron run.
There is a page Troubleshooting cron which contains an SQL query (under "Check for problems with modules") to find nodes that haven't been indexed so one can inspect them for bad content. Unfortunately, this works only with Drupal 7, and I couldn't find a similar page for Drupal 8.
How can I find non-indexed nodes on Drupal 8?
Any other idea on how to track down the problem?
Update: I now found a way to log the nodes being indexed by adding a call to the logger to NodeSearch.php. Which admittedly is a hack.
I can now see that Drupal tries to index the same nodes again and again, without success and without error messages. However, I haven't found out the reason yet.