[compurandom]

Playing with this for months. I'm starting to think the real issue is just the sqlite database itself being slow. Slow can be caused by too many entries in the table, or it can be caused by too much churn of books being added and deleted, leaving the database fragmented.

I've noticed that when sync repeatedly fails and I do a soft database reset (by deleting it and letting all my sideloaded books rescan and then restoring bookmarks and categories with calibre), the calibre metadata sync itself gets much faster and also kobo sync stops failing.

I have to wonder if deleting large categories works because the database is smaller, or if it removes a bunch of fragments, or perhaps both. If it is database access time rather than database size threshold, this might explain some of the seeming randomness of the issue.

So next time I see a sync fail, I'm going to try compressing the database and see if that fixes it and see if it also shortens the calibre metadata sync too.