The article is a great survey of gotchas across the storage stack. However, I think what motivated Dropbox to drop so many Linux filesystems was the need for a

The article is a great survey of gotchas across the storage stack. However, I think what motivated Dropbox to drop so many Linux filesystems was the need for a certain flavor of xattrs, but specifically to detect renames, i.e. to merge create/delete events into renames. Perhaps Dropbox adds internal UIDs via xattrs to every file you add to a Dropbox folder. If you move the file around, no problem, Dropbox can detect the rename via the xattr UID. Relying on heuristics alone to do rename detection can be brittle if you don't do it right, and missing rename events means file version histories get lost, which is a terrible shock for users. Imagine you suddenly can't find last month's version of a file... So and just guessing here but probably those Linux filesystems lacked performant xattr access and were too slow for Dropbox to do hundreds of thousands of xattr lookups when scanning a folder at startup or every N minutes, or whenever inotify tells them something changed. They didn't want the complexity of heuristics, or they found a way to exploit EXT on-disk data structures for faster xattr lookups, something like that. Who knows? Perhaps the real reason is less complicated than that.