You do need access to an index/DB of all files in a bucket in order to delete them in parallel. Otherwise you're stuck paginating with the B2 API.
There are lots of problem spaces where deletion is expensive and so is time shifted not to align with peak system load. Some sort of reaper goes around tidying up as it can.
But I think by far my favorite variant is amortizing deletes across creates. Every call to create a new record pays the cost of deleting N records (if N are available). This keeps you from exhausting your resource, but also keeps read operations fast. And the average and minimum create time is more representative of the actual costs incurred.
Variants of this show up in real-time systems.
IMO an "Empty" button should have been implemented by Backblaze.
A single pass: paginating through all entries in the bucket without deletion, just to build up your index of files. And then using that index to delete objects in parallel.
> no way to empty a bucket.
Backblaze currently recommends you do this by writing a “Lifecycle rule” to hide/delete all files in the bucket, then let Backblaze empty the bucket for you on the server side in 24 hours: https://www.backblaze.com/b2/docs/lifecycle_rules.html