> I am trying to better understand how the cleaner kthread
> (btrfs-cleaner) impacts foreground performance, specifically
> during snapshot deletion.  My experience so far has been that
> it can be dramatically disruptive to foreground I/O.

That's such a warmly innocent and optimistic question! This post
gives the answer, and to an even more general question:


> the response tends to be "use less snapshots," or "disable
> quotas," both of which strike me as intellectually
> unsatisfying answers, especially the former in a filesystem
> where snapshots are supposed to be "first-class citizens."

They are "first class" but not "cost-free".
In particular every extent is linked in a forward map and a
reverse map, and deleting a snapshot involves materializing and
updating a join of the two, which seems to be done with a
classic nested-loop join strategy resulting in N^2 running
time. I suspect that quotas have a similar optimization.
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