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Free extents are stored in two btrees: one sorted by block number, one by size. So if you insert a new extent between two existing extents, you can be modifying two items in the by-block-number tree. And depending on the size of those items, that can result in three items over in the -by-size tree. So that's a 5x multiplier per level. If we're shrinking the tree and adding more freed blocks, we're conceptually dirtying two blocks at each level to merge. (current *2 in the code). But if they fall under the low water mark then one of them is freed, so we can have *3 per level in this case. Signed-off-by: Chris Kirby <ckirby@versity.com>