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## Problem When pageservers do compaction, they frequently create image layers that make earlier layers un-needed for reads, but then keep those earlier layers around for 24 hours waiting for time-based eviction to expire them. Now that we track layer visibility, we can use it as an input to eviction, and avoid the 24 hour "disk bump" that happens around pageserver restarts. ## Summary of changes - During time-based eviction, if a layer is marked Covered, use the eviction period as the threshold: i.e. these layers get to remain resident for at least one iteration of the eviction loop, but then get evicted. With current settings this means they get evicted after 1h instead of 24h. - During disk usage eviction, prioritized evicting covered layers above all other layers. Caveats: - Using the period as the threshold for time based eviction in this case is a bit of a hack, but it avoids adding yet another configuration property, and in any case the value of a new property would be somewhat arbitrary: there's no "right" length of time to keep covered layers around just in case. - We had previously planned on removing time-based eviction: this change would motivate us to keep it around, but we can still simplify the code later to just do the eviction of covered layers, rather than applying a TTL policy to all layers.