document that leveled compaction only looks at top/root collection

[steveyen]

The leveled compaction policy decision only looks at the root collection when making its decision in compactMaybe(), so need to doc that up more clearly somewhere.

[mschoch]

Oh interesting, that seems to present a problem for use cases where nothing goes into the root and everything is in a child collection.

[steveyen]

Indeed -- solving that for real is a harder problem / will create a separate ticket for that.