By default, MongoDB keeps handles open for over a day, even if they aren’t used.

This configuration was chosen because of a disconnect between cache management (where pages of data are pushed out of memory when the cache is full and they aren’t accessed for a while), vs handle management where handles are marked idle regardless of whether they are taking up space in cache.

If we change WT’s handle sweep to only close handles where the trees have already been evicted from cache (i.e., __wt_btree_bytes_evictable returns 0), then it would be fine for MongoDB to run with the normal WiredTiger default (discard handles after 30 seconds being idle and not in cache).