As you saw in the previous chapter, proper indexing is the best way to keep MongoDB responsive and performant. B-tree misses refer to page faults that happen when we try to access a B-tree index. Indexes are usually used frequently, and are relatively small compared to our working set and the memory available, so they should be in the memory at all times.

If we have an increasing number of B-tree misses or ratio of B-tree hits, or if there is a decrease in the number of B-tree misses, it's a sign that our indexes have grown in size and/or are not optimally designed. B-tree misses can also be monitored via MongoDB Cloud Manager, or in the shell.