The balance between features and performance in the block layer

Of course performance matters. Half my job is (after the pager goes off) figuring out why we can't serve 99% of our RPCs within X milliseconds. But surprisingly often, the answer turns out to be "because the client asked us to do something inherently expensive, and we're lumping it in with the cheap requests," and so we end up changing the monitoring rather than improving the hot path (i.e. we change our definition of "good performance" to exclude the expensive operations, or to give them additional time).