There are lots of knobs to tune (row cache, key cache, index sample size, bloom filter buckets, network, disk, heap) Figuring out what to tune under load isn't always obvious. Turning on debug logging under load has heisenberg effect of shifting bottlenecks around. The attached patch is a simple slow query logger to log information about what queries that take longer than some threshold did that took so long. I've found it useful for pinpointing bottlenecks under load and figuring out which knobs need tweaking. This patch only instruments the get_slice command and the read path. Incrementing the other commands/paths is straightforward, and I can update the patch with the write path.