> Last year, we rolled out a new service that changed how data is placed across Magic Pocket. The change reduced write amplification for background writes, so each write triggered fewer backend storage operations. But it also had an unintended side effect: fragmentation increased, pushing storage overhead higher. Most of that growth came from a small number of severely under-filled volumes that consumed a disproporti…
Data in these systems moves slowly and with a lot of inertia, so the effects show up gradually and can lag behind the change itself. On top of that, the impact wasn’t uniform. Most of the overhead came from a small subset of volumes, so it took time to isolate what was actually driving the increase. These systems are hard to test at scale!
