Design Disk-Based Inventory Serving

Design software for a machine that has no SSD storage and only a small amount of RAM. The machine must service inventory requests using spinning disks.

Assume the inventory system stores records keyed by item ID, with values such as available quantity, reserved quantity, and metadata. The system must support reads, updates, and conditional reservation operations such as reserve one unit only if available quantity is greater than zero.

Discuss the data layout, indexing strategy, caching strategy, write path, read path, and how you would handle latency given the constraints.