Nginx Split Proxy Cache Across Multiple Drives

One of the websites we run serves a massive amount of static content. However, each page request needs to query a MySQL database to retrieve the storage location of the files. To reduce the load on our servers, we implemented an Nginx reverse proxy with caching. As our site grew, we quickly outgrew the system we were hosting the proxy on. At its peak time of day, this site is now serving over 600 requests per second for small image files averaging 15KB each. This is generating a massive amount of disk IO to the point where the disk was pegged at 100% utilized all the time, and starting to impact performance.