Our PHP application consists of a single web server that will receive files from clients and perform a CPU-intensive analysis on them. Right now, analysis of a single user upload can take 3sec to conclude and take 100% CPU. This makes our system capacity amount to 1/3 requests per second.

My team's requirement is to increase capacity without a lot of code reengineering. A possible solution would be to set up a load balancer in front of multiple servers running the same app, connecting to a common DB. The problem is that the analysis outputs files on disk.

A load balancer would increase capacity, but then files won't be available between servers so consequent client requests may fail. We are hosted on Rackspace, is there a way to configure some sort of "common" storage for all servers, without having to rewrite our file persistance code? Current code relies on simple fopens etc. What are our options?