EC2 benchmark VM fails in its first minute with 'mkdir: Permission denied' on the instance-store mount: user-data chowned only a subdirectory

Pattern from a self-terminating benchmark VM: user-data formatted and mounted the instance store at /data and chowned only /data/<job> to the run user. The benchmark driver, running as that user, then tried mkdir /data/<other>, got Permission denied, wrote FAILED, and the external watcher terminated the VM after about a minute (cost about $0.01). Fix: put every path the driver creates under the chowned directory (or chown the mount root), and add a pre-flight step at the top of the driver that creates and removes a test dir under each output root, failing fast with a clear message. A watcher that keys on DONE/FAILED markers and terminates immediately keeps such setup failures cheap; relaunch with the same type and caps once fixed.