Onsite Consultancy – The Black Tile Problem
VxWorks Avionic Map Server using NFS
The client had been plagued by what was known as the ‘Black Tile Problem’. A mapping system that read map data via an NFS mount had one missing image, that cause a black tile to appear in the Pilot’s mapping system – very frustrating for them and embarrassing for the customer. This was a problem which would appear and disappear, making it extremely difficult to replicate. Many attempts had been made to fix the problem, but eventually they decided to call on the experts!
Our Top Gun Engineer was sent onsite (to Rome) for a week to fix the problem. A deep dive into the NFS code, and a hand built tool to trawl the NFS server, allowed the corner case of the problem to be discovered. A bug in the VxWorks NFS mapping code was causing the hash table algorithm to fail if the tile was accessed in a certain fixed sequence (i.e. with a particular hash). A fix was implemented, and tested, and documented for the customer. Job done and the customer (and his customer) was extremely happy!