A few years ago I implemented the storage system for a special-purpose diagnostic camera. The specification defined (very long) filenames for the saved images using a timestamp and some other data. I used a mostly off-the-shelf microcontroller/NAND/USB mass storage reference implementation, hooked it up a side-channel to the FPGA, and had everything working pretty nicely. Until the test harness that just continuously commanded pictures to be taken reached 105 iterations. After that, the camera timed out waiting on the storage subsystem to store the image.
The problem turned out to be the code that found the 8.3 filename: it did the longfi~1.bin, checked to see if that file existed and if so, incremented to longfi~2.bin, then checked that... but never did the checksum trick described here, just kept iterating. (bear in mind this was a tiny 8-bit microcontroller that didn't have the RAM to just read all the directory entries at once and keep them around for comparison) Finding the proper 8.3 filename this way took longer than the timeout period after 104 collisions.
Of course, we only cared about the long filename and never saw the 8.3 filename, so my fix was simply to use an appropriate hash of the long filename to ensure a good probability of uniqueness.
The problem turned out to be the code that found the 8.3 filename: it did the longfi~1.bin, checked to see if that file existed and if so, incremented to longfi~2.bin, then checked that... but never did the checksum trick described here, just kept iterating. (bear in mind this was a tiny 8-bit microcontroller that didn't have the RAM to just read all the directory entries at once and keep them around for comparison) Finding the proper 8.3 filename this way took longer than the timeout period after 104 collisions.
Of course, we only cared about the long filename and never saw the 8.3 filename, so my fix was simply to use an appropriate hash of the long filename to ensure a good probability of uniqueness.