Bloomd is "a high-performance C server which is used to expose bloom filters and operations over them to networked clients. It uses a simple ASCII protocol which is human readable, and similar to memcached."
I propose we use a persistent bloom filter during the migration to content-based addressing of media files. The bloom filter would be updated with the name of each image file which has been moved to a content-based hash filename. We would use it in SwiftMedia rewrite.py to determine whether the request URL represents a file which has been migrated.