I've committed the initial modifications to the storage rebuilding code. The changes mostly live in the AUFS and COSS code - the rest of Lusca isn't affected.
The change pushes the rebuild logic itself into external helpers which simply stream swaplog entries to the main process. Lusca doesn't care how the swaplog entries are generated.
The external helper method is big boost for AUFS. Each storedir creates a single rebuild helper process which can block on disk IO without blocking anything else. The original code in Squid will do a little disk IO work at a time - which almost always involved blocking the process until said disk IO completed.
The main motivation of this work was the removal of a lot of really horrible, twisty code and further modularisation of the codebase. The speedups to the rebuild process are a nice side-effect. The next big improvement will be sorting out how the swap logs are written. Fixing that will be key to allowing enormous caches to properly function without log rotation potentially destroying the proxy service.