I've been fixing the COSS code in Squid - its a cyclic-style filesystem with a twist - instead of completely cyclic it implements an on-disk LRU.
I've got it mostly stable. The main problem right now? The linux user-space Posix AIO support only seems to de-queue one op at a time per FD. I think this is severely hampering the disk performance; but there's little I can do about it for now. Grr.