From the latest BCache patches that hit the Linux kernel mailing list on Wednesday, here's the description of this new kernel caching mechanism for Linux:
Bcache does both writethrough and writeback caching. It presents itself as a new block device, a bit like say md. You can cache an arbitrary number of block devices with a single cache device, and attach and detach things at runtime - it's quite flexible.The work is fairly in-depth and spread across 16 patches now up to their 13th revision prior to mainline inclusion. Will the 13th time be the charm and be set for inclusion into the Linux 3.5 kernel? Only time will tell, but from their TODO list: "The userspace interface is going to change before it goes in. The general consensus at LSF was that we don't want yet another interface for probing/managing block devices, and dm exists so we may as well use that. I don't think anyone's started on that yet, though." With user-space breakage, BCache might still be a ways off from hitting the mainline tree.
It's very fast. It uses a b+ tree for the index, along with a journal to coalesce index updates, and a bunch of other cool tricks like auxiliary binary search trees with software floating point keys for searching within btreenodes.
Bcache is solid, production ready code. There are still bugs being found that affect specific configurations, but there haven't been any major issues found in awhile - it's well past time I started working on getting it into mainline.
It's a lot of code - I tried to split it out so that it'd make some sort of sense for reviewing. Let me know if there's anything else I can do to make review easier.