On Wed, May 9, 2012 at 8:07 PM, Kent Overstreet [off-list ref] wrote:

bcache: a cache for arbitrary block devices using an SSD.

Short overview:
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.

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 btree
nodes.

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.

TODO/known issues:

__up_write() needs to be exported for bcache to compile as a module - it's
used for bypassing lockdep when traversing the btree during garbage
collection. If someone else knows a better solution, please let me know.

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.

Might as well mention I've started work on a md conversion.  Nearing
the point of having RFC patches ready to post.

I'm doing it in such a way that it does not preclude a dm target from
coming along as well.  Like I mentioned to Alasdair at LSF, there's no
reason this could not be treated the same as the shared raid
personalities between dm and md.

--
Dan