Bcache: Version 5 - read/write, pretty close to stable, and some numbers

new file mode 100644
index 0000000..53079a7
@@ -0,0 +1,75 @@
+Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
+nice if you could use them as cache... Hence bcache.
+
+It's designed around the performance characteristics of SSDs - it only allocates
+in erase block sized buckets, and it uses a bare minimum btree to track cached
+extants (which can be anywhere from a single sector to the bucket size). It's
+also designed to be very lazy, and use garbage collection to clean stale
+pointers.
+
+Cache devices are used as a pool; all available cache devices are used for all
+the devices that are being cached.  The cache devices store the UUIDs of
+devices they have, allowing caches to safely persist across reboots.  There's
+space allocated for 256 UUIDs right after the superblock - which means for now
+that there's a hard limit of 256 devices being cached.
+
+Currently only writethrough caching is supported; data is transparently added
+to the cache on writes but the write is not returned as completed until it has
+reached the underlying storage. Writeback caching will be supported when
+journalling is implemented.
+
+To protect against stale data, the entire cache is invalidated if it wasn't
+cleanly shutdown, and if caching is turned on or off for a device while it is
+opened read/write, all data for that device is invalidated.
+
+Caching can be transparently enabled and disabled for devices while they are in
+use. All configuration is done via sysfs. To use our SSD sde to cache our
+raid md1:
+
+  make-bcache /dev/sde
+  echo "/dev/sde" > /sys/kernel/bcache/register_cache
+  echo "<UUID> /dev/md1" > /sys/kernel/bcache/register_dev
+
+And that's it.
+
+If md1 was a raid 1 or 10, that's probably all you want to do; there's no point
+in caching multiple copies of the same data. However, if you have a raid 5 or
+6, caching the raw devices will allow the p and q blocks to be cached, which
+will help your random write performance:
+  echo "<UUID> /dev/sda1" > /sys/kernel/bcache/register_dev
+  echo "<UUID> /dev/sda2" > /sys/kernel/bcache/register_dev
+  etc.
+
+To script the UUID lookup, you could do something like:
+  echo  "`find /dev/disk/by-uuid/ -lname "*md1"|cut -d/ -f5` /dev/md1"\
+	  > /sys/kernel/bcache/register_dev 
+
+Of course, if you were already referencing your devices by UUID, you could do:
+  echo "$UUID /dev/disk/by-uiid/$UUID"\
+	  > /sys/kernel/bcache/register_dev 
+
+There are a number of other files in sysfs, some that provide statistics,
+others that allow tweaking of heuristics. Directories are also created
+for both cache devices and devices that are being cached, for per device
+statistics and device removal.
+
+Statistics: cache_hits, cache_misses, cache_hit_ratio
+These should be fairly obvious, they're simple counters.
+
+Cache hit heuristics: cache_priority_seek contributes to the new bucket
+priority once per cache hit; this lets us bias in favor of random IO.
+The file cache_priority_hit is scaled by the size of the cache hit, so
+we can give a 128k cache hit a higher weighting than a 4k cache hit.
+
+When new data is added to the cache, the initial priority is taken from
+cache_priority_initial. Every so often, we must rescale the priorities of
+all the in use buckets, so that the priority of stale data gradually goes to
+zero: this happens every N sectors, taken from cache_priority_rescale. The
+rescaling is currently hard coded at priority *= 7/8.
+
+For cache devices, there are a few more files. Most should be obvious;
+min_priority shows the priority of the bucket that will next be pulled off
+the heap, and tree_depth shows the current btree height.
+
+Writing to the unregister file in a device's directory will trigger the
+closing of that device.

index 9be0b56..4ebc4cc 100644
@@ -77,6 +77,21 @@ config BLK_DEV_INTEGRITY
 	T10/SCSI Data Integrity Field or the T13/ATA External Path
 	Protection.  If in doubt, say N.
 
+config BLK_CACHE
+	tristate "Block device as cache"
+	select SLOW_WORK
+	default m
+	---help---
+	Allows a block device to be used as cache for other devices; uses
+	a btree for indexing and the layout is optimized for SSDs.
+
+	Caches are persistent, and store the UUID of devices they cache.
+	Hence, to open a device as cache, use
+	echo /dev/foo > /sys/kernel/bcache/register_cache
+	And to enable caching for a device
+	echo "<UUID> /dev/bar" > /sys/kernel/bcache/register_dev
+	See Documentation/bcache.txt for details.
+
 endif # BLOCK
 
 config BLOCK_COMPAT

index 0bb499a..617845c 100644
@@ -15,3 +15,5 @@ obj-$(CONFIG_IOSCHED_CFQ)	+= cfq-iosched.o
 
 obj-$(CONFIG_BLOCK_COMPAT)	+= compat_ioctl.o
 obj-$(CONFIG_BLK_DEV_INTEGRITY)	+= blk-integrity.o
+
+obj-$(CONFIG_BLK_CACHE)		+= bcache.o