Add log-structured caching target

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+dm-lc
+=====
+
+dm-lc provides write-back log-structured caching.
+It batches random writes into a big sequential write.
+
+1. Setup
+========
+dm-lc is composed of two target_type instances named lc and lc-mgr.
+
+- lc target is responsible for creating logical volumes and controlling ios.
+- lc-mgr target is reponsible for doing
+  formatting/initializing/destructing cache devices.
+  Operating dm-lc through these native interfaces are not recommended.
+
+Nice userland tools are provided in
+	https://github.com/akiradeveloper/dm-lc
+
+To install the tools, cd Admin directory and run
+	python setup.py install
+and you are a dm-lc admin.
+
+2. Admin example
+================
+Let's create a logical volume named myLV
+backed by /dev/myVg/myBacking and
+uses /dev/myCache as a cache device.
+
+myLV |-- (backing store) /dev/myVg/myBacking
+     |-- (cache device)  /dev/myCache
+
+Note that backing store is limited to a LVM device
+in the current implementation.
+
+1. Format myCache
+Format the on-disk metadata blocks on a device.
+Be careful, this operation erases
+all the existing data on the cache device.
+
+	lc-format-cache /dev/myCache
+
+2. Create myLV
+Create a logical volume simply backed by a existing volume.
+We give device ID 5 to the volume in this example.
+
+As of now, this operation create a logical volume
+with different name from the backing store.
+But some users don't want to change the name
+because the backing store is in operation
+and want to apply dm-lc on the fly.
+This can be technically realizable
+but I haven't implemented it at this time
+because it is too tricky.
+
+	lc-create myLV 5 /dev/myVg/myBacking
+
+3. Resume myCache
+Resuming cache device builds in-memory structures
+such as a hashtable scanned from the on-disk metadata.
+We give cache ID 3 to the device in this example.
+
+Be careful,
+you MUST create all the LVs
+as the destinations of the dirty blocks on the cache device
+before this operation.
+Otherwise, the kernel may crash.
+
+	lc-resume 3 /dev/myCache
+
+4. Attach myCache to myLV
+To start caching writes submitted to the myLV,
+you must attach myLV to myCache.
+This can be done on the fly.
+
+	lc-attach 5 3
+
+5. Start userland daemon
+dm-lc provides daemon program that
+autonomously control the module behavior such as migration.
+
+	lc-daemon start
+
+6. Terminate myLV
+Safely terminating myLV already attached to myCache is fallible
+and that's one of the reasons dm-lc provides these admin tools.
+myLV can not detach from myCache
+until all the dirty caches on myCache are migrated to myBacking.
+
+	lc-detach 5
+	lc-remove 5
+
+7. Terminate myCache
+After detaching all the LVs that is attached to myCache.
+myCache can be terminated.
+
+	lc-daemon stop
+	lc-free-cache 3
+
+3. Sysfs
+========
+dm-lc provides some sysfs interfaces to control the module behavior.
+The sysfs tree is located under /sys/module/dm_lc.
+
+/sys/module/dm_lc
+|
+|-- devices
+|   `-- 5
+|       |-- cache_id
+|       |-- dev
+|       |-- device -> ../../../../devices/virtual/block/dm-0
+|       |-- migrate_threshold
+|       |-- nr_dirty_caches
+|
+|-- caches
+|   `-- 3
+|       |-- allow_migrate
+|       |-- barrier_deadline_ms
+|       |-- commit_super_block
+|       |-- commit_super_block_interval
+|       |-- device -> ../../../../devices/virtual/block/dm-1
+|       |-- flush_current_buffer
+|       |-- flush_current_buffer_interval
+|       |-- force_migrate
+|       |-- last_flushed_segment_id
+|       |-- last_migrated_segment_id
+|       |-- nr_max_batched_migration
+|       `-- update_interval
+
+4. Technical Issues
+===================
+There are not a few technical issues that
+distinguishes dm-lc from other cache softwares.
+
+4.1 RAM buffer and immediate completion
+dm-lc allocated RAM buffers of 64MB in total by default.
+All of the writes are first stored in one of these RAM buffers
+and immediate completion is notified to the upper layer
+that is quite fast in few microseconds.
+
+4.2 Metadata durability
+After RAM buffer gets full or some deadline comes
+dm-lc creates segment log that combines RAM buffer and its metadata.
+Metadata have information such as relation between
+address in the cache device and the counterpart in the backing store.
+As the segment log is finally written to persistent cache device,
+any data will not be lost due to machine failure.
+
+4.3 Asynchronous log flushing
+dm-lc has a background worker called flush daemon.
+Flushing segment log starts from simply queueing the flush task.
+Flush daemon in background periodically checks if the queue has some tasks
+and actually executes the tasks if exists.
+The fact that the upper layer doesn't block in queueing the task
+maximizes the write throughput
+that is measured as 259MB/s random writes
+with cache device of 266MB/s sequential write which is only 3% loss
+and 1.5GB/s theoritically with a fast enough cache like PCI-e SSDs.
+
+4.4 Deferred ack for REQ_FUA or REQ_FLUSH bios
+Some applications such as NFS, journal filesystems
+and databases often submit SYNC write that
+incurs bios flagged with REQ_FUA or REQ_FLUSH.
+Handling these unusual bios immediately and thus synchronously
+desparately deteriorates the whole throughput.
+To address this issue, dm-lc handles acks for these bios
+lazily or in deferred manner.
+Completion related to these bios will not be done until
+they are written persistently to the cache device
+so this storategy doesn't betray the semantics.
+In the worst case scenario, a bio with some of these flags
+is completed in deadline period that is configurable
+in barrier_deadline_ms in sysfs.
+
+4.5 Asynchronous and autonomous migration
+Some time after a log segment is flushed to the cache device
+it will be migrated to the backing store.
+Migrate daemon is also a background worker
+that periodically checks if log segments to migrate exist.
+
+Restlessly migrating highly burdens backing store
+so migration is preferable to execute when the backing store is in lazy time.
+lc-daemon in userland surveils the load of the backing store
+and autonomously turns on and off migration according to the load.

index 30b426e..a9941f4 100644
@@ -290,6 +290,15 @@ config DM_CACHE_CLEANER
          A simple cache policy that writes back all data to the
          origin.  Used when decommissioning a dm-cache.
 
+config DM_LC
+	tristate "Log-structured Caching (EXPERIMENTAL)"
+	depends on BLK_DEV_DM
+	default y
+	---help---
+	  A cache layer that
+	  batches random writes into a big sequential write
+	  to a cache device in log-structured manner.
+
 config DM_MIRROR
        tristate "Mirror target"
        depends on BLK_DEV_DM

index 5ef78ef..691682e 100644
@@ -52,6 +52,7 @@ obj-$(CONFIG_DM_VERITY)		+= dm-verity.o
 obj-$(CONFIG_DM_CACHE)		+= dm-cache.o
 obj-$(CONFIG_DM_CACHE_MQ)	+= dm-cache-mq.o
 obj-$(CONFIG_DM_CACHE_CLEANER)	+= dm-cache-cleaner.o
+obj-$(CONFIG_DM_LC)		+= dm-lc.o
 
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o