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+dm-log-writes
+=============
+
+This target takes 2 devices, one to pass all IO to normally, and one to log all
+of the write operations to.  This is intended for file system developers wishing
+to verify the integrity of metadata or data as the file system is written to.
+There is a log_write_entry written for every WRITE request and the target is
+able to take arbitrary data from userspace to insert into the log.  The data
+that is in the WRITE requests is copied into the log to make the replay happen
+exactly as it happened originally.
+
+Log Ordering
+============
+
+We log things in order of completion once we are sure the write is no longer in
+cache.  This means that normal WRITE requests are not actually logged until the
+next REQ_FLUSH request.  This is to make it easier for userspace to replay the
+log in a way that correlates to what is on disk and not what is in cache, to
+make it easier to detect improper waiting/flushing.
+
+This works by attaching all WRITE requests to a list once the write completes.
+Once we see a REQ_FLUSH request we splice this list onto the request and once
+the FLUSH request completes we log all of the WRITEs and then the FLUSH.  Only
+completed WRITEs, at the time the REQ_FLUSH is issued, are added in order to
+simulate the worst case scenario with regard to power failures.  Consider the
+following example (W means write, C means complete):
+
+W1,W2,W3,C3,C2,Wflush,C1,Cflush
+
+The log would show the following
+
+W3,W2,flush,W1....
+
+Again this is to simulate what is actually on disk, this allows us to detect
+cases where a power failure at a particular point in time would create an
+inconsistent file system.
+
+Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as
+they complete as those requests will obviously bypass the device cache.
+
+Any REQ_DISCARD requests are treated like WRITE requests.  Otherwise we would
+have all the DISCARD requests, and then the WRITE requests and then the FLUSH
+request.  Consider the following example:
+
+WRITE block 1, DISCARD block 1, FLUSH
+
+If we logged DISCARD when it completed, the replay would look like this
+
+DISCARD 1, WRITE 1, FLUSH
+
+which isn't quite what happened and wouldn't be caught during the log replay.
+
+Target interface
+================
+
+i) Constructor
+
+   log-writes <dev_path> <log_dev_path>
+
+   dev_path	: Device that all of the IO will go to normally.
+   log_dev_path : Device where the log entries are written to.
+
+ii) Status
+
+    <#logged entries> <highest allocated sector>
+
+    #logged entries	       : Number of logged entries
+    highest allocated sector   : Highest allocated sector
+
+iii) Messages
+
+    mark <description>
+
+	You can use a dmsetup message to set an arbitrary mark in a log.
+	For example say you want to fsck a file system after every
+	write, but first you need to replay up to the mkfs to make sure
+	we're fsck'ing something reasonable, you would do something like
+	this:
+
+	  mkfs.btrfs -f /dev/mapper/log
+	  dmsetup message log 0 mark mkfs
+	  <run test>
+
+	  This would allow you to replay the log up to the mkfs mark and
+	  then replay from that point on doing the fsck check in the
+	  interval that you want.
+
+	Every log has a mark at the end labeled "dm-log-writes-end".
+
+Userspace component
+===================
+
+There is a userspace tool that will replay the log for you in various ways.
+It can be found here: https://github.com/josefbacik/log-writes
+
+Example usage
+=============
+
+Say you want to test fsync on your file system.  You would do something like
+this:
+
+TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
+dmsetup create log --table "$TABLE"
+mkfs.btrfs -f /dev/mapper/log
+dmsetup message log 0 mark mkfs
+
+mount /dev/mapper/log /mnt/btrfs-test
+<some test that does fsync at the end>
+dmsetup message log 0 mark fsync
+md5sum /mnt/btrfs-test/foo
+umount /mnt/btrfs-test
+
+dmsetup remove log
+replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync
+mount /dev/sdb /mnt/btrfs-test
+md5sum /mnt/btrfs-test/foo
+<verify md5sum's are correct>
+
+Another option is to do a complicated file system operation and verify the file
+system is consistent during the entire operation.  You could do this with:
+
+TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
+dmsetup create log --table "$TABLE"
+mkfs.btrfs -f /dev/mapper/log
+dmsetup message log 0 mark mkfs
+
+mount /dev/mapper/log /mnt/btrfs-test
+<fsstress to dirty the fs>
+btrfs filesystem balance /mnt/btrfs-test
+umount /mnt/btrfs-test
+dmsetup remove log
+
+replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs
+btrfsck /dev/sdb
+replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \
+	--fsck "btrfsck /dev/sdb" --check fua
+
+And that will replay the log until it sees a FUA request, run the fsck command
+and if the fsck passes it will replay to the next FUA, until it is completed or
+the fsck command exists abnormally.