本当に動いたら最善手だと思う　

gistfile1.txt

kira@Kamille:~/src/device-mapper-test-suite/lib/dmtest$ git di
diff --git a/lib/dmtest/tests/writeboost/stack.rb b/lib/dmtest/tests/writeboost/stack.rb
index 56b0d73..1aee66d 100644
--- a/lib/dmtest/tests/writeboost/stack.rb
+++ b/lib/dmtest/tests/writeboost/stack.rb
@@ -78,6 +78,12 @@ class WriteboostStack
               @fast_tvm.table('plog_dev')
              ) do |backing_dev, cache_dev, plog_dev|
       @backing_dev = backing_dev
+      class << @backing_dev
+        def to_s
+          @slow_dev_name
+        end
+      end
+
       @cache_dev = cache_dev
       @plog_dev = plog_dev

@@ -158,6 +164,12 @@ class WriteboostStack
   end
 end

+class WriteboostStackBackingDevice < WriteboostStack
+  def table
+    Table.new(LinearTarget.new(backing_sz, @slow_dev_name, 0)
+  end
+end
+
 class WriteboostStackType0 < WriteboostStack
   def table
     essentials = [0, @backing_dev, @cache_dev]
diff --git a/lib/dmtest/tests/writeboost/tests.rb b/lib/dmtest/tests/writeboost/tests.rb
index 4dab15f..b0d9fe6 100644
--- a/lib/dmtest/tests/writeboost/tests.rb
+++ b/lib/dmtest/tests/writeboost/tests.rb
@@ -205,6 +205,15 @@ module WriteboostTests
   end
 end

+class WriteboostTestsBackingDevice < ThinpTestCase
+  include WriteboostTests
+
+  def setup
+    super
+    @stack_maker = WriteboostStackBackingDevice
+  end
+end
+
 class WriteboostTestsType0 < ThinpTestCase
   include WriteboostTests