BITMAP_GRANULARITY   30 drivers/md/dm-writecache.c #define BITMAP_GRANULARITY	65536
BITMAP_GRANULARITY   32 drivers/md/dm-writecache.c #undef BITMAP_GRANULARITY
BITMAP_GRANULARITY   33 drivers/md/dm-writecache.c #define BITMAP_GRANULARITY	PAGE_SIZE