diff --git a/Makefile.objs b/Makefile.objs
index 1c65087..2521f0d 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -31,6 +31,7 @@ block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
 
 block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
 block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
+block-nested-y += add-cow.o
 block-nested-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
 block-nested-y += qed-check.o
 block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
diff --git a/block.c b/block.c
index e3fe97f..66f2d39 100644
--- a/block.c
+++ b/block.c
@@ -106,7 +106,7 @@ int is_windows_drive(const char *filename)
 #endif
 
 /* check if the path starts with "<protocol>:" */
-static int path_has_protocol(const char *path)
+int path_has_protocol(const char *path)
 {
 #ifdef _WIN32
     if (is_windows_drive(path) ||
@@ -875,7 +875,7 @@ int bdrv_commit(BlockDriverState *bs)
 
     if (!drv)
         return -ENOMEDIUM;
-    
+
     if (!bs->backing_hd) {
         return -ENOTSUP;
     }
diff --git a/block.h b/block.h
index 16bfa0a..8b09f12 100644
--- a/block.h
+++ b/block.h
@@ -256,6 +256,7 @@ char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
 
 char *get_human_readable_size(char *buf, int buf_size, int64_t size);
 int path_is_absolute(const char *path);
+int path_has_protocol(const char *path);
 void path_combine(char *dest, int dest_size,
                   const char *base_path,
                   const char *filename);
diff --git a/block/add-cow.c b/block/add-cow.c
new file mode 100644
index 0000000..d27be6e
--- /dev/null
+++ b/block/add-cow.c
@@ -0,0 +1,403 @@
+#include "qemu-common.h"
+#include "block_int.h"
+#include "module.h"
+
+#define ADD_COW_MAGIC  (((uint64_t)'A' << 56) | ((uint64_t)'D' << 48) | \
+                        ((uint64_t)'D' << 40) | ((uint64_t)'_' << 32) | \
+                        ((uint64_t)'C' << 24) | ((uint64_t)'O' << 16) | \
+                        ((uint64_t)'W' << 8) | 0xFF)
+#define ADD_COW_VERSION 1
+
+typedef struct AddCowHeader {
+    uint64_t magic;
+    uint32_t version;
+    char backing_file[1024];
+    char image_file[1024];
+    uint64_t size;
+    uint32_t sectorsize;
+} QEMU_PACKED AddCowHeader;
+
+typedef struct BDRVAddCowState {
+    CoMutex lock;
+    CoMutex bitmap_lock;
+    char image_file[1024];
+    BlockDriverState *image_hd;
+    uint8_t *bitmap;
+    uint64_t bitmap_size;
+} BDRVAddCowState;
+
+static int add_cow_probe(const uint8_t *buf, int buf_size, const char *filename)
+{
+    const AddCowHeader *header = (const void *)buf;
+
+    if (be64_to_cpu(header->magic) == ADD_COW_MAGIC &&
+        be32_to_cpu(header->version) == ADD_COW_VERSION) {
+        return 100;
+    } else {
+        return 0;
+    }
+}
+
+static int add_cow_open(BlockDriverState *bs, int flags)
+{
+    AddCowHeader header;
+    int64_t size;
+    char image_filename[1024];
+    int image_flags;
+    BlockDriver *image_drv = NULL;
+    int ret;
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+
+    if (bdrv_pread(bs->file, 0, &header, sizeof(header)) != sizeof(header)) {
+        goto fail;
+    }
+
+    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC ||
+        be32_to_cpu(header.version) != ADD_COW_VERSION) {
+        goto fail;
+    }
+
+    size = be64_to_cpu(header.size);
+    bs->total_sectors = size / 512;
+
+    pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+            header.backing_file);
+    pstrcpy(state->image_file, sizeof(state->image_file),
+            header.image_file);
+
+    state->bitmap_size = ((bs->total_sectors + 7) >> 3);
+    if (!state->bitmap) {
+        state->bitmap = g_malloc0(state->bitmap_size);
+    }
+    if (bdrv_pread(bs->file, sizeof(header), state->bitmap,  \
+        state->bitmap_size) != state->bitmap_size) {
+        goto fail;
+    }
+   /* if there is a image_file, must be together with backing_file */
+    if (state->image_file[0] != '\0') {
+        state->image_hd = bdrv_new("");
+        if (path_has_protocol(state->image_file)) {
+            pstrcpy(image_filename, sizeof(image_filename),
+                    state->image_file);
+        } else {
+            path_combine(image_filename, sizeof(image_filename),
+                         bs->filename, state->image_file);
+        }
+
+        image_drv = bdrv_find_format("raw");
+        image_flags =
+             (flags & (~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING))) | BDRV_O_RDWR;
+        state->image_hd->keep_read_only = 0;
+
+        ret = bdrv_open(state->image_hd, image_filename, image_flags, \
+                image_drv);
+        if (ret < 0) {
+            bdrv_close(bs);
+            goto fail;
+        }
+    }
+    return 0;
+ fail:
+    if (state->bitmap) {
+        g_free(state->bitmap);
+        state->bitmap = NULL;
+    }
+    return -1;
+}
+
+static inline void add_cow_set_bit(BlockDriverState *bs, int64_t bitnum)
+{
+    uint64_t offset = sizeof(AddCowHeader) + bitnum / 8;
+    uint8_t bitmap;
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+
+    qemu_co_mutex_lock(&state->bitmap_lock);
+    bitmap = (state->bitmap[offset]) |= (1 << (bitnum % 8));
+    qemu_co_mutex_unlock(&state->bitmap_lock);
+}
+
+static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
+{
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+    qemu_co_mutex_lock(&state->bitmap_lock);
+    uint64_t offset = sizeof(AddCowHeader) + bitnum / 8;
+    qemu_co_mutex_unlock(&state->bitmap_lock);
+
+    return !!(state->bitmap[offset] & (1 << (bitnum % 8)));
+}
+
+static int add_cow_is_allocated(BlockDriverState *bs, int64_t sector_num,
+        int nb_sectors, int *num_same)
+{
+    int changed;
+
+    if (nb_sectors == 0) {
+        *num_same = nb_sectors;
+        return 0;
+    }
+
+    changed = is_bit_set(bs, sector_num);
+    if (changed < 0) {
+        return 0;
+    }
+
+    for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
+        if (is_bit_set(bs, sector_num + *num_same) != changed) {
+            break;
+        }
+    }
+
+    return changed;
+}
+
+static int add_cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
+        int nb_sectors)
+{
+    int error = 0;
+    int i, ret;
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+
+    for (i = 0; i < nb_sectors; i++) {
+        add_cow_set_bit(bs, sector_num + i);
+    }
+    ret = bdrv_pwrite_sync(bs->file, sizeof(AddCowHeader), \
+            state->bitmap, state->bitmap_size);
+    if (ret < 0) {
+        return ret;
+    }
+    return error;
+}
+
+static void add_cow_close(BlockDriverState *bs)
+{
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+    if (state->bitmap) {
+        g_free(state->bitmap);
+        state->bitmap = NULL;
+    }
+}
+
+static int add_cow_create(const char *filename, QEMUOptionParameter *options)
+{
+    AddCowHeader header;
+    int64_t image_sectors = 0;
+    const char *backing_filename = NULL;
+    const char *image_filename = NULL;
+    int ret;
+    BlockDriverState *bs, *image_bs = NULL;
+
+    while (options && options->name) {
+        if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
+            image_sectors = options->value.n / 512;
+        } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
+            backing_filename = options->value.s;
+        } else if (!strcmp(options->name, BLOCK_OPT_IMAGE_FILE)) {
+            image_filename = options->value.s;
+        }
+        options++;
+    }
+
+    if (!backing_filename || !image_filename) {
+        return -EINVAL;
+    }
+    /* Make sure image file exists */
+    BlockDriver *raw_drv = bdrv_find_format("raw");
+    assert(raw_drv != NULL);
+    ret = bdrv_file_open(&image_bs, image_filename, BDRV_O_RDWR | \
+            BDRV_O_CACHE_WB);
+    if (ret < 0) {
+        return ret;
+    }
+    bdrv_close(image_bs);
+    ret = bdrv_create_file(filename, NULL);
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = bdrv_file_open(&bs, filename, BDRV_O_RDWR);
+    if (ret < 0) {
+        return ret;
+    }
+
+    memset(&header, 0, sizeof(header));
+    header.magic = cpu_to_be64(ADD_COW_MAGIC);
+    header.version = cpu_to_be32(ADD_COW_VERSION);
+    pstrcpy(header.backing_file, \
+                sizeof(header.backing_file), backing_filename);
+    pstrcpy(header.image_file, sizeof(header.image_file),
+                image_filename);
+
+    header.sectorsize = cpu_to_be32(512);
+    header.size = cpu_to_be64(image_sectors * 512);
+
+    ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
+    if (ret < 0) {
+        return ret;
+    }
+    bdrv_close(bs);
+
+    BlockDriver *drv = bdrv_find_format("add-cow");
+    assert(drv != NULL);
+    ret = bdrv_open(bs, filename, BDRV_O_RDWR | BDRV_O_NO_FLUSH, drv);
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = bdrv_truncate(bs, ((image_sectors + 7) >> 3));
+    if (ret < 0) {
+        return ret;
+    }
+    return ret;
+}
+
+
+static int add_cow_co_readv(BlockDriverState *bs, int64_t sector_num,
+                         int remaining_sectors, QEMUIOVector *qiov)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int cur_nr_sectors;
+    uint64_t bytes_done = 0;
+    QEMUIOVector hd_qiov;
+    int n, ret = 0;
+
+    qemu_iovec_init(&hd_qiov, qiov->niov);
+    qemu_co_mutex_lock(&s->lock);
+    while (remaining_sectors != 0) {
+        cur_nr_sectors = remaining_sectors;
+        if (add_cow_is_allocated(bs, sector_num, cur_nr_sectors, &n)) {
+            cur_nr_sectors = n;
+            qemu_iovec_reset(&hd_qiov);
+            qemu_iovec_copy(&hd_qiov, qiov, bytes_done,
+                            cur_nr_sectors * 512);
+            qemu_co_mutex_unlock(&s->lock);
+            ret = bdrv_co_readv(s->image_hd, sector_num, n, &hd_qiov);
+            qemu_co_mutex_lock(&s->lock);
+            if (ret < 0) {
+                goto fail;
+            }
+        } else {
+            cur_nr_sectors = n;
+            if (bs->backing_hd) {
+                qemu_iovec_reset(&hd_qiov);
+                qemu_iovec_copy(&hd_qiov, qiov, bytes_done,
+                            cur_nr_sectors * 512);
+                qemu_co_mutex_unlock(&s->lock);
+                ret = bdrv_co_readv(bs->backing_hd, sector_num,
+                                    n, &hd_qiov);
+                qemu_co_mutex_lock(&s->lock);
+                if (ret < 0) {
+                    goto fail;
+                }
+            } else {
+                qemu_iovec_memset(&hd_qiov, 0, 512 * cur_nr_sectors);
+            }
+        }
+        remaining_sectors -= cur_nr_sectors;
+        sector_num += cur_nr_sectors;
+        bytes_done += cur_nr_sectors * 512;
+    }
+fail:
+    qemu_co_mutex_unlock(&s->lock);
+    qemu_iovec_destroy(&hd_qiov);
+    return ret;
+}
+
+static int add_cow_co_writev(BlockDriverState *bs, int64_t sector_num,
+                          int remaining_sectors, QEMUIOVector *qiov)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int ret;
+    int cur_nr_sectors = remaining_sectors;
+    QEMUIOVector hd_qiov;
+    uint64_t bytes_done = 0;
+    qemu_iovec_init(&hd_qiov, qiov->niov);
+    qemu_co_mutex_lock(&s->lock);
+
+    qemu_iovec_reset(&hd_qiov);
+    qemu_iovec_copy(&hd_qiov, qiov, bytes_done, cur_nr_sectors * 512);
+    qemu_co_mutex_unlock(&s->lock);
+    ret = bdrv_co_writev(s->image_hd,
+                     sector_num,
+                     cur_nr_sectors, &hd_qiov);
+    qemu_co_mutex_lock(&s->lock);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    qemu_co_mutex_unlock(&s->lock);
+    add_cow_update_bitmap(bs, sector_num, remaining_sectors);
+    ret = 0;
+fail:
+    qemu_iovec_destroy(&hd_qiov);
+    return ret;
+}
+
+static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t offset)
+{
+    int ret;
+    ret = bdrv_truncate(bs->file, offset + sizeof(AddCowHeader));
+    if (ret < 0) {
+        return ret;
+    }
+
+    return 0;
+}
+
+static QEMUOptionParameter add_cow_create_options[] = {
+    {
+        .name = BLOCK_OPT_SIZE,
+        .type = OPT_SIZE,
+        .help = "Virtual disk size"
+    },
+    {
+        .name = BLOCK_OPT_BACKING_FILE,
+        .type = OPT_STRING,
+        .help = "File name of a base image"
+    },
+    {
+        .name = BLOCK_OPT_IMAGE_FILE,
+        .type = OPT_STRING,
+        .help = "File name of a image file"
+    },
+    { NULL }
+};
+
+static int add_cow_flush(BlockDriverState *bs)
+{
+    BDRVAddCowState *state = bs->opaque;
+    return bdrv_flush(bs->file) | bdrv_flush(state->image_hd);
+}
+
+static BlockDriverAIOCB *add_cow_aio_flush(BlockDriverState *bs,
+        BlockDriverCompletionFunc *cb, void *opaque)
+{
+    BDRVAddCowState *state = bs->opaque;
+    bdrv_aio_flush(state->image_hd, cb, opaque);
+    return bdrv_aio_flush(bs->file, cb, opaque);
+
+}
+
+static BlockDriver bdrv_add_cow = {
+    .format_name        = "add-cow",
+    .instance_size      = sizeof(BDRVAddCowState),
+    .bdrv_probe         = add_cow_probe,
+    .bdrv_open          = add_cow_open,
+    .bdrv_close         = add_cow_close,
+    .bdrv_create        = add_cow_create,
+    .bdrv_is_allocated  = add_cow_is_allocated,
+
+    .bdrv_co_readv      = add_cow_co_readv,
+    .bdrv_co_writev     = add_cow_co_writev,
+    .bdrv_truncate      = bdrv_add_cow_truncate,
+
+    .create_options     = add_cow_create_options,
+    .bdrv_flush         = add_cow_flush,
+    .bdrv_aio_flush     = add_cow_aio_flush,
+};
+
+static void bdrv_add_cow_init(void)
+{
+    bdrv_register(&bdrv_add_cow);
+}
+
+block_init(bdrv_add_cow_init);
diff --git a/block_int.h b/block_int.h
index 8c3b863..2b26fdc 100644
--- a/block_int.h
+++ b/block_int.h
@@ -42,6 +42,7 @@
 #define BLOCK_OPT_TABLE_SIZE    "table_size"
 #define BLOCK_OPT_PREALLOC      "preallocation"
 #define BLOCK_OPT_SUBFMT        "subformat"
+#define BLOCK_OPT_IMAGE_FILE    "image_file"
 
 typedef struct AIOPool {
     void (*cancel)(BlockDriverAIOCB *acb);
diff --git a/qemu-img.c b/qemu-img.c
index 6a39731..6335234 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -746,7 +746,7 @@ static int img_convert(int argc, char **argv)
         ret = -1;
         goto out;
     }
-        
+
     qemu_progress_init(progress, 2.0);
     qemu_progress_print(0, 100);
 
