diff --git a/Makefile.objs b/Makefile.objs
index 70c5c79..10c5c52 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -52,6 +52,7 @@ block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vv
 block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.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 += add-cow.o add-cow-cache.o
 block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
 block-nested-y += stream.o
 block-nested-$(CONFIG_WIN32) += raw-win32.o
diff --git a/block.c b/block.c
index 43c794c..206860c 100644
--- a/block.c
+++ b/block.c
@@ -196,7 +196,7 @@ static void bdrv_io_limits_intercept(BlockDriverState *bs,
 }
 
 /* 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) ||
diff --git a/block.h b/block.h
index f163e54..f74c79e 100644
--- a/block.h
+++ b/block.h
@@ -319,6 +319,7 @@ 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-cache.c b/block/add-cow-cache.c
new file mode 100644
index 0000000..3ae23c1
--- /dev/null
+++ b/block/add-cow-cache.c
@@ -0,0 +1,193 @@
+/*
+ * Cache For QEMU ADD-COW Disk Format
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include "block_int.h"
+#include "qemu-common.h"
+#include "add-cow.h"
+
+/* Based on qcow2-cache.c */
+AddCowCache *add_cow_cache_create(BlockDriverState *bs, int num_tables,
+    bool writethrough)
+{
+    AddCowCache *c;
+    int i;
+
+    c = g_malloc0(sizeof(*c));
+    c->size = num_tables;
+    c->entries = g_malloc0(sizeof(*c->entries) * num_tables);
+    c->writethrough = writethrough;
+    c->entry_size = ADD_COW_CACHE_ENTRY_SIZE;
+
+    for (i = 0; i < c->size; i++) {
+        c->entries[i].table = qemu_blockalign(bs, c->entry_size);
+        c->entries[i].offset = -1;
+    }
+
+    return c;
+}
+
+int add_cow_cache_destroy(BlockDriverState *bs, AddCowCache *c)
+{
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        qemu_vfree(c->entries[i].table);
+    }
+
+    g_free(c->entries);
+    g_free(c);
+
+    return 0;
+}
+
+static int add_cow_cache_entry_flush(BlockDriverState *bs,
+    AddCowCache *c,
+    int i)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int ret = 0;
+
+    if (!c->entries[i].dirty || -1 == c->entries[i].offset) {
+        return ret;
+    }
+
+    ret = bdrv_pwrite(bs->file, sizeof(AddCowHeader) + c->entries[i].offset,
+        c->entries[i].table,
+        MIN(c->entry_size, s->bitmap_size - c->entries[i].offset));
+    if (ret < 0) {
+        return ret;
+    }
+
+    c->entries[i].dirty = false;
+
+    return 0;
+}
+
+int add_cow_cache_flush(BlockDriverState *bs, AddCowCache *c)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int result = 0;
+    int ret;
+    int i;
+
+    ret = bdrv_flush(s->image_hd);
+    if (ret < 0) {
+        return result;
+    }
+
+    for (i = 0; i < c->size; i++) {
+        ret = add_cow_cache_entry_flush(bs, c, i);
+        if (ret < 0 && result != -ENOSPC) {
+            result = ret;
+        }
+    }
+
+    if (result == 0) {
+        ret = bdrv_flush(bs->file);
+        if (ret < 0) {
+            result = ret;
+        }
+    }
+
+    return result;
+}
+
+static int add_cow_cache_find_entry_to_replace(AddCowCache *c)
+{
+    int i;
+    int min_count = INT_MAX;
+    int min_index = -1;
+
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].cache_hits < min_count) {
+            min_index = i;
+            min_count = c->entries[i].cache_hits;
+        }
+
+        c->entries[i].cache_hits /= 2;
+    }
+
+    if (min_index == -1) {
+        abort();
+    }
+    return min_index;
+}
+
+static int add_cow_cache_do_get(BlockDriverState *bs, AddCowCache *c,
+    uint64_t offset, void **table)
+{
+    int i, ret;
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].offset == offset) {
+            goto found;
+        }
+    }
+
+    i = add_cow_cache_find_entry_to_replace(c);
+    if (i < 0) {
+        return i;
+    }
+
+    ret = add_cow_cache_entry_flush(bs, c, i);
+    if (ret < 0) {
+        return ret;
+    }
+    c->entries[i].offset = -1;
+    ret = bdrv_pread(bs->file, sizeof(AddCowHeader) + offset,
+        c->entries[i].table, c->entry_size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    c->entries[i].cache_hits = 32;
+    c->entries[i].offset = offset;
+
+found:
+    c->entries[i].cache_hits++;
+    *table = c->entries[i].table;
+
+    return 0;
+}
+
+int add_cow_cache_get(BlockDriverState *bs, AddCowCache *c, uint64_t sector_num,
+    void **table)
+{
+    /* each byte in bitmap indicates 8 * SECTORS_PER_CLUSTER clusters */
+    uint64_t offset = offset_in_bitmap(sector_num) & (~(c->entry_size - 1));
+    return add_cow_cache_do_get(bs, c, offset, table);
+}
+
+void add_cow_cache_entry_mark_dirty(AddCowCache *c, void *table)
+{
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].table == table) {
+            goto found;
+        }
+    }
+    abort();
+
+found:
+    c->entries[i].dirty = true;
+}
+
+bool add_cow_cache_set_writethrough(BlockDriverState *bs, AddCowCache *c,
+    bool enable)
+{
+    bool old = c->writethrough;
+
+    if (!old && enable) {
+        add_cow_cache_flush(bs, c);
+    }
+
+    c->writethrough = enable;
+    return old;
+}
diff --git a/block/add-cow.c b/block/add-cow.c
new file mode 100644
index 0000000..8bf27ab
--- /dev/null
+++ b/block/add-cow.c
@@ -0,0 +1,446 @@
+/*
+ * QEMU ADD-COW Disk Format
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include "qemu-common.h"
+#include "block_int.h"
+#include "module.h"
+#include "add-cow.h"
+
+static int add_cow_probe(const uint8_t *buf, int buf_size, const char *filename)
+{
+    const AddCowHeader *header = (const AddCowHeader *)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_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 / BDRV_SECTOR_SIZE;
+        } 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) {
+        error_report("Both backing_file and image_file should be given.");
+        return -EINVAL;
+    }
+
+    ret = bdrv_file_open(&image_bs, image_filename, BDRV_O_RDWR
+            | BDRV_O_CACHE_WB);
+    if (ret < 0) {
+        return ret;
+    }
+    image_sectors = image_bs->total_sectors;
+    bdrv_delete(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);
+
+    ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
+    if (ret < 0) {
+        bdrv_delete(bs);
+        return ret;
+    }
+
+    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) {
+        bdrv_delete(bs);
+        return ret;
+    }
+
+    ret = bdrv_truncate(bs, image_sectors * BDRV_SECTOR_SIZE);
+    bdrv_delete(bs);
+    return ret;
+}
+
+static int add_cow_open(BlockDriverState *bs, int flags)
+{
+    AddCowHeader        header;
+    char                image_filename[ADD_COW_FILE_LEN];
+    BlockDriver         *image_drv = NULL;
+    int                 ret;
+    bool                writethrough;
+    int                 sector_per_byte;
+    BDRVAddCowState     *s = bs->opaque;
+
+    ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
+    if (ret != sizeof(header)) {
+        goto fail;
+    }
+
+    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC) {
+        ret = -EINVAL;
+        goto fail;
+    }
+    if (be32_to_cpu(header.version) != ADD_COW_VERSION) {
+        char version[64];
+        snprintf(version, sizeof(version), "ADD-COW version %d",
+            be32_to_cpu(header.version));
+        qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
+            bs->device_name, "add-cow", version);
+        ret = -ENOTSUP;
+        goto fail;
+    }
+
+    QEMU_BUILD_BUG_ON(sizeof(bs->backing_file) != sizeof(header.backing_file));
+    pstrcpy(bs->backing_file, sizeof(bs->backing_file), header.backing_file);
+
+    if (header.image_file[0] == '\0') {
+        ret = -ENOENT;
+        goto fail;
+    }
+    header.image_file[ADD_COW_FILE_LEN - 1] = '\0';
+    s->image_hd = bdrv_new("");
+    if (path_has_protocol(header.image_file)) {
+        pstrcpy(image_filename, sizeof(image_filename), header.image_file);
+    } else {
+        path_combine(image_filename, sizeof(image_filename),
+                     bs->filename, header.image_file);
+    }
+
+    image_drv = bdrv_find_format("raw");
+    ret = bdrv_open(s->image_hd, image_filename, flags, image_drv);
+    if (ret < 0) {
+        bdrv_delete(s->image_hd);
+        goto fail;
+    }
+    bs->total_sectors = s->image_hd->total_sectors;
+    s->cluster_size = ADD_COW_CLUSTER_SIZE;
+    sector_per_byte = SECTORS_PER_CLUSTER * 8;
+    s->bitmap_size =
+        (bs->total_sectors + sector_per_byte - 1) / sector_per_byte;
+    writethrough = ((flags & BDRV_O_CACHE_WB) == 0);
+    s->bitmap_cache =
+        add_cow_cache_create(bs, ADD_COW_CACHE_SIZE, writethrough);
+
+    qemu_co_mutex_init(&s->lock);
+    return 0;
+fail:
+    if (s->bitmap_cache) {
+        add_cow_cache_destroy(bs, s->bitmap_cache);
+    }
+    return ret;
+}
+
+static void add_cow_close(BlockDriverState *bs)
+{
+    BDRVAddCowState *s = bs->opaque;
+    add_cow_cache_destroy(bs, s->bitmap_cache);
+    bdrv_delete(s->image_hd);
+}
+
+static bool is_allocated(BlockDriverState *bs, int64_t sector_num)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int64_t cluster_num = sector_num / SECTORS_PER_CLUSTER;
+    uint8_t *table = NULL;
+    int ret = add_cow_cache_get(bs, s->bitmap_cache,
+        sector_num, (void **)&table);
+
+    if (ret < 0) {
+        return 0;
+    }
+    return table[cluster_num / 8 % ADD_COW_CACHE_ENTRY_SIZE]
+        & (1 << (cluster_num % 8));
+}
+
+static coroutine_fn int add_cow_is_allocated(BlockDriverState *bs,
+        int64_t sector_num, int nb_sectors, int *num_same)
+{
+    int changed;
+    int64_t cluster_num;
+
+    if (nb_sectors == 0) {
+        *num_same = 0;
+        return 0;
+    }
+
+    cluster_num = sector_num / SECTORS_PER_CLUSTER;
+    changed = is_allocated(bs, sector_num);
+    *num_same =
+        MIN(nb_sectors, (cluster_num + 1) * SECTORS_PER_CLUSTER - sector_num);
+
+    for (cluster_num = sector_num / SECTORS_PER_CLUSTER + 1;
+        cluster_num <= (sector_num + nb_sectors - 1) / SECTORS_PER_CLUSTER;
+        cluster_num++) {
+        if (is_allocated(bs, cluster_num * SECTORS_PER_CLUSTER) != changed) {
+            break;
+        }
+        *num_same = MIN(nb_sectors,
+            (cluster_num + 1) * SECTORS_PER_CLUSTER - sector_num);
+    }
+
+    return changed;
+}
+
+static coroutine_fn 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 * BDRV_SECTOR_SIZE);
+            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 * BDRV_SECTOR_SIZE);
+                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,
+                    BDRV_SECTOR_SIZE * cur_nr_sectors);
+            }
+        }
+        remaining_sectors -= cur_nr_sectors;
+        sector_num += cur_nr_sectors;
+        bytes_done += cur_nr_sectors * BDRV_SECTOR_SIZE;
+    }
+fail:
+    qemu_co_mutex_unlock(&s->lock);
+    qemu_iovec_destroy(&hd_qiov);
+    return ret;
+}
+
+static int coroutine_fn copy_sectors(BlockDriverState *bs,
+                                     int n_start, int n_end)
+{
+    BDRVAddCowState *s = bs->opaque;
+    QEMUIOVector qiov;
+    struct iovec iov;
+    int n, ret;
+
+    n = n_end - n_start;
+    if (n <= 0) {
+        return 0;
+    }
+
+    iov.iov_len = n * BDRV_SECTOR_SIZE;
+    iov.iov_base = qemu_blockalign(bs, iov.iov_len);
+
+    qemu_iovec_init_external(&qiov, &iov, 1);
+
+    ret = bdrv_co_readv(bs->backing_hd, n_start, n, &qiov);
+    if (ret < 0) {
+        goto out;
+    }
+    ret = bdrv_co_writev(s->image_hd, n_start, n, &qiov);
+    if (ret < 0) {
+        goto out;
+    }
+
+    ret = 0;
+out:
+    qemu_vfree(iov.iov_base);
+    return ret;
+}
+
+static coroutine_fn int add_cow_co_writev(BlockDriverState *bs,
+        int64_t sector_num, int remaining_sectors, QEMUIOVector *qiov)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int ret = 0, i;
+    QEMUIOVector hd_qiov;
+    uint8_t *table;
+    bool changed = false;
+
+    qemu_co_mutex_lock(&s->lock);
+    qemu_iovec_init(&hd_qiov, qiov->niov);
+    ret = bdrv_co_writev(s->image_hd,
+                     sector_num,
+                     remaining_sectors, qiov);
+
+    if (ret < 0) {
+        goto fail;
+    }
+    /* copy content of unmodified sectors */
+    if (!is_cluster_head(sector_num) && !is_allocated(bs, sector_num)) {
+        qemu_co_mutex_unlock(&s->lock);
+        ret = copy_sectors(bs, sector_num & ~(SECTORS_PER_CLUSTER - 1),
+            sector_num);
+        qemu_co_mutex_lock(&s->lock);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    if (!is_cluster_tail(sector_num + remaining_sectors - 1)
+        && !is_allocated(bs, sector_num + remaining_sectors - 1)) {
+        qemu_co_mutex_unlock(&s->lock);
+        ret = copy_sectors(bs, sector_num + remaining_sectors,
+            ((sector_num + remaining_sectors) | (SECTORS_PER_CLUSTER - 1)) + 1);
+        qemu_co_mutex_lock(&s->lock);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    for (i = sector_num / SECTORS_PER_CLUSTER;
+        i <= (sector_num + remaining_sectors - 1) / SECTORS_PER_CLUSTER;
+        i++) {
+        ret = add_cow_cache_get(bs, s->bitmap_cache,
+            i * SECTORS_PER_CLUSTER, (void **)&table);
+        if (ret < 0) {
+            return 0;
+        }
+        if ((table[i / 8] & (1 << (i % 8))) == 0) {
+            table[i / 8] |= (1 << (i % 8));
+            changed = true;
+            add_cow_cache_entry_mark_dirty(s->bitmap_cache, table);
+        }
+
+    }
+    ret = 0;
+fail:
+    if (changed) {
+        ret = add_cow_cache_flush(bs, s->bitmap_cache);
+    }
+    qemu_co_mutex_unlock(&s->lock);
+    qemu_iovec_destroy(&hd_qiov);
+    return ret;
+}
+
+static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t size)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int sector_per_byte = SECTORS_PER_CLUSTER * 8;
+    int ret;
+    int64_t old_image_sector = s->image_hd->total_sectors;
+    int64_t bitmap_size =
+        (size / BDRV_SECTOR_SIZE + sector_per_byte - 1) / sector_per_byte;
+
+    ret = bdrv_truncate(bs->file,
+        sizeof(AddCowHeader) + bitmap_size);
+    if (ret < 0) {
+        bdrv_truncate(s->image_hd, old_image_sector * BDRV_SECTOR_SIZE);
+        return ret;
+    }
+    return 0;
+}
+
+static coroutine_fn int add_cow_co_flush(BlockDriverState *bs)
+{
+    BDRVAddCowState *s = bs->opaque;
+    int ret;
+
+    qemu_co_mutex_lock(&s->lock);
+    ret = add_cow_cache_flush(bs, s->bitmap_cache);
+    qemu_co_mutex_unlock(&s->lock);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return bdrv_co_flush(bs->file);
+}
+
+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"
+    },
+    {
+        .name = BLOCK_OPT_BACKING_FMT,
+        .type = OPT_STRING,
+        .help = "Image format of the base image"
+    },
+    { NULL }
+};
+
+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_co_readv              = add_cow_co_readv,
+    .bdrv_co_writev             = add_cow_co_writev,
+    .bdrv_truncate              = bdrv_add_cow_truncate,
+    .bdrv_co_is_allocated       = add_cow_is_allocated,
+
+    .create_options             = add_cow_create_options,
+    .bdrv_co_flush_to_disk      = add_cow_co_flush,
+};
+
+static void bdrv_add_cow_init(void)
+{
+    bdrv_register(&bdrv_add_cow);
+}
+
+block_init(bdrv_add_cow_init);
diff --git a/block/add-cow.h b/block/add-cow.h
new file mode 100644
index 0000000..46567bb
--- /dev/null
+++ b/block/add-cow.h
@@ -0,0 +1,83 @@
+/*
+ * QEMU ADD-COW Disk Format
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#ifndef BLOCK_ADD_COW_H
+#define BLOCK_ADD_COW_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
+#define ADD_COW_FILE_LEN           1024
+#define ADD_COW_CACHE_SIZE         16
+#define ADD_COW_CACHE_ENTRY_SIZE   65536
+#define ADD_COW_CLUSTER_SIZE       65536
+#define SECTORS_PER_CLUSTER         (ADD_COW_CLUSTER_SIZE / BDRV_SECTOR_SIZE)
+
+typedef struct AddCowHeader {
+    uint64_t        magic;
+    uint32_t        version;
+    char            backing_file[ADD_COW_FILE_LEN];
+    char            image_file[ADD_COW_FILE_LEN];
+    char            reserved[500];
+} QEMU_PACKED AddCowHeader;
+
+typedef struct AddCowCachedTable {
+    void    *table;
+    int64_t offset;
+    bool    dirty;
+    int     cache_hits;
+} AddCowCachedTable;
+
+typedef struct AddCowCache {
+    AddCowCachedTable       *entries;
+    int                     entry_size;
+    int                     size;
+    bool                    writethrough;
+} AddCowCache;
+
+typedef struct BDRVAddCowState {
+    BlockDriverState    *image_hd;
+    CoMutex             lock;
+    int                 cluster_size;
+    AddCowCache         *bitmap_cache;
+    uint64_t            bitmap_size;
+} BDRVAddCowState;
+
+/* convert sector_num to offset in bitmap */
+static inline int64_t offset_in_bitmap(int64_t sector_num)
+{
+    int64_t cluster_num = sector_num / SECTORS_PER_CLUSTER;
+    return cluster_num / 8;
+}
+
+static inline bool is_cluster_head(int64_t sector_num)
+{
+    return sector_num % SECTORS_PER_CLUSTER == 0;
+}
+
+static inline bool is_cluster_tail(int64_t sector_num)
+{
+    return (sector_num + 1) % SECTORS_PER_CLUSTER == 0;
+}
+
+AddCowCache *add_cow_cache_create(BlockDriverState *bs, int num_tables,
+    bool writethrough);
+int add_cow_cache_destroy(BlockDriverState *bs, AddCowCache *c);
+void add_cow_cache_entry_mark_dirty(AddCowCache *c, void *table);
+int add_cow_cache_get(BlockDriverState *bs, AddCowCache *c, uint64_t offset,
+    void **table);
+int add_cow_cache_flush(BlockDriverState *bs, AddCowCache *c);
+bool add_cow_cache_set_writethrough(BlockDriverState *bs, AddCowCache *c,
+    bool enable);
+#endif
diff --git a/block_int.h b/block_int.h
index 086832a..83e75ea 100644
--- a/block_int.h
+++ b/block_int.h
@@ -51,6 +51,7 @@
 #define BLOCK_OPT_PREALLOC      "preallocation"
 #define BLOCK_OPT_SUBFMT        "subformat"
 #define BLOCK_OPT_COMPAT_LEVEL  "compat"
+#define BLOCK_OPT_IMAGE_FILE    "image_file"
 
 typedef struct BdrvTrackedRequest BdrvTrackedRequest;
 
diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
new file mode 100644
index 0000000..28085d4
--- /dev/null
+++ b/docs/specs/add-cow.txt
@@ -0,0 +1,68 @@
+== General ==
+
+Raw file format does not support backing_file and copy on write feature.
+The add-cow image format makes it possible to use backing files with raw
+image by keeping a separate .add-cow metadata file.  Once all sectors
+have been written to in the raw image it is safe to discard the .add-cow
+and backing files and instead use the raw image directly.
+
+When using add-cow, procedures may like this:
+(ubuntu.img is a disk image which has been installed OS.)
+    1)  Create a raw image with the same size of ubuntu.img
+            qemu-img create -f raw test.raw 8G
+    2)  Create a add-cow image which will store dirty bitmap
+            qemu-img create -f add-cow test.add-cow -o backing_file=ubuntu.img,image_file=test.raw
+    3)  Run qemu with add-cow image
+            qemu -drive if=virtio,file=test.add-cow
+
+=Specification=
+
+The file format looks like this:
+
+ +---------------+--------------------------+
+ |     Header    |         COW bitmap       |
+ +---------------+--------------------------+
+
+All numbers in add-cow are stored in Big Endian byte order.
+
+== Header ==
+
+The Header is included in the first bytes:
+
+    Byte  0 -  7:       magic
+                        add-cow magic string ("ADD_COW\xff")
+
+          8 -  11:      version
+                        Version number (only valid value is 1 now)
+
+          12 - 1035:    backing_file
+                        backing_file file name related to add-cow file. All
+                        unused bytes are padded with zeros. Must not be longer
+                        than 1023 bytes.
+
+         1036 - 2059:   image_file
+                        image_file is a raw file. All unused bytes are padded
+                        with zeros. Must not be longer than 1023 bytes.
+
+         2060  - 2559:   The Reserved field is used to make sure Data field starts
+                        at the multiple of 512, not used currently. All bytes are
+                        filled with 0.
+
+== COW bitmap ==
+
+The "COW bitmap" field starts at the 2560th byte, stores a bitmap related to
+backing_file and image_file. The bitmap will track whether the sector in
+backing_file is dirty or not.
+
+Each bit in the bitmap indicates one cluster's status. One cluster includes 128
+sectors, then each bit indicates 512 * 128 = 64k bytes, So the size of bitmap is
+calculated according to virtual size of image_file. In each byte, bit 0 to 7
+will track the 1st to 7th cluster in sequence, bit orders in one byte look like:
+ +----+----+----+----+----+----+----+----+
+ | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
+ +----+----+----+----+----+----+----+----+
+
+If the bit is 0, indicates the sector has not been allocated in image_file, data
+should be loaded from backing_file while reading; if the bit is 1,  indicates the
+related sector has been dirty, should be loaded from image_file while reading.
+Writing to a sector causes the corresponding bit to be set to 1.
diff --git a/qemu-img.c b/qemu-img.c
index 0ae543c..29865b5 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -628,7 +628,9 @@ static int img_convert(int argc, char **argv)
     int c, ret = 0, n, n1, bs_n, bs_i, compress, cluster_size, cluster_sectors;
     int progress = 0, flags;
     const char *fmt, *out_fmt, *cache, *out_baseimg, *out_filename;
+    char image_filename[1024], backing_filename[1024];
     BlockDriver *drv, *proto_drv;
+    BlockDriver *image_drv, *backing_drv;
     BlockDriverState **bs = NULL, *out_bs = NULL;
     int64_t total_sectors, nb_sectors, sector_num, bs_offset;
     uint64_t bs_sectors;
@@ -637,6 +639,7 @@ static int img_convert(int argc, char **argv)
     BlockDriverInfo bdi;
     QEMUOptionParameter *param = NULL, *create_options = NULL;
     QEMUOptionParameter *out_baseimg_param;
+    QEMUOptionParameter *image_param = NULL;
     char *options = NULL;
     const char *snapshot_name = NULL;
     float local_progress;
@@ -797,6 +800,7 @@ static int img_convert(int argc, char **argv)
         out_baseimg = out_baseimg_param->value.s;
     }
 
+    image_param = get_option_parameter(param, BLOCK_OPT_SIZE);
     /* Check if compression is supported */
     if (compress) {
         QEMUOptionParameter *encryption =
@@ -828,6 +832,41 @@ static int img_convert(int argc, char **argv)
     }
 
     /* Create the new image */
+
+    if (0 == strcmp(out_fmt, "add-cow")) {
+        image_drv = bdrv_find_format("raw");
+        if (!drv) {
+            ret = -1;
+            goto out;
+        }
+        snprintf(image_filename, sizeof(image_filename),
+            "%s"".ct.raw", out_filename);
+        ret = bdrv_create(image_drv, image_filename, image_param);
+        if (ret < 0) {
+            error_report("%s: error while creating image_file: %s",
+                     image_filename, strerror(-ret));
+            goto out;
+        }
+        set_option_parameter(param, BLOCK_OPT_IMAGE_FILE, image_filename);
+
+        if (!out_baseimg) {
+            backing_drv = bdrv_find_format("qcow2");
+            if (!drv) {
+                ret = -1;
+                goto out;
+            }
+            snprintf(backing_filename, sizeof(backing_filename),
+                "%s"".ct.qcow2", out_filename);
+            ret = bdrv_create(backing_drv, backing_filename, image_param);
+            if (ret < 0) {
+                error_report("%s: error while creating backing_file: %s",
+                         backing_filename, strerror(-ret));
+                goto out;
+            }
+            set_option_parameter(param, BLOCK_OPT_BACKING_FILE,
+                backing_filename);
+        }
+    }
     ret = bdrv_create(drv, out_filename, param);
     if (ret < 0) {
         if (ret == -ENOTSUP) {
