Patchwork [v3] add add-cow file format

login
register
mail settings
Submitter Robert Wang
Date Oct. 12, 2011, 4:23 p.m.
Message ID <1318436596-6952-1-git-send-email-wdongxu@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/119251/
State New
Headers show

Comments

Robert Wang - Oct. 12, 2011, 4:23 p.m.
Add add-cow file format

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
---
 Makefile.objs          |    1 +
 block.c                |    2 +-
 block.h                |    1 +
 block/add-cow.c        |  412 ++++++++++++++++++++++++++++++++++++++++++++++++
 block_int.h            |    1 +
 docs/specs/add-cow.txt |   45 ++++++
 6 files changed, 461 insertions(+), 1 deletions(-)
 create mode 100644 block/add-cow.c
 create mode 100644 docs/specs/add-cow.txt
Robert Wang - Oct. 13, 2011, 7:41 a.m.
On Thu 13 Oct 2011 12:23:16 AM CST, Dong Xu Wang wrote:
> Add add-cow file format
>
> Signed-off-by: Dong Xu Wang<wdongxu@linux.vnet.ibm.com>
> ---
>   Makefile.objs          |    1 +
>   block.c                |    2 +-
>   block.h                |    1 +
>   block/add-cow.c        |  412 ++++++++++++++++++++++++++++++++++++++++++++++++
>   block_int.h            |    1 +
>   docs/specs/add-cow.txt |   45 ++++++
>   6 files changed, 461 insertions(+), 1 deletions(-)
>   create mode 100644 block/add-cow.c
>   create mode 100644 docs/specs/add-cow.txt
>
> diff --git a/Makefile.objs b/Makefile.objs
> index c849e51..624c04c 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 e865fab..c25241d 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) ||
> 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..d2538a2
> --- /dev/null
> +++ b/block/add-cow.c
> @@ -0,0 +1,412 @@
> +#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;
> +} QEMU_PACKED AddCowHeader;
> +
> +typedef struct BDRVAddCowState {
> +    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);
> +
> +    ret = bdrv_pread(bs->file, 0,&header, sizeof(header));
> +    if (ret != sizeof(header)) {
> +        goto fail;
> +    }
> +
> +    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC ||
> +        be32_to_cpu(header.version) != ADD_COW_VERSION) {
> +        ret = -1;
> +        goto fail;
> +    }
> +
> +    size = be64_to_cpu(header.size);
> +    bs->total_sectors = size / BDRV_SECTOR_SIZE;
> +
> +    QEMU_BUILD_BUG_ON(sizeof(state->image_file) != sizeof(header.image_file));
> +    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);
> +    state->bitmap = g_malloc0(state->bitmap_size);
> +
> +    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
> +            state->bitmap_size);
> +    if (ret != 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("");
> +        /* Relative to image or working dir, need discussion */
> +        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) {
> +            goto fail;
> +        }
> +    } else {
> +        ret = -ENOENT;
> +        goto fail;
> +    }
> +    return 0;
> + fail:
> +    if (state->bitmap) {
> +        g_free(state->bitmap);
> +        state->bitmap = NULL;
> +    }
> +    return ret;
> +}
> +
> +static inline void add_cow_set_bit(BlockDriverState *bs, int64_t bitnum)
> +{
> +    uint64_t offset = bitnum / 8;
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    state->bitmap[offset] |= (1<<  (bitnum % 8));
> +}
> +
> +static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
> +{
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    uint64_t offset = bitnum / 8;
> +    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;
> +    uint64_t bitmap_size = ((BDRVAddCowState *)(bs->opaque))->bitmap_size;
> +
> +    /* Beyond the end of bitmap, return error or read from backing_file? */
> +    if (((sector_num + nb_sectors + 7) / 8)>  bitmap_size) {
> +        return 0;
> +    }
> +
> +    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 i, ret = 0;
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    uint64_t start_pos = sector_num / 8;
> +    uint64_t end_pos = (sector_num + nb_sectors - 1) / 8;
> +
> +    if (start_pos>  state->bitmap_size) {
> +        return -1;
> +    }
> +
> +    for (i = 0; i<  nb_sectors; i++) {
> +        add_cow_set_bit(bs, sector_num + i);
> +    }
> +    ret = bdrv_pwrite_sync(bs->file,
> +        sizeof(AddCowHeader) + start_pos,
> +        state->bitmap + start_pos,
> +        MIN(((end_pos - start_pos)&  (~512)) + 512, state->bitmap_size - start_pos));
> +    return ret;
> +}
> +
> +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 / 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) {
> +        return -EINVAL;
> +    }
> +    /* Make sure image file exists */
> +    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.size = cpu_to_be64(image_sectors * BDRV_SECTOR_SIZE);
> +
> +    ret = bdrv_pwrite(bs, 0,&header, sizeof(header));
> +    if (ret<  0) {
> +        bdrv_close(bs);
> +        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 * BDRV_SECTOR_SIZE);
> +fail:
Sorry, "fail" label is not used. While testing, after apply the patch, 
please delete this line.
The patch has too many lines, so I did not re-send the new one. 
> +    bdrv_close(bs);
> +    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);
> +    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);
> +            ret = bdrv_co_readv(s->image_hd, sector_num, n,&hd_qiov);
> +            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);
> +                ret = bdrv_co_readv(bs->backing_hd, sector_num,
> +                                    n,&hd_qiov);
> +                if (ret<  0) {
> +                    goto fail;
> +                }
> +            } else {
> +                qemu_iovec_reset(&hd_qiov);
> +                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_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 = 0;
> +    QEMUIOVector hd_qiov;
> +    qemu_iovec_init(&hd_qiov, qiov->niov);
> +
> +    qemu_iovec_reset(&hd_qiov);
> +    qemu_iovec_copy(&hd_qiov, qiov, 0, remaining_sectors * BDRV_SECTOR_SIZE);
> +    ret = bdrv_co_writev(s->image_hd,
> +                     sector_num,
> +                     remaining_sectors,&hd_qiov);
> +    if (ret<  0) {
> +        goto fail;
> +    }
> +
> +    ret = add_cow_update_bitmap(bs, sector_num, remaining_sectors);
> +    if (ret<  0) {
> +        goto fail;
> +    }
> +fail:
> +    qemu_iovec_destroy(&hd_qiov);
> +    return ret;
> +}
> +
> +static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t offset)
> +{
> +    int ret = 0;
> +    int64_t image_sectors = offset/BDRV_SECTOR_SIZE;
> +    int64_t be_offset = cpu_to_be64(offset);
> +    BDRVAddCowState *state = bs->opaque;
> +    ret = bdrv_truncate(bs->file, ((image_sectors + 7)>>  3)
> +        + sizeof(AddCowHeader));
> +    if (ret<  0) {
> +        return ret;
> +    }
> +
> +    ret = bdrv_pwrite_sync(bs->file, offsetof(AddCowHeader, size),
> +&be_offset, sizeof(uint64_t));
> +    if (ret<  0) {
> +        return ret;
> +    }
> +
> +    ret = bdrv_truncate(state->image_hd, offset);
> +    return ret;
> +}
> +
> +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;
> +    int ret = bdrv_flush(state->image_hd);
> +    if (ret<  0) {
> +        return ret;
> +    }
> +    return bdrv_flush(bs->file);
> +}
> +
> +static BlockDriverAIOCB *add_cow_aio_flush(BlockDriverState *bs,
> +        BlockDriverCompletionFunc *cb, void *opaque)
> +{
> +    BDRVAddCowState *state = bs->opaque;
> +    int ret = bdrv_flush(state->image_hd);
> +    if (ret<  0) {
> +        return NULL;
> +    }
> +
> +    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/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
> new file mode 100644
> index 0000000..0443c6c
> --- /dev/null
> +++ b/docs/specs/add-cow.txt
> @@ -0,0 +1,45 @@
> +== General ==
> +
> +Raw file format does not support backing_file and copy on write feature. Then
> +you can use add-cow file realize these features.
> +
> +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  |   Data   |   Data   | ... |
> + +----------+----------+----------+-----+
> +
> + 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. While
> +                        using backing_file, must together with image_file.
> +
> +         1036 - 2059:   image_file
> +                        image_file is a raw file, While using image_file, must
> +                        together with image_file.
> +
> +         2060 - 2067:   size
> +                        Virtual disk size of image_file in bytes.
shu ming - Oct. 23, 2011, 3:28 p.m.
On 2011-10-13 0:23, Dong Xu Wang wrote:
> Add add-cow file format
>
> Signed-off-by: Dong Xu Wang<wdongxu@linux.vnet.ibm.com>
> ---
>   Makefile.objs          |    1 +
>   block.c                |    2 +-
>   block.h                |    1 +
>   block/add-cow.c        |  412 ++++++++++++++++++++++++++++++++++++++++++++++++
>   block_int.h            |    1 +
>   docs/specs/add-cow.txt |   45 ++++++
>   6 files changed, 461 insertions(+), 1 deletions(-)
>   create mode 100644 block/add-cow.c
>   create mode 100644 docs/specs/add-cow.txt
>
> diff --git a/Makefile.objs b/Makefile.objs
> index c849e51..624c04c 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 e865fab..c25241d 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) ||
> 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..d2538a2
> --- /dev/null
> +++ b/block/add-cow.c
> @@ -0,0 +1,412 @@
> +#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];
1024 is a magic number for me.  Can we have a meaningful macro?


> +    uint64_t size;
> +} QEMU_PACKED AddCowHeader;
> +
> +typedef struct BDRVAddCowState {
> +    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);
> +
> +    ret = bdrv_pread(bs->file, 0,&header, sizeof(header));
> +    if (ret != sizeof(header)) {
> +        goto fail;
> +    }
> +
> +    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC ||
> +        be32_to_cpu(header.version) != ADD_COW_VERSION) {
> +        ret = -1;
> +        goto fail;
> +    }
> +
> +    size = be64_to_cpu(header.size);
> +    bs->total_sectors = size / BDRV_SECTOR_SIZE;
> +
> +    QEMU_BUILD_BUG_ON(sizeof(state->image_file) != sizeof(header.image_file));
> +    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);
> +    state->bitmap = g_malloc0(state->bitmap_size);
> +
> +    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
> +            state->bitmap_size);
> +    if (ret != 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("");
> +        /* Relative to image or working dir, need discussion */
> +        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) {
> +            goto fail;
> +        }
> +    } else {
> +        ret = -ENOENT;
> +        goto fail;
> +    }
> +    return 0;
> + fail:
> +    if (state->bitmap) {
> +        g_free(state->bitmap);
> +        state->bitmap = NULL;
> +    }
> +    return ret;
> +}
> +
> +static inline void add_cow_set_bit(BlockDriverState *bs, int64_t bitnum)
> +{
> +    uint64_t offset = bitnum / 8;
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    state->bitmap[offset] |= (1<<  (bitnum % 8));
> +}
> +
> +static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
> +{
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    uint64_t offset = bitnum / 8;
> +    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;
> +    uint64_t bitmap_size = ((BDRVAddCowState *)(bs->opaque))->bitmap_size;
> +
> +    /* Beyond the end of bitmap, return error or read from backing_file? */
> +    if (((sector_num + nb_sectors + 7) / 8)>  bitmap_size) {
> +        return 0;
> +    }
> +
> +    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 i, ret = 0;
> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
> +    uint64_t start_pos = sector_num / 8;
> +    uint64_t end_pos = (sector_num + nb_sectors - 1) / 8;
> +
> +    if (start_pos>  state->bitmap_size) {
> +        return -1;
> +    }
> +
> +    for (i = 0; i<  nb_sectors; i++) {
> +        add_cow_set_bit(bs, sector_num + i);
> +    }
> +    ret = bdrv_pwrite_sync(bs->file,
> +        sizeof(AddCowHeader) + start_pos,
> +        state->bitmap + start_pos,
> +        MIN(((end_pos - start_pos)&  (~512)) + 512, state->bitmap_size - start_pos));
> +    return ret;
> +}
> +
> +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 / 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) {
> +        return -EINVAL;
> +    }
> +    /* Make sure image file exists */
> +    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.size = cpu_to_be64(image_sectors * BDRV_SECTOR_SIZE);
> +
> +    ret = bdrv_pwrite(bs, 0,&header, sizeof(header));
> +    if (ret<  0) {
> +        bdrv_close(bs);
> +        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 * BDRV_SECTOR_SIZE);
> +fail:
"fail" label is not used in this function.


> +    bdrv_close(bs);
> +    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);
> +    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);
> +            ret = bdrv_co_readv(s->image_hd, sector_num, n,&hd_qiov);
> +            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);
> +                ret = bdrv_co_readv(bs->backing_hd, sector_num,
> +                                    n,&hd_qiov);
> +                if (ret<  0) {
> +                    goto fail;
> +                }
> +            } else {
> +                qemu_iovec_reset(&hd_qiov);
> +                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_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 = 0;
> +    QEMUIOVector hd_qiov;
> +    qemu_iovec_init(&hd_qiov, qiov->niov);
> +

It looks like that the memory where "bs" is pointing to should be 
protected by a lock. Because "bs"
was passed to several BlockDriver entries like ".bdrv_co_writev', 
'bdrv_co_readv'&etc which can be executed
in parallel to access the memory where "bs" is pointing to.

> +    qemu_iovec_reset(&hd_qiov);
> +    qemu_iovec_copy(&hd_qiov, qiov, 0, remaining_sectors * BDRV_SECTOR_SIZE);
> +    ret = bdrv_co_writev(s->image_hd,
> +                     sector_num,
> +                     remaining_sectors,&hd_qiov);
> +    if (ret<  0) {
> +        goto fail;
> +    }
> +
> +    ret = add_cow_update_bitmap(bs, sector_num, remaining_sectors);
> +    if (ret<  0) {
> +        goto fail;
> +    }
> +fail:
> +    qemu_iovec_destroy(&hd_qiov);
> +    return ret;
> +}
> +
> +static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t offset)
> +{
> +    int ret = 0;
> +    int64_t image_sectors = offset/BDRV_SECTOR_SIZE;
> +    int64_t be_offset = cpu_to_be64(offset);
> +    BDRVAddCowState *state = bs->opaque;
> +    ret = bdrv_truncate(bs->file, ((image_sectors + 7)>>  3)
> +        + sizeof(AddCowHeader));
> +    if (ret<  0) {
> +        return ret;
> +    }
> +
> +    ret = bdrv_pwrite_sync(bs->file, offsetof(AddCowHeader, size),
> +&be_offset, sizeof(uint64_t));
> +    if (ret<  0) {
> +        return ret;
> +    }
> +
> +    ret = bdrv_truncate(state->image_hd, offset);
> +    return ret;
> +}
> +
> +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;
> +    int ret = bdrv_flush(state->image_hd);
> +    if (ret<  0) {
> +        return ret;
> +    }
> +    return bdrv_flush(bs->file);
> +}
> +
> +static BlockDriverAIOCB *add_cow_aio_flush(BlockDriverState *bs,
> +        BlockDriverCompletionFunc *cb, void *opaque)
> +{
> +    BDRVAddCowState *state = bs->opaque;
> +    int ret = bdrv_flush(state->image_hd);
> +    if (ret<  0) {
> +        return NULL;
> +    }
> +
> +    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/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
> new file mode 100644
> index 0000000..0443c6c
> --- /dev/null
> +++ b/docs/specs/add-cow.txt
> @@ -0,0 +1,45 @@
> +== General ==
> +
> +Raw file format does not support backing_file and copy on write feature. Then
> +you can use add-cow file realize these features.
> +
> +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  |   Data   |   Data   | ... |
> + +----------+----------+----------+-----+
> +
> + 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. While
> +                        using backing_file, must together with image_file.
> +
> +         1036 - 2059:   image_file
> +                        image_file is a raw file, While using image_file, must
> +                        together with image_file.
> +
> +         2060 - 2067:   size
> +                        Virtual disk size of image_file in bytes.

Patch

diff --git a/Makefile.objs b/Makefile.objs
index c849e51..624c04c 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 e865fab..c25241d 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) ||
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..d2538a2
--- /dev/null
+++ b/block/add-cow.c
@@ -0,0 +1,412 @@ 
+#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;
+} QEMU_PACKED AddCowHeader;
+
+typedef struct BDRVAddCowState {
+    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);
+
+    ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
+    if (ret != sizeof(header)) {
+        goto fail;
+    }
+
+    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC ||
+        be32_to_cpu(header.version) != ADD_COW_VERSION) {
+        ret = -1;
+        goto fail;
+    }
+
+    size = be64_to_cpu(header.size);
+    bs->total_sectors = size / BDRV_SECTOR_SIZE;
+
+    QEMU_BUILD_BUG_ON(sizeof(state->image_file) != sizeof(header.image_file));
+    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);
+    state->bitmap = g_malloc0(state->bitmap_size);
+
+    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
+            state->bitmap_size);
+    if (ret != 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("");
+        /* Relative to image or working dir, need discussion */
+        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) {
+            goto fail;
+        }
+    } else {
+        ret = -ENOENT;
+        goto fail;
+    }
+    return 0;
+ fail:
+    if (state->bitmap) {
+        g_free(state->bitmap);
+        state->bitmap = NULL;
+    }
+    return ret;
+}
+
+static inline void add_cow_set_bit(BlockDriverState *bs, int64_t bitnum)
+{
+    uint64_t offset = bitnum / 8;
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+    state->bitmap[offset] |= (1 << (bitnum % 8));
+}
+
+static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
+{
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+    uint64_t offset = bitnum / 8;
+    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;
+    uint64_t bitmap_size = ((BDRVAddCowState *)(bs->opaque))->bitmap_size;
+
+    /* Beyond the end of bitmap, return error or read from backing_file? */
+    if (((sector_num + nb_sectors + 7) / 8) > bitmap_size) {
+        return 0;
+    }
+
+    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 i, ret = 0;
+    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
+    uint64_t start_pos = sector_num / 8;
+    uint64_t end_pos = (sector_num + nb_sectors - 1) / 8;
+
+    if (start_pos > state->bitmap_size) {
+        return -1;
+    }
+
+    for (i = 0; i < nb_sectors; i++) {
+        add_cow_set_bit(bs, sector_num + i);
+    }
+    ret = bdrv_pwrite_sync(bs->file,
+        sizeof(AddCowHeader) + start_pos,
+        state->bitmap + start_pos,
+        MIN(((end_pos - start_pos) & (~512)) + 512, state->bitmap_size - start_pos));
+    return ret;
+}
+
+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 / 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) {
+        return -EINVAL;
+    }
+    /* Make sure image file exists */
+    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.size = cpu_to_be64(image_sectors * BDRV_SECTOR_SIZE);
+
+    ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
+    if (ret < 0) {
+        bdrv_close(bs);
+        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 * BDRV_SECTOR_SIZE);
+fail:
+    bdrv_close(bs);
+    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);
+    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);
+            ret = bdrv_co_readv(s->image_hd, sector_num, n, &hd_qiov);
+            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);
+                ret = bdrv_co_readv(bs->backing_hd, sector_num,
+                                    n, &hd_qiov);
+                if (ret < 0) {
+                    goto fail;
+                }
+            } else {
+                qemu_iovec_reset(&hd_qiov);
+                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_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 = 0;
+    QEMUIOVector hd_qiov;
+    qemu_iovec_init(&hd_qiov, qiov->niov);
+
+    qemu_iovec_reset(&hd_qiov);
+    qemu_iovec_copy(&hd_qiov, qiov, 0, remaining_sectors * BDRV_SECTOR_SIZE);
+    ret = bdrv_co_writev(s->image_hd,
+                     sector_num,
+                     remaining_sectors, &hd_qiov);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = add_cow_update_bitmap(bs, sector_num, remaining_sectors);
+    if (ret < 0) {
+        goto fail;
+    }
+fail:
+    qemu_iovec_destroy(&hd_qiov);
+    return ret;
+}
+
+static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t offset)
+{
+    int ret = 0;
+    int64_t image_sectors = offset/BDRV_SECTOR_SIZE;
+    int64_t be_offset = cpu_to_be64(offset);
+    BDRVAddCowState *state = bs->opaque;
+    ret = bdrv_truncate(bs->file, ((image_sectors + 7) >> 3)
+        + sizeof(AddCowHeader));
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = bdrv_pwrite_sync(bs->file, offsetof(AddCowHeader, size),
+        &be_offset, sizeof(uint64_t));
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = bdrv_truncate(state->image_hd, offset);
+    return ret;
+}
+
+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;
+    int ret = bdrv_flush(state->image_hd);
+    if (ret < 0) {
+        return ret;
+    }
+    return bdrv_flush(bs->file);
+}
+
+static BlockDriverAIOCB *add_cow_aio_flush(BlockDriverState *bs,
+        BlockDriverCompletionFunc *cb, void *opaque)
+{
+    BDRVAddCowState *state = bs->opaque;
+    int ret = bdrv_flush(state->image_hd);
+    if (ret < 0) {
+        return NULL;
+    }
+
+    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/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
new file mode 100644
index 0000000..0443c6c
--- /dev/null
+++ b/docs/specs/add-cow.txt
@@ -0,0 +1,45 @@ 
+== General ==
+
+Raw file format does not support backing_file and copy on write feature. Then
+you can use add-cow file realize these features.
+
+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  |   Data   |   Data   | ... |
+ +----------+----------+----------+-----+
+
+ 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. While
+                        using backing_file, must together with image_file.
+
+         1036 - 2059:   image_file
+                        image_file is a raw file, While using image_file, must
+                        together with image_file.
+
+         2060 - 2067:   size
+                        Virtual disk size of image_file in bytes.