I re-sent the patch and use bdrv_co_flush now, thanks.

2011/10/27 shu ming <shum...@linux.vnet.ibm.com>:
> Looks good to me.  A nit, it seems that bdrv_flush is not supported anymore
> in upstream.
> bdrv_co_flush should be used instead if you update your workspace to latest
> one.
> On 2011-10-26 18:08, Robert Wang wrote:
>>
>> Please find version 4 in the attachment.
>>
>> 2011/10/23 shu ming<shum...@linux.vnet.ibm.com>:
>>>
>>> On 2011-10-13 0:23, Dong Xu Wang wrote:
>>>>
>>>> Add add-cow file format
>>>>
>>>> Signed-off-by: Dong Xu Wang<wdon...@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.
>>>
>>>
>>>
>>
>>
>
>
>



-- 
Regards
Robert Wang

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