Re: [Qemu-devel] [PATCH V12 1/6] docs: document for add-cow file format

[Dong Xu Wang]

On Fri, Sep 7, 2012 at 1:27 AM, Michael Roth <address@hidden> wrote:
> On Fri, Aug 10, 2012 at 11:39:40PM +0800, Dong Xu Wang wrote:
>> Document for add-cow format, the usage and spec of add-cow are introduced.
>>
>> Signed-off-by: Dong Xu Wang <address@hidden>
>> ---
>>  docs/specs/add-cow.txt |  123 
>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 123 insertions(+), 0 deletions(-)
>>  create mode 100644 docs/specs/add-cow.txt
>>
>> diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
>> new file mode 100644
>> index 0000000..d5a7a68
>> --- /dev/null
>> +++ b/docs/specs/add-cow.txt
>> @@ -0,0 +1,123 @@
>> +== General ==
>> +
>> +The raw file format does not support backing files or 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 into the raw image it is safe to discard the .add-cow
>> +and backing files, then we can use the raw image directly.
>> +
>> +An example usage of add-cow would look like::
>> +(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 an 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
>> +
>> +test.raw may be larger than ubuntu.img, in that case, the size of 
>> test.add-cow
>> +will be calculated from the size of test.raw.
>> +
>> +=Specification=
>> +
>> +The file format looks like this:
>> +
>> + +---------------+-------------+-----------------+
>> + |     Header    |   Reserved  |    COW bitmap   |
>> + +---------------+-------------+-----------------+
>> +
>> +All numbers in add-cow are stored in Little Endian byte order.
>> +
>> +== Header ==
>> +
>> +The Header is included in the first bytes:
>> +(#define HEADER_SIZE (4096 * header_pages_size))
>> +    Byte    0 -  7:     magic
>> +                        add-cow magic string ("ADD_COW\xff").
>> +
>> +            8 -  11:    version
>> +                        Version number (only valid value is 1 now).
>> +
>> +            12 - 15:    backing file name offset
>> +                        Offset in the add-cow file at which the backing file
>> +                        name is stored (NB: The string is not 
>> nul-terminated).
>> +                        If backing file name does NOT exist, this field 
>> will be
>> +                        0. Must be between 80 and [HEADER_SIZE - 2](a file 
>> name
>> +                        must be at least 1 byte).
>> +
>> +            16 - 19:    backing file name size
>> +                        Length of the backing file name in bytes. It will 
>> be 0
>> +                        if the backing file name offset is 0. If backing 
>> file
>> +                        name offset is non-zero, then it must be non-zero. 
>> Must
>> +                        be less than [HEADER_SIZE - 80] to fit in the 
>> reserved
>> +                        part of the header.
>> +
>> +            20 - 23:    image file name offset
>> +                        Offset in the add-cow file at which the image file 
>> name
>> +                        is stored (NB: The string is not null terminated). 
>> It
>> +                        must be between 80 and [HEADER_SIZE - 2].
>> +
>> +            24 - 27:    image file name size
>> +                        Length of the image file name in bytes.
>> +                        Must be less than [HEADER_SIZE - 80] to fit in the 
>> reserved
>> +                        part of the header.
>> +
>> +            28 - 35:    features
>> +                        Currently only 1 feature bit is used:
>> +                        Feature bits:
>> +                            * ADD_COW_F_All_ALLOCATED   = 0x01.
>> +
>> +            36 - 43:    optional features
>> +                        Not used now. Reserved for future use. It must be 
>> set to 0.
>> +
>> +            44 - 47:    header pages size
>> +                        The header field is variable-sized. This field 
>> indicates
>> +                        how many pages(4k) will be used to store add-cow 
>> header.
>> +                        In add-cow v1, it is fixed to 1, so the header size 
>> will
>> +                        be 4k * 1 = 4096 bytes.
>> +
>> +            48 - 63:    backing file format
>> +                        format of backing file. It will be filled with 0 if
>> +                        backing file name offset is 0. If backing file name
>> +                        offset is non-zero, it must be non-zero. It is coded
>> +                        in free-form ASCII, and is not NUL-terminated.
>> +
>> +            64 - 79:    image file format
>> +                        format of image file. It must be non-zero. It is 
>> coded
>> +                        in free-form ASCII, and is not NUL-terminated.
>> +
>> +            80 - [HEADER_SIZE - 1]:
>> +                        It is used to make sure COW bitmap field starts at 
>> the
>> +                        HEADER_SIZE byte, backing file name and image file 
>> name
>> +                        will be stored here. The bytes that is not pointing 
>> to
>> +                        backing file and image file names will bet set to 0.
>> +
>> +== COW bitmap ==
>> +
>> +The "COW bitmap" field starts at offset HEADER_SIZE, 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. the size of bitmap 
>> is
>> +calculated according to virtual size of image file, and it also should be 
>> multipe
>> +of 65536, the bits not used will be set to 0. Within each byte, the least
>> +significant bit covers the first cluster. 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.
>> +
>> +If raw image is not an even multiple of cluster bytes, bits that correspond 
>> to
>> +bytes beyond the raw file size in add-cow will be 0.
>> +
>> +Image file name and backing file name must NOT be the same, we prevent this
>> +while creating add-cow files.
>> +
>> +Image file and backing file are interpreted relative to the qcow2 file, not
>
> Relative to the add-cow file?
Ah, yes..
>
>> +to the current working directory of the process that opened the qcow2 file.

>> --
>> 1.7.1
>>
>>
>