Re: [Qemu-devel] [PATCH V17 1/6] docs: document for add-cow file format

[Dong Xu Wang]

On Mon, Dec 10, 2012 at 11:39 PM, Kevin Wolf <address@hidden> wrote:
> Am 06.12.2012 07:51, schrieb Dong Xu Wang:
>> 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 |  154 
>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 154 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..24e9a11
>> --- /dev/null
>> +++ b/docs/specs/add-cow.txt
>> @@ -0,0 +1,154 @@
>> +== 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 a 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::
>
> Double colon.

Okay.

>
>> +(ubuntu.img is a disk image which has an 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.
>> +
>> +image_fmt can be omitted, in that case image_fmt should be set as "raw".
>
> By "should be set as" you mean "is assumed to be"?
>
Okay. Will fix.
>> +backing_fmt can also be omitted, add-cow should do a probe operation and 
>> determine
>
> This line takes more than 80 characters. More follow, I won't comment on
> each.
>
Okay, will fix.
>> +what the backing file's format is.
>> +
>> +=Specification=
>> +
>> +The file format looks like this:
>> +
>> + +---------------+-------------------------------+
>> + |     Header    |           COW bitmap          |
>> + +---------------+-------------------------------+
>> +
>> +All numbers in add-cow are stored in Little Endian byte order.
>> +
>> +== Header ==
>> +
>> +The Header is included in the first bytes:
>> +(HEADER_SIZE is defined in 44-47 bytes.)
>> +    Byte    0  -  3:    magic
>> +                        add-cow magic string ("ACOW").
>> +
>> +            4  -  7:    version
>> +                        Version number (only valid value is 1 now).
>> +
>> +            8  - 11:    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).
>> +
>> +            12 - 15:    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. Backing file name offset + size
>> +                        must be no more than HEADER_SIZE.
>> +
>> +            16 - 19:    image file name offset
>> +                        Offset in the add-cow file at which the image file 
>> name
>> +                        is stored (NB: The string is not NUL-terminated). It
>> +                        must be between 80 and [HEADER_SIZE - 2]. Image file
>> +                        name size + offset must be no more than HEADER_SIZE.
>> +
>> +            20 - 23:    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.
>> +
>> +            24 - 27:    cluster bits
>> +                        Number of bits that are used for addressing an 
>> offset
>> +                        within a cluster (1 << cluster_bits is the cluster 
>> size).
>> +                        Must not be less than 9 (i.e. 512 byte clusters).
>> +
>> +                        Note: qemu as of today has an implementation limit 
>> of 2 MB
>> +                        as the maximum cluster size and won't be able to 
>> open images
>> +                        with larger cluster sizes.
>> +
>> +            28 - 35:    features
>> +                        Bitmask of features. If a feature bit is set but 
>> not recognized,
>> +                        the add-cow file should be dropped. They are not 
>> used in v1.
>
> Does v1 mean header.version = 1? I think this is wrong, we will want to
> add incompatible feature flags without increasing header.version (that's
> the whole point of them)
Okay, will fix.
>
>> +
>> +                        Bits 0-63:  Reserved (set to 0)
>> +
>> +            36 - 43:    compatible features
>> +                        Bitmask of compatible features. An implementation 
>> can
>> +                        safely ignore any unknown bits that are set.
>> +                        Bit 0:      All allocated bit.  If this bit is set 
>> then
>> +                                    backing file and COW bitmap will not be 
>> used,
>> +                                    and can read from or write to image 
>> file directly.
>> +
>> +                        Bits 1-63:  Reserved (set to 0)
>> +
>> +            44 - 47:    HEADER_SIZE
>> +                        The header field is variable-sized. This field 
>> indicates
>> +                        how many bytes will be used to store add-cow header.
>> +                        In add-cow v1, it is fixed to 4096.
>
> Same question about v1. If it's fixed, why have a field for it?
Okay, will make more clear in next version.

>
>> +
>> +            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-empty, it must be non-empty. It is 
>> coded
>> +                        in free-form ASCII, and is not NUL-terminated. Zero
>> +                        padded on the right.
>> +
>> +            64 - 79:    image file format
>> +                        Format of image file. It must be non-empty. It is 
>> coded
>> +                        in free-form ASCII, and is not NUL-terminated. Zero
>> +                        padded on the right.
>> +
>> +            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 are not 
>> pointing to
>> +                        backing file and image file names must be set to 0.
>> +
>> +== COW bitmap ==
>> +
>> +The "COW bitmap" field starts at offset HEADER_SIZE, stores a bitmap 
>> related to
>> +backing file and image file.  It is tracking whether the sector in image 
>> file
>> +is allocated or not.
>> +
>> +Each bit in the bitmap tracks one cluster's status. For example, if cluster
>> +bit is 16, then each bit tracks one cluster, (1 << 16) = 65536 bytes. The
>
> clusters bit_s_
>
Okay. "cluster_bits is".
>> +image file size is rounded up to cluster size (where any bytes in the
>> +last cluster that do not fit in the image are ignored), then if the
>> +number of clusters is not a multiple of 8, then remaining bits in the
>> +bitmap will be set to 0.
>> +
>> +The size of bitmap is calculated according to virtual size of image file, 
>> and
>> +the size of bitmap should be multiple of add-cow file's cluster size, 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, it indicates the sector has not been allocated in image 
>> file,
>
> s/sector/cluster/
>
> More instances follow, not commenting on each.
>
Okay.

>> +data should be loaded from backing file while reading; if the bit is 1, it
>> +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 there is no backing file, or if the image file is larger than the backing
>> +file and the offset is beyond the end of the backing file, then the data 
>> should
>> +be read as all zero bytes instead.
>> +
>> +If raw image is not an even multiple of cluster bytes, bits that correspond 
>> to
>> +bytes beyond the raw file size in add-cow must be written as 0 and must be
>> +ignored when reading.
>
> Don't refer to a "raw image", it could be any image format.
>
Okay, will fix.
>> +
>> +Image file name and backing file name must NOT be the same, we prevent this
>> +while creating add-cow files via qemu-img. If image file name and backing 
>> file
>> +name are the same, the add-cow image must be treated as invalid.
>
> Kevin
>