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

[Kevin Wolf]

Am 10.08.2012 17:39, 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 |  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:

What happens when opening a file with an unknown bit set? How must
unknown bits be initialised?

> +                        Feature bits:
> +                            * ADD_COW_F_All_ALLOCATED   = 0x01.

What does this flag mean, and is it required to be set on that
condition? Also, please use ALL_CAPS.

> +
> +            36 - 43:    optional features
> +                        Not used now. Reserved for future use. It must be 
> set to 0.

And must be ignored when reading.

> +
> +            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.

Why arbitrarily defined "pages" instead of bytes or at least clusters?

> +
> +            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.

Zero padded on the right, I guess?

Also defining that a string must be "non-zero" looks odd, should
probably be "non-empty".

> +
> +            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.

Same here.

> +
> +            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.

"will be set to 0" describes the behaviour of qemu. A spec should
describe the file format, not a specific implementation. Make it "must"
or "should".

> +
> +== 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.

Should we make the cluster size configurable?

> the size of bitmap is
> +calculated according to virtual size of image file, and it also should be 
> multipe

Typo: multiple

Sure you mean "should", or should it be "must"?

> +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.

"must be written as 0 and must be ignored when reading" or something
like that.

> +Image file name and backing file name must NOT be the same, we prevent this
> +while creating add-cow files.

What we do is irrelevant for a spec.

> +Image file and backing file are interpreted relative to the qcow2 file, not
> +to the current working directory of the process that opened the qcow2 file.

Kevin