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 <wdon...@linux.vnet.ibm.com>
> ---
> 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
