On Mon, Dec 10, 2012 at 11:39 PM, Kevin Wolf <kw...@redhat.com> 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 <wdon...@linux.vnet.ibm.com>
>> ---
>> 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
>
