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+== 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::
+(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.
+
+=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 null 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 set that can not
+                        be recognized, the add-cow file should be droped. They are not
+                        used in v1.
+
+                        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.
+
+            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 is 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. The bitmap will track whether the sector in
+backing file is dirty 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
+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,
+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.
+
+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.
