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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 image by keeping a separate .add-cow metadata file.
+Once all clusters have been written into the image it is safe to
+discard the .add-cow and backing files, then we can use the image
+An example usage of add-cow would look like:
+(ubuntu.img is a disk image which has an installed OS.)
+ 1) Create a image, such as raw format, with the same size of
+ 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 is assumed to be
+"raw". backing_fmt can also be omitted, add-cow should do a probe
+operation and determine what the backing file's format is.
+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 40-43 bytes.)
+ Byte 0 - 3: magic
+ add-cow magic string ("ACOW").
+ 4 - 7: backing file name offset
+ Offset in the add-cow file at which the backing
+ file name is stored (NB: The string is not
+ If backing file name does NOT exist, this field
+ will be 0. Must be between 76 and [HEADER_SIZE
+ - 2](a file name must be at least 1 byte).
+ 8 - 11: 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 - 76] to fit in the reserved
+ part of the header. Backing file name offset
+ + size must be no more than HEADER_SIZE.
+ 12 - 15: 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 76 and
+ [HEADER_SIZE - 2]. Image file name size + offset
+ must be no more than HEADER_SIZE.
+ 16 - 19: image file name size
+ Length of the image file name in bytes.
+ Must be less than [HEADER_SIZE - 76] to fit in
+ the reserved part of the header.
+ 20 - 23: 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
+ 24 - 31: 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 now.
+ Bits 0-63: Reserved (set to 0)
+ 32 - 39: 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)
+ 40 - 43: HEADER_SIZE
+ The header field is variable-sized. This field
+ indicates how many bytes will be used to store
+ add-cow header. By default, it is maximum value
+ of 4096 and cluster size value.
+ 44 - 59: 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.
+ 60 - 75: 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.
+ 76 - [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
+cluster 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 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 cluster 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 cluster has been dirty, should
+be loaded from image file while reading. Writing to a cluster 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 image file is not an even multiple of cluster bytes, bits that
+correspond to bytes beyond the image 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