[Qemu-devel] [PATCH V18 1/6] docs: document for add-cow file format

[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>
---
V17->V18:
1) remove version field.
2) header size is maximum value and cluster size value.
3) fix type.
 docs/specs/add-cow.txt | 165 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)
 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..151028b
--- /dev/null
+++ b/docs/specs/add-cow.txt
@@ -0,0 +1,165 @@
+== 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
+directly.
+
+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
+        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 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.
+
+=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 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
+                        lNUL-terminated).
+                        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
+                        sizes.
+
+            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
+invalid.
-- 
1.7.11.7