docs/interop/prl-xml: description of Parallels Disk format

This patch adds main information about Parallels Disk
format, which consists of DiskDescriptor.xml and other files.

Signed-off-by: Edgar Kaziakhmedov <edgar.kaziakhmedov@virtuozzo.com>
Signed-off-by: Klim Kireev <klim.kireev@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Message-id: 20180112090122.1702-2-klim.kireev@virtuozzo.com
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/interop/prl-xml.txt

+= License =
+
+Copyright (c) 2015-2017, Virtuozzo, Inc.
+Authors:
+        2015 Denis Lunev <den@openvz.org>
+        2015 Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
+        2016-2017 Klim Kireev <klim.kireev@virtuozzo.com>
+        2016-2017 Edgar Kaziakhmedov <edgar.kaziakhmedov@virtuozzo.com>
+
+This work is licensed under the terms of the GNU GPL, version 2 or later.
+See the COPYING file in the top-level directory.
+
+This specification contains minimal information about Parallels Disk Format,
+which is enough to proper work with QEMU. Nevertheless, Parallels Cloud Server
+and Parallels Desktop are able to add some unspecified nodes to xml and use
+them, but they are for internal work and don't affect functionality. Also it
+uses auxiliary xml "Snapshot.xml", which allows to store optional snapshot
+information, but it doesn't influence open/read/write functionality. QEMU and
+other software should not use fields not covered in this document and
+Snapshot.xml file and must leave them as is.
+
+= Parallels Disk Format =
+
+Parallels disk consists of two parts: the set of snapshots and the disk
+descriptor file, which stores information about all files and snapshots.
+
+== Definitions ==
+    Snapshot       a record of the contents captured at a particular time,
+                   capable of storing current state. A snapshot has UUID and
+                   parent UUID.
+
+ Snapshot image    an overlay representing the difference between this
+                   snapshot and some earlier snapshot.
+
+    Overlay        an image storing the different sectors between two captured
+                   states.
+
+   Root image      snapshot image with no parent, the root of snapshot tree.
+
+    Storage        the backing storage for a subset of the virtual disk. When
+                   there is more than one storage in a Parallels disk then that
+                   is referred to as a split image. In this case every storage
+                   covers specific address space area of the disk and has its
+                   particular root image. Split images are not considered here
+                   and are not supported. Each storage consists of disk
+                   parameters and a list of images. The list of images always
+                   contains a root image and may also contain overlays. The
+                   root image can be an expandable Parallels image file or
+                   plain. Overlays must be expandable.
+
+  Description      DiskDescriptor.xml stores information about disk parameters,
+     file          snapshots, storages.
+
+     Top           The overlay between actual state and some previous snapshot.
+   Snapshot        It is not a snapshot in the classical sense because it
+                   serves as the active image that the guest writes to.
+
+    Sector         a 512-byte data chunk.
+
+== Description file ==
+All information is placed in a single XML element Parallels_disk_image.
+The element has only one attribute "Version", that must be 1.0.
+Schema of DiskDescriptor.xml:
+
+<Parallels_disk_image Version="1.0">
+    <Disk_Parameters>
+        ...
+    </Disk_Parameters>
+    <StorageData>
+        ...
+    </StorageData>
+    <Snapshots>
+        ...
+    </Snapshots>
+</Parallels_disk_image>
+
+== Disk_Parameters element ==
+The Disk_Parameters element describes the physical layout of the virtual disk
+and some general settings.
+
+The Disk_Parameters element MUST contain the following child elements:
+    * Disk_size - number of sectors in the disk,
+                  desired size of the disk.
+    * Cylinders - number of the disk cylinders.
+    * Heads     - number of the disk heads.
+    * Sectors   - number of the disk sectors per cylinder
+                  (sector size is 512 bytes)
+                  Limitation: Product of the Heads, Sectors and Cylinders
+                  values MUST be equal to the value of the Disk_size parameter.
+    * Padding   - must be 0. Parallels Cloud Server and Parallels Desktop may
+                  use padding set to 1, however this case is not covered
+                  by this spec, QEMU and other software should not open
+                  such disks and should not create them.
+
+== StorageData element ==
+This element of the file describes the root image and all snapshot images.
+
+The StorageData element consists of the Storage child element, as shown below:
+<StorageData>
+    <Storage>
+        ...
+    </Storage>
+</StorageData>
+
+A Storage element has following child elements:
+    * Start     - start sector of the storage, in case of non split storage
+                  equals to 0.
+    * End       - number of sector following the last sector, in case of non
+                  split storage equals to Disk_size.
+    * Blocksize - storage cluster size, number of sectors per one cluster.
+                  Cluster size for each "Compressed" (see below) image in
+                  parallels disk must be equal to this field. Note: cluster
+                  size for Parallels Expandable Image is in 'tracks' field of
+                  its header (see docs/interop/parallels.txt).
+    * Several Image child elements.
+
+Each Image element has following child elements:
+    * GUID - image identifier, UUID in curly brackets.
+             For instance, {12345678-9abc-def1-2345-6789abcdef12}.
+             The GUID is used by the Snapshots element to reference images
+             (see below)
+    * Type - image type of the element. It can be:
+             "Plain" for raw files.
+             "Compressed" for expanding disks.
+    * File - path to image file. Path can be relative to DiskDecriptor.xml or
+             absolute.
+
+== Snapshots element ==
+The Snapshots element describes the snapshot relations with the snapshot tree.
+
+The element contains the set of Shot child elements, as shown below:
+<Snapshots>
+    <TopGUID> ... </TopGUID> /* Optional child element */
+    <Shot>
+        ...
+    </Shot>
+    <Shot>
+        ...
+    </Shot>
+    ...
+</Snapshots>
+
+Each Shot element contains the following child elements:
+    * GUID       - an image GUID.
+    * ParentGUID - GUID of the image of the parent snapshot.
+
+The software may traverse snapshots from child to parent using <ParentGUID>
+field as reference. ParentGUID of root snapshot is
+{00000000-0000-0000-0000-000000000000}. There should be only one root
+snapshot. Top snapshot could be described via two ways: via TopGUID child
+element of the Snapshots element or via predefined GUID
+{5fbaabe3-6958-40ff-92a7-860e329aab41}. If TopGUID is defined, predefined GUID is
+interpreted as usual GUID. All snapshot images (except Top Snapshot) should be
+opened read-only. There is another predefined GUID,
+BackupID = {704718e1-2314-44c8-9087-d78ed36b0f4e}, which is used by original and
+some third-party software for backup, QEMU and other software may operate with
+images with GUID = BackupID as usual, however, it is not recommended to use this
+GUID for new disks. Top snapshot cannot have this GUID.