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