Parallels Disk Format — QEMU documentation

# Parallels Disk Format

This specification contains minimal information about Parallels Disk Format, which is enough to properly work with QEMU. Nevertheless, Parallels Cloud Server and Parallels Desktop are able to add some unspecified nodes to the xml and use them, but they are for internal work and don’t affect functionality. Also it uses auxiliary xml `Snapshot.xml`, which allows storage of optional snapshot information, but this doesn’t influence open/read/write functionality. QEMU and other software should not use fields not covered in this document or the `Snapshot.xml` file, and must leave them as is.

A 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 a UUID and a 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

  a snapshot image with no parent, the root of the 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 a 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 file

  `DiskDescriptor.xml` stores information about disk parameters, snapshots, and storages.

Top Snapshot

  The overlay between actual state and some previous 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`, which must be `1.0`.

The 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: The 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 specification. 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 the 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. The cluster size for each “Compressed” (see below) image in a parallels disk must be equal to this field. Note: the cluster size for a Parallels Expandable Image is in the `tracks` field of its header (see Parallels Expandable Image File Format).

- Several `Image` child elements.

Each `Image` element has the 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. The path can be relative to `DiskDescriptor.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 the `<ParentGUID>` field as reference. The `ParentGUID` of the root snapshot is `{00000000-0000-0000-0000-000000000000}`. There should be only one root snapshot.

The Top snapshot could be described via two ways: via the `TopGUID` child element of the `Snapshots` element, or via the predefined GUID `{5fbaabe3-6958-40ff-92a7-860e329aab41}`. If `TopGUID` is defined, the predefined GUID is interpreted as a normal GUID. All snapshot images (except the 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. The Top snapshot cannot have this GUID.