[1/5] docs/interop/prl-xml: description of Parallels Disk format
diff mbox

Message ID 1513595351-5899-2-git-send-email-den@openvz.org
State New
Headers show

Commit Message

Denis V. Lunev Dec. 18, 2017, 11:09 a.m. UTC
From: Klim Kireev <klim.kireev@virtuozzo.com>

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>
CC: Stefan Hajnoczi <stefanha@redhat.com>
---
 docs/interop/prl-xml.txt | 155 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 155 insertions(+)
 create mode 100644 docs/interop/prl-xml.txt

Comments

Stefan Hajnoczi Jan. 4, 2018, 11:34 a.m. UTC | #1
On Mon, Dec 18, 2017 at 02:09:07PM +0300, Denis V. Lunev wrote:
> From: Klim Kireev <klim.kireev@virtuozzo.com>
> 
> 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>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/interop/prl-xml.txt | 155 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 155 insertions(+)
>  create mode 100644 docs/interop/prl-xml.txt
> 
> diff --git a/docs/interop/prl-xml.txt b/docs/interop/prl-xml.txt
> new file mode 100644
> index 0000000..8ccb91a
> --- /dev/null
> +++ b/docs/interop/prl-xml.txt
> @@ -0,0 +1,155 @@
> += License =
> +
> +Copyright (c) 2015 Denis Lunev
> +Copyright (c) 2015 Vladimir Sementsov-Ogievskiy
> +Copyright (c) 2016-2017 Klim Kireev
> +Copyright (c) 2016-2017 Edgar Kaziakhmedov
> +
> +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 unspecified here fields and Snapshot.xml file

s/unspecified here fields/fields not covered in this document/

> +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 without parent, the root of snapshot tree.

s/without parent/with no parent/

> +
> +    Storage        a special conception of data, which consists of disk
> +                   parameters and a list of images. One of this image is root,
> +                   others are overlays. Images must be expandable (parallels
> +                   image file), however root image could be expandable or
> +                   plain. There may be more then one storage in the Parallels

s/then/than/

> +                   disk and it is defined as a split image.

I was having trouble understanding this paragraph.  At this point I can
begin to see that split images consist of multiple storages.  That makes
the concept of storage clearer.

Perhaps rephrase this paragraph as:

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.  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.

> +                   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 isn't supported

s/isn't/aren't/

> +                   in QEMU.
> +
> +  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 classical meaning.

To make this clearer the last line could be:

  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 single XML section Parallels_disk_image.
> +The section has only one attribute "Version", that must be 1.0.
> +General structure of DiskDescriptor.xml:
> +
> +<Parallels_disk_image Version="1.0">
> +    <Disk_Parameters>
> +        ...
> +    </Disk_Parameters>
> +    <StorageData>
> +        ...
> +    </StorageData>
> +    <Snapshots>
> +        ...
> +    </Snapshots>
> +</Parallels_disk_image>
> +
> +== Disk_Parameters section ==
> +The Disk_Parameters section describes the physical layout of the virtual disk
> +and some general settings.
> +
> +The Disk_Parameters section MUST contain the following subsections:
> +    * 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.
> +                  Attention: this field affects the read/write functionality.

What does "Attention: this field affects the read/write functionality" mean?

> +
> +== StorageData section ==
> +This section of the file describes root image and all snapshot images.

s/root image/the root image/

> +
> +The StorageData section consists of the Storage subsection, as shown below:
> +<StorageData>
> +    <Storage>
> +        ...
> +    </Storage>
> +</StorageData>
> +
> +A Storage descriptor consists of the following subsections:
> +    * Start     - start sector of the storage, equals to 0.
> +    * End       - number of sectors in storage, equals to Disk_size.

Just to be clear, if Start > 0 then End is still Disk_size and not Start
+ Disk_size?  Please tweak the wording to make this clear.

> +    * 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).

What is the purpose of this field if the Parallels Expandable Image
header also has the same value?

> +    * Several Image subsections.
> +
> +Each Image section consists of the following subsections:

I'm confused by the use of "section" and "subsections" here.  This spec
is ambiguous and does not describe the XML clearly.  Is a "section"
always an XML tag and a "subsection" is a child XML tag?

In other words, are we talking about:

  <Image>
      <GUID>{12345678-9abc-def1-2345-6789abcdef12}</GUID>
      <Type>Plain</Type>
      <File>my_file</File>
  </Image>

Or could it be:

  <Image GUID="{12345678-9abc-def1-2345-6789abcdef12}"
         Type="Plain"
	 File="my_file" />

?

Please use XML terminology ("tag", "attribute", etc) instead of
"section"/"subsection".

> +    * GUID - image identifier, UUID in curly brackets.
> +             For instance, {12345678-9abc-def1-2345-6789abcdef12}.

How is the GUID used?  Does it need to be validated against the GUID in
the image file?

> +    * Type - image type of the element. It can be:
> +             "Plain" for plain disks.

What is a plain disk?  Is this a raw file or a Parallels Expandable
Image file?

> +             "Compressed" for expanding disks.
> +    * File - path to image file. Path can be relative to DiskDecriptor.xml or
> +             absolute.
> +
> +== Snapshots section ==
> +The Snapshots section describes the snapshot relations with the snapshot tree.
> +
> +The section contains the set of Shot subsections, as shown below:
> +<Snapshots>
> +    <TopGUID> ... </TopGUID> //Optional subsection
> +    <Shot>
> +        ...
> +    </Shot>
> +    <Shot>
> +        ...
> +    </Shot>
> +    ...
> +</Snapshots>
> +
> +Each Shot section contains the following subsections:
> +    * 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 one root
> +snapshot. Top snapshot could be described via two ways: via TopGUID subsection
> +in the Snapshots section 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) sould be

s/sould/should/

> +opened read-only.
> +There is another predefined GIUD,

s/GIUD/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.

> +NOTE: To address top snapshot QEMU supports only predefined GUID mode.

This sounds like an implementation limitation and a spec violation
(since the spec says <TopGUID> may be used).  Please implement <TopGUID>
support unless it's really hard to do, or drop <TopGUID> from this spec
so that it's clear this feature doesn't need to be supported by QEMU and
other open source software.
Klim Kireev Jan. 10, 2018, 4:23 p.m. UTC | #2
On 01/04/2018 02:34 PM, Stefan Hajnoczi wrote:
> On Mon, Dec 18, 2017 at 02:09:07PM +0300, Denis V. Lunev wrote:
>> From: Klim Kireev <klim.kireev@virtuozzo.com>
>>
>> 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>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>   docs/interop/prl-xml.txt | 155 +++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 155 insertions(+)
>>   create mode 100644 docs/interop/prl-xml.txt
>>
>> diff --git a/docs/interop/prl-xml.txt b/docs/interop/prl-xml.txt
>> new file mode 100644
>> index 0000000..8ccb91a
>> --- /dev/null
>> +++ b/docs/interop/prl-xml.txt
>> @@ -0,0 +1,155 @@
>> += License =
>> +
>> +Copyright (c) 2015 Denis Lunev
>> +Copyright (c) 2015 Vladimir Sementsov-Ogievskiy
>> +Copyright (c) 2016-2017 Klim Kireev
>> +Copyright (c) 2016-2017 Edgar Kaziakhmedov
>> +
>> +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 unspecified here fields and Snapshot.xml file
> s/unspecified here fields/fields not covered in this document/
>
>> +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 without parent, the root of snapshot tree.
> s/without parent/with no parent/
>
>> +
>> +    Storage        a special conception of data, which consists of disk
>> +                   parameters and a list of images. One of this image is root,
>> +                   others are overlays. Images must be expandable (parallels
>> +                   image file), however root image could be expandable or
>> +                   plain. There may be more then one storage in the Parallels
> s/then/than/
>
>> +                   disk and it is defined as a split image.
> I was having trouble understanding this paragraph.  At this point I can
> begin to see that split images consist of multiple storages.  That makes
> the concept of storage clearer.
>
> Perhaps rephrase this paragraph as:
>
> 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.  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.
>
>> +                   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 isn't supported
> s/isn't/aren't/
>
>> +                   in QEMU.
>> +
>> +  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 classical meaning.
> To make this clearer the last line could be:
>
>    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 single XML section Parallels_disk_image.
>> +The section has only one attribute "Version", that must be 1.0.
>> +General structure of DiskDescriptor.xml:
>> +
>> +<Parallels_disk_image Version="1.0">
>> +    <Disk_Parameters>
>> +        ...
>> +    </Disk_Parameters>
>> +    <StorageData>
>> +        ...
>> +    </StorageData>
>> +    <Snapshots>
>> +        ...
>> +    </Snapshots>
>> +</Parallels_disk_image>
>> +
>> +== Disk_Parameters section ==
>> +The Disk_Parameters section describes the physical layout of the virtual disk
>> +and some general settings.
>> +
>> +The Disk_Parameters section MUST contain the following subsections:
>> +    * 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.
>> +                  Attention: this field affects the read/write functionality.
> What does "Attention: this field affects the read/write functionality" mean?
>
>> +
>> +== StorageData section ==
>> +This section of the file describes root image and all snapshot images.
> s/root image/the root image/
>
>> +
>> +The StorageData section consists of the Storage subsection, as shown below:
>> +<StorageData>
>> +    <Storage>
>> +        ...
>> +    </Storage>
>> +</StorageData>
>> +
>> +A Storage descriptor consists of the following subsections:
>> +    * Start     - start sector of the storage, equals to 0.
>> +    * End       - number of sectors in storage, equals to Disk_size.
> Just to be clear, if Start > 0 then End is still Disk_size and not Start
> + Disk_size?  Please tweak the wording to make this clear.
>
>> +    * 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).
> What is the purpose of this field if the Parallels Expandable Image
> header also has the same value?
I suppose that it is needed for creating snapshots over an raw root image.
>> +    * Several Image subsections.
>> +
>> +Each Image section consists of the following subsections:
> I'm confused by the use of "section" and "subsections" here.  This spec
> is ambiguous and does not describe the XML clearly.  Is a "section"
> always an XML tag and a "subsection" is a child XML tag?
>
> In other words, are we talking about:
>
>    <Image>
>        <GUID>{12345678-9abc-def1-2345-6789abcdef12}</GUID>
>        <Type>Plain</Type>
>        <File>my_file</File>
>    </Image>
>
> Or could it be:
>
>    <Image GUID="{12345678-9abc-def1-2345-6789abcdef12}"
>           Type="Plain"
> 	 File="my_file" />
>
> ?
>
> Please use XML terminology ("tag", "attribute", etc) instead of
> "section"/"subsection".
>
>> +    * GUID - image identifier, UUID in curly brackets.
>> +             For instance, {12345678-9abc-def1-2345-6789abcdef12}.
> How is the GUID used?  Does it need to be validated against the GUID in
> the image file?
Parallels images don't contain GUID, what do you mean?
>
>> +    * Type - image type of the element. It can be:
>> +             "Plain" for plain disks.
> What is a plain disk?  Is this a raw file or a Parallels Expandable
> Image file?
>
>> +             "Compressed" for expanding disks.
>> +    * File - path to image file. Path can be relative to DiskDecriptor.xml or
>> +             absolute.
>> +
>> +== Snapshots section ==
>> +The Snapshots section describes the snapshot relations with the snapshot tree.
>> +
>> +The section contains the set of Shot subsections, as shown below:
>> +<Snapshots>
>> +    <TopGUID> ... </TopGUID> //Optional subsection
>> +    <Shot>
>> +        ...
>> +    </Shot>
>> +    <Shot>
>> +        ...
>> +    </Shot>
>> +    ...
>> +</Snapshots>
>> +
>> +Each Shot section contains the following subsections:
>> +    * 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 one root
>> +snapshot. Top snapshot could be described via two ways: via TopGUID subsection
>> +in the Snapshots section 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) sould be
> s/sould/should/
>
>> +opened read-only.
>> +There is another predefined GIUD,
> s/GIUD/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.
>> +NOTE: To address top snapshot QEMU supports only predefined GUID mode.
> This sounds like an implementation limitation and a spec violation
> (since the spec says <TopGUID> may be used).  Please implement <TopGUID>
> support unless it's really hard to do, or drop <TopGUID> from this spec
> so that it's clear this feature doesn't need to be supported by QEMU and
> other open source software.
Stefan Hajnoczi Jan. 11, 2018, 3:33 p.m. UTC | #3
On Wed, Jan 10, 2018 at 07:23:29PM +0300, klim wrote:
> On 01/04/2018 02:34 PM, Stefan Hajnoczi wrote:
> > On Mon, Dec 18, 2017 at 02:09:07PM +0300, Denis V. Lunev wrote:
> > > +    * GUID - image identifier, UUID in curly brackets.
> > > +             For instance, {12345678-9abc-def1-2345-6789abcdef12}.
> > How is the GUID used?  Does it need to be validated against the GUID in
> > the image file?
> Parallels images don't contain GUID, what do you mean?

At this point in the spec nothing has used the GUID yet, so I was
wondering what purpose it serves.  After reading the Snapshots section
below I saw that the GUID is used there.

It would be nice to include something like "The GUID is used by the
Snapshots section to reference images (see below)" so that the reader
knows why this field is needed.

Patch
diff mbox

diff --git a/docs/interop/prl-xml.txt b/docs/interop/prl-xml.txt
new file mode 100644
index 0000000..8ccb91a
--- /dev/null
+++ b/docs/interop/prl-xml.txt
@@ -0,0 +1,155 @@ 
+= License =
+
+Copyright (c) 2015 Denis Lunev
+Copyright (c) 2015 Vladimir Sementsov-Ogievskiy
+Copyright (c) 2016-2017 Klim Kireev
+Copyright (c) 2016-2017 Edgar Kaziakhmedov
+
+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 unspecified here fields 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 without parent, the root of snapshot tree.
+
+    Storage        a special conception of data, which consists of disk
+                   parameters and a list of images. One of this image is root,
+                   others are overlays. Images must be expandable (parallels
+                   image file), however root image could be expandable or
+                   plain. There may be more then one storage in the Parallels
+                   disk and it is defined 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 isn't supported
+                   in QEMU.
+
+  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 classical meaning.
+
+    Sector         a 512-byte data chunk.
+
+== Description file ==
+All information is placed in single XML section Parallels_disk_image.
+The section has only one attribute "Version", that must be 1.0.
+General structure of DiskDescriptor.xml:
+
+<Parallels_disk_image Version="1.0">
+    <Disk_Parameters>
+        ...
+    </Disk_Parameters>
+    <StorageData>
+        ...
+    </StorageData>
+    <Snapshots>
+        ...
+    </Snapshots>
+</Parallels_disk_image>
+
+== Disk_Parameters section ==
+The Disk_Parameters section describes the physical layout of the virtual disk
+and some general settings.
+
+The Disk_Parameters section MUST contain the following subsections:
+    * 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.
+                  Attention: this field affects the read/write functionality.
+
+== StorageData section ==
+This section of the file describes root image and all snapshot images.
+
+The StorageData section consists of the Storage subsection, as shown below:
+<StorageData>
+    <Storage>
+        ...
+    </Storage>
+</StorageData>
+
+A Storage descriptor consists of the following subsections:
+    * Start     - start sector of the storage, equals to 0.
+    * End       - number of sectors in 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 subsections.
+
+Each Image section consists of the following subsections:
+    * GUID - image identifier, UUID in curly brackets.
+             For instance, {12345678-9abc-def1-2345-6789abcdef12}.
+    * Type - image type of the element. It can be:
+             "Plain" for plain disks.
+             "Compressed" for expanding disks.
+    * File - path to image file. Path can be relative to DiskDecriptor.xml or
+             absolute.
+
+== Snapshots section ==
+The Snapshots section describes the snapshot relations with the snapshot tree.
+
+The section contains the set of Shot subsections, as shown below:
+<Snapshots>
+    <TopGUID> ... </TopGUID> //Optional subsection
+    <Shot>
+        ...
+    </Shot>
+    <Shot>
+        ...
+    </Shot>
+    ...
+</Snapshots>
+
+Each Shot section contains the following subsections:
+    * 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 one root
+snapshot. Top snapshot could be described via two ways: via TopGUID subsection
+in the Snapshots section 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) sould be
+opened read-only.
+There is another predefined GIUD,
+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.
+
+NOTE: To address top snapshot QEMU supports only predefined GUID mode.