[PATCH v2] Document qemu-img options data_file and data_file_raw

[Connor Kuehl <ckuehl@redhat.com>]

The contents of this patch were initially developed and posted by Han
Han[1], however, it appears the original patch was not applied. Since
then, the relevant documentation has been moved and adapted to a new
format.

I've taken most of the original wording and tweaked it according to
some of the feedback from the original patch submission. I've also
adapted it to restructured text, which is the format the documentation
currently uses.

[1] https://lists.nongnu.org/archive/html/qemu-block/2019-10/msg01253.html

Fixes: https://bugzilla.redhat.com/1763105
Signed-off-by: Han Han <hhan@redhat.com>
Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
---
Changes since v1:
  * Clarify different behaviors with these options when using qemu-img
    create vs amend (Max)
  * Touch on the negative case of how the file becomes inconsistent
    (John)

 docs/tools/qemu-img.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index c9efcfaefc..87b4a65535 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -866,6 +866,26 @@ Supported image file formats:
     issue ``lsattr filename`` to check if the NOCOW flag is set or not
     (Capital 'C' is NOCOW flag).
 
+  ``data_file``
+    Filename where all guest data will be stored. If this option is used,
+    the qcow2 file will only contain the image's metadata.
+
+    Note: Data loss will occur if the given filename already exists when
+    using this option with ``qemu-img create`` since ``qemu-img`` will create
+    the data file anew, overwriting the file's original contents. To simply
+    update the reference to point to the given pre-existing file, use
+    ``qemu-img amend``.
+
+  ``data_file_raw``
+    If this option is set to ``on``, QEMU will always keep the external
+    data file consistent as a standalone read-only raw image. It does
+    this by forwarding updates through to the raw image in addition to
+    updating the image metadata. If set to ``off``, QEMU will only
+    update the image metadata without forwarding the changes through
+    to the raw image. The default value is ``off``.
+
+    This option can only be enabled if ``data_file`` is set.
+
 ``Other``
 
   QEMU also supports various other image file formats for
-- 
2.30.2