[Qemu-block] [PATCH 1/2] docs: Describe using images in writing iotests

[Fam Zheng]

Signed-off-by: Fam Zheng <address@hidden>
---
 docs/devel/testing.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index 5e19cd50da..8e1fa3a66e 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -255,6 +255,17 @@ comparable library support for invoking and interacting 
with QEMU programs. If
 you opt for Python, it is strongly recommended to write Python 3 compatible
 code.
 
+Both Python and Bash frameworks in iotests provide helpers to manage test
+images. They can be used to create and clean up images under the test
+directory. If no I/O or any protocol specific feature is needed, it is often
+more convenient to use the pseudo block driver, ``null-co://``, as the test
+image, which doesn't require image creation or cleaning up. Avoid system-wide
+devices or files whenever possible, such as ``/dev/null`` or ``/dev/zero``.
+Otherwise, image locking implications have to be considered.  For example,
+another application on the host may have locked the file, possibly leading to a
+test failure.  If using such devices are explicitly desired, consider adding
+``locking=off`` option to disable image locking.
+
 Docker based tests
 ==================
 
-- 
2.17.1