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+Testing - unit tests
+====================
+
+The Ceph GitHub repository has two types of tests: unit tests (also called
+``make check`` tests) and integration tests. Strictly speaking, the
+``make check`` tests are not "unit tests", but rather tests that can be run
+easily on a single build machine after compiling Ceph from source, whereas
+integration tests require package installation and multi-machine clusters to
+run.
+
+.. _make-check:
+
+What does "make check" mean?
+----------------------------
+
+After compiling Ceph, the code can be run through a battery of tests. For
+historical reasons, this is often referred to as ``make check`` even though
+the actual command used to run the tests is now ``ctest``. To be included in
+this group of tests, a test must:
+
+* bind ports that do not conflict with other tests
+* not require root access
+* not require more than one machine to run
+* complete within a few minutes
+
+For the sake of simplicity, this class of tests is referred to as "make
+check tests" or "unit tests". This is meant to distinguish these tests from
+the more complex "integration tests" that are run via the `teuthology
+framework`_.
+
+While it is possible to run ``ctest`` directly, it can be tricky to correctly
+set up your environment for it. Fortunately, there is a script that makes it
+easy to run the unit tests on your code. This script can be run from the
+top-level directory of the Ceph source tree by invoking:
+
+  .. prompt:: bash $
+
+     ./run-make-check.sh
+
+You will need a minimum of 8GB of RAM and 32GB of free drive space for this
+command to complete successfully on x86_64 architectures; other architectures
+may have different requirements. Depending on your hardware, it can take from
+twenty minutes to three hours to complete.
+
+
+How unit tests are declared
+---------------------------
+
+Unit tests are declared in the ``CMakeLists.txt`` file, which is found in the
+``./src`` directory. The ``add_ceph_test`` and ``add_ceph_unittest`` CMake
+functions are used to declare unit tests.  ``add_ceph_test`` and
+``add_ceph_unittest`` are themselves defined in
+``./cmake/modules/AddCephTest.cmake``. 
+
+Some unit tests are scripts and other unit tests are binaries that are
+compiled during the build process.  
+
+* ``add_ceph_test`` function - used to declare unit test scripts 
+* ``add_ceph_unittest`` function - used for unit test binaries
+
+Unit testing of CLI tools
+-------------------------
+Some of the CLI tools are tested using special files ending with the extension
+``.t`` and stored under ``./src/test/cli``. These tests are run using a tool
+called `cram`_ via a shell script called ``./src/test/run-cli-tests``.
+`cram`_ tests that are not suitable for ``make check`` can also be run by
+teuthology using the `cram task`_.
+
+.. _`cram`: https://bitheap.org/cram/
+.. _`cram task`: https://github.com/ceph/ceph/blob/master/qa/tasks/cram.py
+
+Tox-based testing of Python modules
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Some of the Python modules in Ceph use `tox <https://tox.readthedocs.io/en/latest/>`_ 
+to run their unit tests.
+
+Most of these Python modules can be found in the directory ``./src/pybind/``.
+
+Currently (December 2020) the following modules use **tox**:
+
+* Cephadm (``./src/cephadm/tox.ini``)
+* Ceph Manager Python API (``./src/pybind/mgr``)
+
+  * ``./src/pybind/mgr/tox.ini``
+    
+  * ``./src/pybind/mgr/dashboard/tox.ini``
+
+  * ``./src/pybind/tox.ini``
+
+* Dashboard (``./src/pybind/mgr/dashboard``)
+* Python common (``./src/python-common/tox.ini``)
+* CephFS (``./src/tools/cephfs/tox.ini``)
+* ceph-volume
+
+  * ``./src/ceph-volume/tox.ini``
+
+  * ``./src/ceph-volume/plugin/zfs/tox.ini``
+
+  * ``./src/ceph-volume/ceph_volume/tests/functional/batch/tox.ini``
+
+  * ``./src/ceph-volume/ceph_volume/tests/functional/simple/tox.ini``
+
+  * ``./src/ceph-volume/ceph_volume/tests/functional/lvm/tox.ini``
+
+Configuring Tox environments and tasks 
+""""""""""""""""""""""""""""""""""""""
+Most tox configurations support multiple environments and tasks. 
+
+The list of environments and tasks that are supported is in the ``tox.ini``
+file, under ``envlist``. For example, here are the first three lines of
+``./src/cephadm/tox.ini``::
+
+   [tox]
+   envlist = py3, mypy
+   skipsdist=true
+
+In this example, the ``Python 3`` and ``mypy`` environments are specified.
+
+The list of environments can be retrieved with the following command:
+
+  .. prompt:: bash $
+
+     tox --list
+
+Or:
+
+  .. prompt:: bash $
+
+     tox -l
+
+Running Tox
+"""""""""""
+To run **tox**, just execute ``tox`` in the directory containing
+``tox.ini``.  If you do not specify any environments (for example, ``-e
+$env1,$env2``), then ``tox`` will run all environments. Jenkins will run
+``tox`` by executing ``./src/script/run_tox.sh``.
+
+Here are some examples from Ceph Dashboard that show how to specify different
+environments and run options::
+
+  ## Run Python 2+3 tests+lint commands:
+  $ tox -e py27,py3,lint,check
+
+  ## Run Python 3 tests+lint commands:
+  $ tox -e py3,lint,check
+
+  ## To run it as Jenkins would:  
+  $ ../../../script/run_tox.sh --tox-env py3,lint,check
+
+Manager core unit tests
+"""""""""""""""""""""""
+
+Currently only doctests_ inside ``mgr_util.py`` are run.
+
+To add more files to be tested inside the core of the manager, open the
+``tox.ini`` file and add the files to be tested  at the end of the line that
+includes ``mgr_util.py``.
+
+.. _doctests: https://docs.python.org/3/library/doctest.html
+
+Unit test caveats
+-----------------
+
+#. Unlike the various Ceph daemons and ``ceph-fuse``, the unit tests are
+   linked against the default memory allocator (glibc) unless they are
+   explicitly linked against something else. This enables tools such as
+   **valgrind** to be used in the tests.
+
+#. Google Test unit testing library hides the client output from the shell.
+   In order to debug the client after setting the desired debug level
+   (e.g ``ceph config set client debug_rbd 20``), the debug log file can
+   be found at ``build/out/client.admin.<pid>.log``.
+   This can also be handy when examining teuthology failed unit test
+   jobs, the job's debug level can be set at the relevant yaml file.
+
+.. _make check:
+.. _teuthology framework: https://github.com/ceph/teuthology