===========
 Snapshots
===========

.. index:: Ceph Block Device; snapshots

A snapshot is a read-only logical copy of an image at a particular point in
time: a checkpoint. One of the advanced features of Ceph block devices is that
you can create snapshots of images to retain point-in-time state history.  Ceph
also supports snapshot layering, which allows you to clone images (for example,
VM images) quickly and easily. Ceph block device snapshots are managed using
the ``rbd`` command and several higher-level interfaces, including `QEMU`_,
`libvirt`_, `OpenStack`_, `OpenNebula`_ and `CloudStack`_.

.. important:: To use RBD snapshots, you must have a running Ceph cluster.


.. note:: Because RBD is unaware of any file system within an image (volume),
   snapshots are merely `crash-consistent` unless they are coordinated within
   the mounting (attaching) operating system. We therefore recommend that you
   pause or stop I/O before taking a snapshot.

   If the volume contains a file system, the file system should be in an
   internally consistent state before a snapshot is taken. Snapshots taken
   without write quiescing could need an `fsck` pass before they are mounted
   again. To quiesce I/O you can use `fsfreeze` command. See the `fsfreeze(8)`
   man page for more details.

   For virtual machines, `qemu-guest-agent` can be used to automatically freeze
   file systems when creating a snapshot.

.. ditaa::

           +------------+         +-------------+
           | {s}        |         | {s} c999    |
           |   Active   |<-------*|   Snapshot  |
           |   Image    |         |   of Image  |
           | (stop i/o) |         | (read only) |
           +------------+         +-------------+


Cephx Notes
===========

When :ref:`cephx <rados-cephx-config-ref>` authentication is enabled (it is by default), you must specify a
user name or ID and a path to the keyring containing the corresponding key. See
:ref:`User Management <user-management>` for details.

.. prompt:: bash $

	rbd --id {user-ID} --keyring /path/to/secret [commands]
	rbd --name {username} --keyring /path/to/secret [commands]

For example:

.. prompt:: bash $

	rbd --id admin --keyring /etc/ceph/ceph.keyring [commands]
	rbd --name client.admin --keyring /etc/ceph/ceph.keyring [commands]

.. tip:: Add the user and secret to the ``CEPH_ARGS`` environment variable to
   avoid re-entry of these parameters.


Snapshot Basics
===============

The following procedures demonstrate how to create, list, and remove
snapshots using the ``rbd`` command.

Create Snapshot
---------------

To create a snapshot, use the ``rbd snap create`` command and specify the pool
name, the image name, and the snap name:

.. prompt:: bash $

   rbd snap create {pool-name}/{image-name}@{snap-name}

For example:

.. prompt:: bash $

   rbd snap create rbd/foo@snapname


List Snapshots
--------------

To list the snapshots of an image, use the ``rbd snap ls`` command and specify
the pool name and the image name:

.. prompt:: bash $

   rbd snap ls {pool-name}/{image-name}

For example:

.. prompt:: bash $

   rbd snap ls rbd/foo


Roll back Snapshot
------------------

To roll back to a snapshot,  use the ``rbd snap rollback`` command and specify
the pool name, the image name, and the snap name:

.. prompt:: bash $

   rbd snap rollback {pool-name}/{image-name}@{snap-name}

For example:

.. prompt:: bash $

   rbd snap rollback rbd/foo@snapname


.. note:: Rolling back an image to a snapshot means overwriting the current
   version of the image with data from a snapshot. The time it takes to execute
   a rollback increases with the size of the image. It is **faster to clone**
   from a snapshot **than to roll back** an image to a snapshot. Cloning from a
   snapshot is the preferred method of returning to a pre-existing state.


Delete a Snapshot
-----------------

To delete a snapshot, use the ``rbd snap rm`` command and specify the pool
name, the image name, and the snap name:

.. prompt:: bash $

   rbd snap rm {pool-name}/{image-name}@{snap-name}

For example:

.. prompt:: bash $

   rbd snap rm rbd/foo@snapname


.. note:: Ceph OSDs delete data asynchronously, so deleting a snapshot  does
   not immediately free up the capacity of the underlying OSDs. This process is
   known as "snaptrim", and is referred to as such in ``ceph status`` output.

Purge Snapshots
---------------

To delete all snapshots, use the ``rbd snap purge`` command and specify the
pool name and the image name:

.. prompt:: bash $

   rbd snap purge {pool-name}/{image-name}

For example:

.. prompt:: bash $

   rbd snap purge rbd/foo


.. index:: Ceph Block Device; snapshot layering

Layering
========

Ceph supports the ability to create many copy-on-write (COW) clones of a block
device snapshot. Snapshot layering enables Ceph block device clients to create
images very quickly. For example, you might create a block device image with a
Linux VM written to it, snapshot the image, protect the snapshot, and create as
many copy-on-write clones as you like. A snapshot is read-only, so cloning a
snapshot simplifies semantics, making it possible to create clones rapidly.


.. ditaa::

           +-------------+              +-------------+
           | {s} c999    |              | {s}         |
           |  Snapshot   | Child refers |  COW Clone  |
           |  of Image   |<------------*| of Snapshot |
           |             |  to Parent   |             |
           | (read only) |              | (writable)  |
           +-------------+              +-------------+

               Parent                        Child

.. note:: The terms "parent" and "child" refer to a Ceph block device snapshot
   (parent) and the corresponding image cloned from the snapshot (child).
   These terms are important for the command line usage below.

Each cloned image (child) stores a reference to its parent image, which enables
the cloned image to open the parent snapshot and read it.

A copy-on-write clone of a snapshot behaves exactly like any other Ceph
block device image. You can read to, write from, clone, and resize cloned
images. There are no special restrictions with cloned images. However, the
copy-on-write clone of a snapshot depends on the snapshot, so you must
protect the snapshot before you clone it. The diagram below depicts this
process.

.. note:: Ceph supports the cloning of only "RBD format 2" images (that is,
   images created without specifying ``--image-format 1``). The Linux kernel
   client supports cloned images beginning with the 3.10 release.

Getting Started with Layering
-----------------------------

Ceph block device layering is a simple process. You must have an image. You
must create a snapshot of the image. You must protect the snapshot. After you
have performed these steps, you can begin cloning the snapshot.

.. ditaa::

           +----------------------------+        +-----------------------------+
           |                            |        |                             |
           | Create Block Device Image  |------->|      Create a Snapshot      |
           |                            |        |                             |
           +----------------------------+        +-----------------------------+
                                                                |
                         +--------------------------------------+
                         |
                         v
           +----------------------------+        +-----------------------------+
           |                            |        |                             |
           |   Protect the Snapshot     |------->|     Clone the Snapshot      |
           |                            |        |                             |
           +----------------------------+        +-----------------------------+


The cloned image has a reference to the parent snapshot, and includes the pool
ID, the image ID, and the snapshot ID. The inclusion of the pool ID means that
you may clone snapshots from one pool to images in another pool.

#. **Image Template:** A common use case for block device layering is to create
   a base image and a snapshot that serves as a template for clones. For
   example: a user may create an image for a Linux distribution (for example,
   Ubuntu 22.04) and create a snapshot of it. The user may occasionally update
   the image and create a new snapshot (by using such commands as ``sudo
   apt-get update``, ``sudo apt-get upgrade``, or ``sudo apt-get dist-upgrade``
   followed by ``rbd snap create``). As the image matures, the user can clone
   any one of the snapshots.

#. **Extended Template:** A more advanced use case includes extending a
   template image to provide more information than a base image. For
   example, a user may clone an image (for example, a VM template) and install
   other software (for example, a database, a content management system, an
   analytics system) and then snapshot the extended image, which may itself be
   updated just like the base image.

#. **Template Pool:** One way to use block device layering is to create a pool
   that contains (1) base images that act as templates and (2) snapshots of
   those templates. You may then extend read-only privileges to users so that
   they may clone the snapshots even though they do not have permissions that
   allow them to write or execute within the pool.

#. **Image Migration/Recovery:** One way to use block device layering is to
   migrate or recover data from one pool into another pool.

Protecting a Snapshot
---------------------

Clones access the parent snapshots. All clones would break if a user
inadvertently deleted the parent snapshot. To prevent data loss, you must
protect the snapshot before you can clone it:

.. prompt:: bash $

   rbd snap protect {pool-name}/{image-name}@{snapshot-name}

For example:

.. prompt:: bash $

   rbd snap protect rbd/foo@snapname

.. note:: You cannot delete a protected snapshot.

Cloning a Snapshot
------------------

To clone a snapshot, specify the parent pool, the parent image, and the parent
snapshot; and also the child pool together with the image name. You must
protect the snapshot before you can clone it:

.. prompt:: bash $

   rbd clone {pool-name}/{parent-image-name}@{snap-name} {pool-name}/{child-image-name}

For example:

.. prompt:: bash $

   rbd clone rbd/foo@snapname rbd/bar


.. note:: You may clone a snapshot from one pool to an image in another pool.
   For example, you may maintain read-only images and snapshots as templates in
   one pool, and writeable clones in another pool.

Unprotecting a Snapshot
-----------------------

Before you can delete a snapshot, you must first unprotect it. Additionally,
you may *NOT* delete snapshots that have references from clones. You must
flatten or delete each clone of a snapshot before you can unprotect the
snapshot:

.. prompt:: bash $

   rbd snap unprotect {pool-name}/{image-name}@{snapshot-name}

For example:

.. prompt:: bash $

   rbd snap unprotect rbd/foo@snapname


Listing Children of a Snapshot
------------------------------

To list the children of a snapshot, use the ``rbd children`` command and
specify the pool name, the image name, and the snap name:

.. prompt:: bash $

   rbd children {pool-name}/{image-name}@{snapshot-name}

For example:

.. prompt:: bash $

   rbd children rbd/foo@snapname


Flattening a Cloned Image
-------------------------

Cloned images retain a reference to the parent snapshot. When you remove the
reference to the parent snapshot from the clone, you effectively "flatten" the
clone by copying the data stored in the snapshot to the clone.  The time it
takes to flatten a clone increases with the size of the snapshot.  To delete a
snapshot, you must first flatten the child images (or delete them):

.. prompt:: bash $

   rbd flatten {pool-name}/{image-name}

For example:

.. prompt:: bash $

   rbd flatten rbd/bar

.. note:: Since a flattened image contains all the data stored in the snapshot,
   a flattened image takes up more storage space than a layered clone does.


.. _QEMU: ../qemu-rbd/
.. _OpenStack: ../rbd-openstack/
.. _OpenNebula: https://docs.opennebula.io/stable/management_and_operations/vm_management/vm_instances.html?highlight=ceph#managing-disk-snapshots
.. _CloudStack: ../rbd-cloudstack/
.. _libvirt: ../libvirt/
