ceph/doc/rbd/rbd-snapshot.rst

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

.. index:: Ceph Block Device; snapshots

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

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

.. note:: Because RBD does not know about the filesystem, snapshots are
	  `crash-consistent` if they are not coordinated with the mounting
	  computer. So, we recommend you stop `I/O` before taking a snapshot of
	  an image. If the image contains a filesystem, the filesystem must be
	  in a consistent state before taking a snapshot or you may have to run
	  `fsck`. To stop `I/O` you can use `fsfreeze` command. See
	  `fsfreeze(8)` man page for more details.
	  For virtual machines, `qemu-guest-agent` can be used to automatically
	  freeze filesystems when creating a snapshot.

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


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

When `cephx`_ is enabled (it is by default), you must specify a user name or ID
and a path to the keyring containing the corresponding key for the user. See
:ref:`User Management <user-management>` for details. You may also add the ``CEPH_ARGS`` environment
variable to avoid re-entry of the following parameters. ::

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

For example:: 

	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 so that you don't need to enter them each time.


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

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

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

To create a snapshot with ``rbd``, specify the ``snap create`` option,  the pool
name and the image name.  ::

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

For example:: 

	rbd snap create rbd/foo@snapname
	

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

To list snapshots of an image, specify the pool name and the image name. ::

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

For example::

	rbd snap ls rbd/foo


Rollback Snapshot
-----------------

To rollback to a snapshot with ``rbd``, specify the ``snap rollback`` option, the
pool name, the image name and the snap name. ::

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

For example::

	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 rollback** 
   an image to a snapshot, and it is the preferred method of returning
   to a pre-existing state.


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

To delete a snapshot with ``rbd``, specify the ``snap rm`` option, the pool
name, the image name and the snap name. ::

	rbd snap rm {pool-name}/{image-name}@{snap-name}
	
For example:: 

	rbd snap rm rbd/foo@snapname


.. note:: Ceph OSDs delete data asynchronously, so deleting a snapshot 
   doesn't free up the disk space immediately.

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

To delete all snapshots for an image with ``rbd``, specify the ``snap purge``
option and the image name. ::

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

For example:: 

	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; then, 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" mean 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 COW 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 refers to the snapshot, so you **MUST** protect the snapshot before
you clone it. The following diagram depicts the process.

.. note:: Ceph only supports cloning for format 2 images (i.e., created with
   ``rbd create --image-format 2``).  The kernel client supports cloned images
   since kernel 3.10.

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. Once 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,  image ID and 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
   master image and a snapshot that serves as a template for clones. For example, 
   a user may create an image for a Linux distribution (e.g., Ubuntu 12.04), and 
   create a snapshot for it. Periodically, the user may update the image and create
   a new snapshot (e.g., ``sudo apt-get update``, ``sudo apt-get upgrade``,
   ``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 that provides more information than a base image. For example, a user may
   clone an image (e.g., a VM template) and install other software (e.g., a database,
   a content management system, an analytics system, etc.) and then snapshot the 
   extended image, which itself may be updated just like the base image.

#. **Template Pool:** One way to use block device layering is to create a 
   pool that contains master images that act as templates, and snapshots of those
   templates. You may then extend read-only privileges to users so that they 
   may clone the snapshots without the ability 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. ::

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

For example::

	rbd snap protect rbd/my-image@my-snapshot

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

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

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

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

	rbd clone rbd/my-image@my-snapshot rbd/new-image
	
.. 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 unprotect it first. Additionally,
you may *NOT* delete snapshots that have references from clones. You must
flatten each clone of a snapshot, before you can delete the snapshot. :: 

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

For example::

	rbd snap unprotect rbd/my-image@my-snapshot


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

To list the children of a snapshot, execute the following::

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

For example::

	rbd children rbd/my-image@my-snapshot


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

Cloned images retain a reference to the parent snapshot. When you remove the
reference from the child clone to the parent snapshot, you effectively "flatten"
the image by copying the information from 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 flatten the child images first. ::

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

For example:: 

	rbd flatten rbd/new-image

.. note:: Since a flattened image contains all the information from the snapshot, 
   a flattened image will take up more storage space than a layered clone.


.. _cephx: ../../rados/configuration/auth-config-ref/
.. _QEMU: ../qemu-rbd/
.. _OpenStack: ../rbd-openstack/
.. _CloudStack: ../rbd-cloudstack/
.. _libvirt: ../libvirt/