--- /dev/null
+===========
+ 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:: If a snapshot is taken while `I/O` is still in progress in a image, the
+ snapshot might not get the exact or latest data of the image and the snapshot
+ may have to be cloned to a new image to be mountable. So, we recommend to 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. 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
+`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 shapshot. 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
+ 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/my-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/
+.. _User Management: ../../operations/user-management
+.. _QEMU: ../qemu-rbd/
+.. _OpenStack: ../rbd-openstack/
+.. _CloudStack: ../rbd-cloudstack/
+.. _libvirt: ../libvirt/
Code Review / stor4nfv.git / blobdiff