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Refine the text in rbd-snapshot.rst https://tracker.ceph.com/issues/57001 Signed-off-by: Zac Dover <zac.dover@gmail.com>
369 lines
12 KiB
ReStructuredText
369 lines
12 KiB
ReStructuredText
===========
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Snapshots
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===========
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.. index:: Ceph Block Device; snapshots
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A snapshot is a read-only logical copy of an image at a particular point in
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time: a checkpoint. One of the advanced features of Ceph block devices is that
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you can create snapshots of images to retain point-in-time state history. Ceph
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also supports snapshot layering, which allows you to clone images (for example,
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VM images) quickly and easily. Ceph block device snapshots are managed using
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the ``rbd`` command and several higher-level interfaces, including `QEMU`_,
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`libvirt`_, `OpenStack`_, and `CloudStack`_.
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.. important:: To use RBD snapshots, you must have a running Ceph cluster.
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.. note:: Because RBD is unaware of any file system within an image (volume),
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snapshots are merely `crash-consistent` unless they are coordinated within
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the mounting (attaching) operating system. We therefore recommend that you
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pause or stop I/O before taking a snapshot.
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If the volume contains a file system, the file system should be in an
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internally consistent state before a snapshot is taken. Snapshots taken
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without write quiescing could need an `fsck` pass before they are mounted
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again. To quiesce I/O you can use `fsfreeze` command. See the `fsfreeze(8)`
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man page for more details.
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For virtual machines, `qemu-guest-agent` can be used to automatically freeze
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file systems when creating a snapshot.
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.. ditaa::
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+------------+ +-------------+
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| {s} | | {s} c999 |
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| Active |<-------*| Snapshot |
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| Image | | of Image |
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| (stop i/o) | | (read only) |
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+------------+ +-------------+
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Cephx Notes
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===========
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When `cephx`_ authentication is enabled (it is by default), you must specify a
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user name or ID and a path to the keyring containing the corresponding key. See
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:ref:`User Management <user-management>` for details.
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.. prompt:: bash $
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rbd --id {user-ID} --keyring /path/to/secret [commands]
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rbd --name {username} --keyring /path/to/secret [commands]
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For example:
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.. prompt:: bash $
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rbd --id admin --keyring /etc/ceph/ceph.keyring [commands]
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rbd --name client.admin --keyring /etc/ceph/ceph.keyring [commands]
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.. tip:: Add the user and secret to the ``CEPH_ARGS`` environment variable to
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avoid re-entry of these parameters.
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Snapshot Basics
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===============
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The following procedures demonstrate how to create, list, and remove
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snapshots using the ``rbd`` command.
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Create Snapshot
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---------------
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To create a snapshot, use the ``rbd snap create`` command and specify the pool
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name, the image name, and the snap name:
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.. prompt:: bash $
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rbd snap create {pool-name}/{image-name}@{snap-name}
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For example:
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.. prompt:: bash $
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rbd snap create rbd/foo@snapname
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List Snapshots
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--------------
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To list the snapshots of an image, use the ``rbd snap ls`` command and specify
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the pool name and the image name:
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.. prompt:: bash $
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rbd snap ls {pool-name}/{image-name}
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For example:
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.. prompt:: bash $
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rbd snap ls rbd/foo
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Roll back Snapshot
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------------------
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To roll back to a snapshot, use the ``rbd snap rollback`` command and specify
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the pool name, the image name, and the snap name:
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.. prompt:: bash $
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rbd snap rollback {pool-name}/{image-name}@{snap-name}
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For example:
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.. prompt:: bash $
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rbd snap rollback rbd/foo@snapname
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.. note:: Rolling back an image to a snapshot means overwriting the current
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version of the image with data from a snapshot. The time it takes to execute
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a rollback increases with the size of the image. It is **faster to clone**
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from a snapshot **than to roll back** an image to a snapshot. Cloning from a
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snapshot is the preferred method of returning to a pre-existing state.
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Delete a Snapshot
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-----------------
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To delete a snapshot, use the ``rbd snap rm`` command and specify the pool
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name, the image name, and the snap name:
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.. prompt:: bash $
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rbd snap rm {pool-name}/{image-name}@{snap-name}
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For example:
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.. prompt:: bash $
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rbd snap rm rbd/foo@snapname
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.. note:: Ceph OSDs delete data asynchronously, so deleting a snapshot does
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not immediately free up the capacity of the underlying OSDs. This process is
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known as "snaptrim", and is referred to as such in ``ceph status`` output.
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Purge Snapshots
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---------------
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To delete all snapshots, use the ``rbd snap purge`` command and specify the
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pool name and the image name:
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.. prompt:: bash $
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rbd snap purge {pool-name}/{image-name}
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For example:
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.. prompt:: bash $
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rbd snap purge rbd/foo
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.. index:: Ceph Block Device; snapshot layering
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Layering
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========
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Ceph supports the ability to create many copy-on-write (COW) clones of a block
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device snapshot. Snapshot layering enables Ceph block device clients to create
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images very quickly. For example, you might create a block device image with a
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Linux VM written to it, snapshot the image, protect the snapshot, and create as
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many copy-on-write clones as you like. A snapshot is read-only, so cloning a
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snapshot simplifies semantics, making it possible to create clones rapidly.
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.. ditaa::
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+-------------+ +-------------+
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| {s} c999 | | {s} |
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| Snapshot | Child refers | COW Clone |
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| of Image |<------------*| of Snapshot |
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| | to Parent | |
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| (read only) | | (writable) |
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+-------------+ +-------------+
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Parent Child
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.. note:: The terms "parent" and "child" refer to a Ceph block device snapshot
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(parent) and the corresponding image cloned from the snapshot (child).
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These terms are important for the command line usage below.
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Each cloned image (child) stores a reference to its parent image, which enables
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the cloned image to open the parent snapshot and read it.
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A copy-on-write clone of a snapshot behaves exactly like any other Ceph
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block device image. You can read to, write from, clone, and resize cloned
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images. There are no special restrictions with cloned images. However, the
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copy-on-write clone of a snapshot depends on the snapshot, so you must
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protect the snapshot before you clone it. The diagram below depicts this
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process.
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.. note:: Ceph supports the cloning of only "RBD format 2" images (that is,
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images created without specifying ``--image-format 1``). The Linux kernel
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client supports cloned images beginning with the 3.10 release.
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Getting Started with Layering
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-----------------------------
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Ceph block device layering is a simple process. You must have an image. You
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must create a snapshot of the image. You must protect the snapshot. After you
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have performed these steps, you can begin cloning the snapshot.
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.. ditaa::
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+----------------------------+ +-----------------------------+
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| | | |
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| Create Block Device Image |------->| Create a Snapshot |
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+----------------------------+ +-----------------------------+
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+--------------------------------------+
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v
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+----------------------------+ +-----------------------------+
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| Protect the Snapshot |------->| Clone the Snapshot |
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+----------------------------+ +-----------------------------+
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The cloned image has a reference to the parent snapshot, and includes the pool
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ID, the image ID, and the snapshot ID. The inclusion of the pool ID means that
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you may clone snapshots from one pool to images in another pool.
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#. **Image Template:** A common use case for block device layering is to create
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a base image and a snapshot that serves as a template for clones. For
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example: a user may create an image for a Linux distribution (for example,
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Ubuntu 22.04) and create a snapshot of it. The user may occasionally update
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the image and create a new snapshot (by using such commands as ``sudo
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apt-get update``, ``sudo apt-get upgrade``, or ``sudo apt-get dist-upgrade``
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followed by ``rbd snap create``). As the image matures, the user can clone
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any one of the snapshots.
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#. **Extended Template:** A more advanced use case includes extending a
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template image to provide more information than a base image. For
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example, a user may clone an image (for example, a VM template) and install
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other software (for example, a database, a content management system, an
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analytics system) and then snapshot the extended image, which may itself be
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updated just like the base image.
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#. **Template Pool:** One way to use block device layering is to create a pool
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that contains (1) base images that act as templates and (2) snapshots of
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those templates. You may then extend read-only privileges to users so that
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they may clone the snapshots even though they do not have permissions that
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allow them to write or execute within the pool.
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#. **Image Migration/Recovery:** One way to use block device layering is to
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migrate or recover data from one pool into another pool.
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Protecting a Snapshot
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---------------------
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Clones access the parent snapshots. All clones would break if a user
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inadvertently deleted the parent snapshot. To prevent data loss, you must
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protect the snapshot before you can clone it:
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.. prompt:: bash $
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rbd snap protect {pool-name}/{image-name}@{snapshot-name}
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For example:
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.. prompt:: bash $
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rbd snap protect rbd/foo@snapname
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.. note:: You cannot delete a protected snapshot.
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Cloning a Snapshot
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------------------
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To clone a snapshot, specify the parent pool, the parent image, and the parent
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snapshot; and also the child pool together with the image name. You must
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protect the snapshot before you can clone it:
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.. prompt:: bash $
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rbd clone {pool-name}/{parent-image-name}@{snap-name} {pool-name}/{child-image-name}
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For example:
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.. prompt:: bash $
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rbd clone rbd/foo@snapname rbd/bar
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.. note:: You may clone a snapshot from one pool to an image in another pool.
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For example, you may maintain read-only images and snapshots as templates in
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one pool, and writeable clones in another pool.
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Unprotecting a Snapshot
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-----------------------
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Before you can delete a snapshot, you must first unprotect it. Additionally,
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you may *NOT* delete snapshots that have references from clones. You must
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flatten or delete each clone of a snapshot before you can unprotect the
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snapshot:
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.. prompt:: bash $
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rbd snap unprotect {pool-name}/{image-name}@{snapshot-name}
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For example:
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.. prompt:: bash $
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rbd snap unprotect rbd/foo@snapname
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Listing Children of a Snapshot
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------------------------------
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To list the children of a snapshot, use the ``rbd children`` command and
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specify the pool name, the image name, and the snap name:
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.. prompt:: bash $
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rbd children {pool-name}/{image-name}@{snapshot-name}
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For example:
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.. prompt:: bash $
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rbd children rbd/foo@snapname
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Flattening a Cloned Image
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-------------------------
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Cloned images retain a reference to the parent snapshot. When you remove the
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reference to the parent snapshot from the clone, you effectively "flatten" the
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clone by copying the data stored in the snapshot to the clone. The time it
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takes to flatten a clone increases with the size of the snapshot. To delete a
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snapshot, you must first flatten the child images (or delete them):
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.. prompt:: bash $
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rbd flatten {pool-name}/{image-name}
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For example:
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.. prompt:: bash $
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rbd flatten rbd/bar
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.. note:: Since a flattened image contains all the data stored in the snapshot,
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a flattened image takes up more storage space than a layered clone does.
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.. _cephx: ../../rados/configuration/auth-config-ref/
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.. _QEMU: ../qemu-rbd/
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.. _OpenStack: ../rbd-openstack/
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.. _CloudStack: ../rbd-cloudstack/
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.. _libvirt: ../libvirt/
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