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317 lines
11 KiB
ReStructuredText
317 lines
11 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
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that you can create snapshots of images to retain point-in-time state history.
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Ceph also supports snapshot layering, which allows you to clone images (e.g., a
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VM image) quickly and easily. Ceph block device snapshots are managed using the
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``rbd`` command and multiple 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 does not know about any filesystem within an image
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(volume), snapshots are not `crash-consistent` unless they are
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coordinated within the mounting (attaching) operating system.
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We therefore recommend that you pause or stop I/O before taking a snapshot.
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If the volume contains a filesystem, it must be in an internally
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consistent state before taking a snapshot. Snapshots taken at
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inconsistent points may need a `fsck` pass before subsequent
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mounting. To stop `I/O` you can use `fsfreeze` command. See
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`fsfreeze(8)` man page for more details.
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For virtual machines, `qemu-guest-agent` can be used to automatically
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freeze 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. You may also set the
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``CEPH_ARGS`` environment variable to avoid re-entry of these parameters. ::
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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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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
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variable so that you don't need to enter them each time.
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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 with ``rbd``, specify the ``snap create`` option, the pool
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name and the image name. ::
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rbd snap create {pool-name}/{image-name}@{snap-name}
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For example::
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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 snapshots of an image, specify the pool name and the image name. ::
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rbd snap ls {pool-name}/{image-name}
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For example::
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rbd snap ls rbd/foo
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Rollback Snapshot
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-----------------
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To rollback to a snapshot with ``rbd``, specify the ``snap rollback`` option, the
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pool name, the image name and the snap name. ::
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rbd snap rollback {pool-name}/{image-name}@{snap-name}
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For example::
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rbd snap rollback rbd/foo@snapname
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.. note:: Rolling back an image to a snapshot means overwriting
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the current version of the image with data from a snapshot. The
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time it takes to execute a rollback increases with the size of the
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image. It is **faster to clone** from a snapshot **than to rollback**
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an image to a snapshot, and is the preferred method of returning
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to a pre-existing state.
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Delete a Snapshot
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-----------------
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To delete a snapshot with ``rbd``, specify the ``snap rm`` subcommand, the pool
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name, the image name and the snap name. ::
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rbd snap rm {pool-name}/{image-name}@{snap-name}
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For example::
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rbd snap rm rbd/foo@snapname
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.. note:: Ceph OSDs delete data asynchronously, so deleting a snapshot
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doesn't immediately free up the underlying OSDs' capacity.
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Purge Snapshots
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---------------
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To delete all snapshots for an image with ``rbd``, specify the ``snap purge``
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subcommand and the image name. ::
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rbd snap purge {pool-name}/{image-name}
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For example::
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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; then, snapshot the image, protect the snapshot, and
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create as many copy-on-write clones as you like. A snapshot is read-only,
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so cloning a snapshot simplifies semantics--making it possible to create
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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 (parent),
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and the corresponding image cloned from the snapshot (child). These terms are
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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 COW clone of a snapshot behaves exactly like any other Ceph block device
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image. You can read to, write from, clone, and resize cloned images. There are
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no special restrictions with cloned images. However, the copy-on-write clone of
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a snapshot depends on the snapshot, so you **MUST** protect the snapshot before
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you clone it. The following diagram depicts the process.
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.. note:: Ceph only supports cloning of RBD format 2 images (i.e., created with
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``rbd create --image-format 2``). The kernel client supports cloned images
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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 must
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create a snapshot of the image. You must protect the snapshot. Once you have
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performed these steps, you can begin cloning the snapshot.
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.. ditaa::
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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, image ID and snapshot ID. The inclusion of the pool ID means that you may
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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 a
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master image and a snapshot that serves as a template for clones. For example,
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a user may create an image for a Linux distribution (e.g., Ubuntu 12.04), and
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create a snapshot for it. Periodically, the user may update the image and create
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a new snapshot (e.g., ``sudo apt-get update``, ``sudo apt-get upgrade``,
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``sudo apt-get dist-upgrade`` followed by ``rbd snap create``). As the image
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matures, the user can clone any one of the snapshots.
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#. **Extended Template:** A more advanced use case includes extending a template
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image that provides more information than a base image. For example, a user may
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clone an image (e.g., a VM template) and install other software (e.g., a database,
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a content management system, an analytics system, etc.) and then snapshot the
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extended image, which itself may be updated just like the base image.
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#. **Template Pool:** One way to use block device layering is to create a
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pool that contains master images that act as templates, and snapshots of those
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templates. You may then extend read-only privileges to users so that they
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may clone the snapshots without the ability to write or execute within the pool.
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#. **Image Migration/Recovery:** One way to use block device layering is to migrate
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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 inadvertently
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deleted the parent snapshot. To prevent data loss, you **MUST** protect the
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snapshot before you can clone it. ::
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rbd snap protect {pool-name}/{image-name}@{snapshot-name}
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For example::
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rbd snap protect rbd/my-image@my-snapshot
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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 you need to specify the parent pool, image and
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snapshot; and, the child pool and image name. You must protect the snapshot
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before you can clone it. ::
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rbd clone {pool-name}/{parent-image}@{snap-name} {pool-name}/{child-image-name}
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For example::
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rbd clone rbd/my-image@my-snapshot rbd/new-image
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.. note:: You may clone a snapshot from one pool to an image in another pool. For example,
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you may maintain read-only images and snapshots as templates in one pool, and writeable
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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 unprotect it first. Additionally,
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you may *NOT* delete snapshots that have references from clones. You must
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flatten each clone of a snapshot, before you can delete the snapshot. ::
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rbd snap unprotect {pool-name}/{image-name}@{snapshot-name}
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For example::
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rbd snap unprotect rbd/my-image@my-snapshot
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Listing Children of a Snapshot
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------------------------------
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To list the children of a snapshot, execute the following::
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rbd children {pool-name}/{image-name}@{snapshot-name}
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For example::
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rbd children rbd/my-image@my-snapshot
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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 from the child clone to the parent snapshot, you effectively "flatten"
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the image by copying the information from the snapshot to the clone. The time
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it takes to flatten a clone increases with the size of the snapshot. To delete
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a snapshot, you must flatten the child images first. ::
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rbd flatten {pool-name}/{image-name}
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For example::
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rbd flatten rbd/new-image
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.. note:: Since a flattened image contains all the information from the snapshot,
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a flattened image will take up more storage space than a layered clone.
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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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