.. 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 (e.g., a
-VM image) quickly and easily. Ceph block device snapshots are managed using the
-``rbd`` command and multiple higher level interfaces, including `QEMU`_,
-`libvirt`_, `OpenStack`_ and `CloudStack`_.
+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`_, and `CloudStack`_.
.. important:: To use RBD snapshots, you must have a running Ceph cluster.
-.. note:: Because RBD does not know about any filesystem within an image
- (volume), snapshots are only `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 filesystem, it should be in an internally
- consistent state before taking a snapshot. Snapshots taken without
- write quiescing may need an `fsck` pass before subsequent
- mounting. To quiesce `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 file systems when creating a snapshot.
+
+.. 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::
When `cephx`_ 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. You may also set the
-``CEPH_ARGS`` environment variable to avoid re-entry of these parameters. ::
+: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]
- rbd --id {user-ID} --keyring=/path/to/secret [commands]
- rbd --name {username} --keyring=/path/to/secret [commands]
+For example:
-For example::
+.. prompt:: bash $
- rbd --id admin --keyring=/etc/ceph/ceph.keyring [commands]
- rbd --name client.admin --keyring=/etc/ceph/ceph.keyring [commands]
+ 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.
+.. tip:: Add the user and secret to the ``CEPH_ARGS`` environment variable to
+ avoid re-entry of these parameters.
Snapshot Basics
Create Snapshot
---------------
-To create a snapshot with ``rbd``, specify the ``snap create`` option, the pool
-name and the image name. ::
+To create a snapshot, use the ``rbd snap create`` command and specify the pool
+name, the image name, and the snap name:
- rbd snap create {pool-name}/{image-name}@{snap-name}
+.. prompt:: bash $
-For example::
+ rbd snap create {pool-name}/{image-name}@{snap-name}
- rbd snap create rbd/foo@snapname
+For example:
+
+.. prompt:: bash $
+
+ rbd snap create rbd/foo@snapname
List Snapshots
--------------
-To list snapshots of an image, specify the pool name and the image name. ::
+To list the snapshots of an image, use the ``rbd snap ls`` command and specify
+the pool name and the image name:
- rbd snap ls {pool-name}/{image-name}
+.. prompt:: bash $
-For example::
+ rbd snap ls {pool-name}/{image-name}
- rbd snap ls rbd/foo
+For example:
+.. prompt:: bash $
-Rollback Snapshot
------------------
+ rbd snap ls rbd/foo
-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}
+Roll back Snapshot
+------------------
-For example::
+To roll back to a snapshot, use the ``rbd snap rollback`` command and specify
+the pool name, the image name, and the snap name:
- rbd snap rollback rbd/foo@snapname
+.. prompt:: bash $
+ rbd snap rollback {pool-name}/{image-name}@{snap-name}
-.. 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 is the preferred method of returning
- to a pre-existing state.
+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 with ``rbd``, specify the ``snap rm`` subcommand, the pool
-name, the image name and the snap name. ::
+To delete a snapshot, use the ``rbd snap rm`` command and specify the pool
+name, the image name, and the snap name:
- rbd snap rm {pool-name}/{image-name}@{snap-name}
+.. prompt:: bash $
+
+ rbd snap rm {pool-name}/{image-name}@{snap-name}
-For example::
+For example:
+
+.. prompt:: bash $
- rbd snap rm rbd/foo@snapname
+ rbd snap rm rbd/foo@snapname
-.. note:: Ceph OSDs delete data asynchronously, so deleting a snapshot
- doesn't immediately free up the underlying OSDs' capacity.
+.. 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 for an image with ``rbd``, specify the ``snap purge``
-subcommand and the image name. ::
+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}
- rbd snap purge {pool-name}/{image-name}
+For example:
-For example::
+.. prompt:: bash $
- rbd snap purge rbd/foo
+ rbd snap purge rbd/foo
.. index:: Ceph Block Device; snapshot 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.
+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::
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.
+.. 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.
+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 depends on the snapshot, so you **MUST** protect the snapshot before
-you clone it. The following diagram depicts the process.
+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 only supports cloning of RBD format 2 images (i.e., created with
- ``rbd create --image-format 2``). The kernel client supports cloned images
- beginning with the 3.10 release.
+.. 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. Once you have
-performed these steps, you can begin cloning the snapshot.
+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::
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.
+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. ::
+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}
+ rbd snap protect {pool-name}/{image-name}@{snapshot-name}
-For example::
+For example:
- rbd snap protect rbd/my-image@my-snapshot
+.. prompt:: bash $
+
+ rbd snap protect rbd/foo@snapname
.. 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. ::
+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}@{snap-name} {pool-name}/{child-image-name}
+ rbd clone {pool-name}/{parent-image-name}@{snap-name} {pool-name}/{child-image-name}
-For example::
+For example:
- rbd clone rbd/my-image@my-snapshot rbd/new-image
+.. 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.
+
+.. 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,
+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 each clone of a snapshot, before you can delete the snapshot. ::
+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}
- rbd snap unprotect {pool-name}/{image-name}@{snapshot-name}
+For example:
-For example::
+.. prompt:: bash $
- rbd snap unprotect rbd/my-image@my-snapshot
+ rbd snap unprotect rbd/foo@snapname
Listing Children of a Snapshot
------------------------------
-To list the children of a snapshot, execute the following::
+To list the children of a snapshot, use the ``rbd children`` command and
+specify the pool name, the image name, and the snap name:
- rbd children {pool-name}/{image-name}@{snapshot-name}
+.. prompt:: bash $
-For example::
+ rbd children {pool-name}/{image-name}@{snapshot-name}
- rbd children rbd/my-image@my-snapshot
+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 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. ::
+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}
- rbd flatten {pool-name}/{image-name}
+For example:
-For example::
+.. prompt:: bash $
- rbd flatten rbd/new-image
+ rbd flatten rbd/bar
-.. 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.
+.. 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.
.. _cephx: ../../rados/configuration/auth-config-ref/
projects / ceph.git / blobdiff