=========== 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:: **STOP I/O BEFORE** snapshotting an image. If the image contains a filesystem, the filesystem must be in a consistent state **BEFORE** snapshotting. .. ditaa:: +------------+ +-------------+ | {s} | | {s} c999 | | Active |<-------*| Snapshot | | Image | | of Image | | (stop i/o) | | (read only) | +------------+ +-------------+ Cephx Notes =========== When `cephx`_ is enabled, you must specify a user and a secret file on the command line, or use the ``CEPH_ARGS`` environment variable to avoid re-entry of the following parameters. :: rbd --id {user-name} --keyring=/path/to/secret [commands] For example:: rbd --id 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 --pool {pool-name} snap create --snap {snap-name} {image-name} rbd snap create {pool-name}/{image-name}@{snap-name} For example:: rbd --pool rbd snap create --snap snapname foo rbd snap create rbd/foo@snapname List Snapshots -------------- To list snapshots of an image, specify the pool name and the image name. :: rbd --pool {pool-name} snap ls {image-name} rbd snap ls {pool-name}/{image-name} For example:: rbd --pool rbd snap ls foo 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 --pool {pool-name} snap rollback --snap {snap-name} {image-name} rbd snap rollback {pool-name}/{image-name}@{snap-name} For example:: rbd --pool rbd snap rollback --snap snapname foo rbd snap rollback rbd/foo@snapname For the rollback section, you could mention that rollback means overwriting the current version with data from a snapshot, and takes longer with larger images. So cloning is preferable for fast recovery. .. 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 username. :: rbd --pool {pool-name} snap rm --snap {snap-name} {image-name} rbd snap rm {pool-name}/{image-name}@{snap-name} For example:: rbd --pool rbd snap rm --snap snapname foo 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 --pool {pool-name} snap purge {image-name} rbd snap purge {pool-name}/{image-name} For example:: rbd --pool rbd snap purge foo rbd snap purge rbd/foo 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 --format 2``), and is not yet supported by the kernel ``rbd`` module. So you MUST use QEMU/KVM or ``librbd`` directly to access clones in the current 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. .. 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 priveleges 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 inadvertantly deleted the parent snapshot. To prevent data loss, you **MUST** protect the snapshot before you can clone it. :: rbd --pool {pool-name} snap protect --image {image-name} --snap {snapshot-name} rbd snap protect {pool-name}/{image-name}@{snapshot-name} For example:: rbd --pool rbd snap protect --image my-image --snap my-snapshot 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 --pool {pool-name} --image {parent-image} --snap {snap-name} --dest-pool {pool-name} --dest {child-image} 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 --pool {pool-name} snap unprotect --image {image-name} --snap {snapshot-name} rbd snap unprotect {pool-name}/{image-name}@{snapshot-name} For example:: rbd --pool rbd snap unprotect --image my-image --snap my-snapshot rbd snap unprotect rbd/my-image@my-snapshot Listing Children of a Snapshot ------------------------------ To list the children of a snapshot, execute the following:: rbd --pool {pool-name} snap children --image {image-name} --snap {snap-name} rbd snap children {pool-name}/{image-name}@{snapshot-name} For example:: rbd --pool rbd snap children --image my-image --snap my-snapshot rbd snap 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 --pool {pool-name} flatten --image {image-name} rbd flatten {pool-name}/{image-name} For example:: rbd --pool rbd flatten --image my-image 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/operations/authentication/ .. _QEMU: ../qemu-rbd/ .. _OpenStack: ../rbd-openstack/ .. _CloudStack: ../rbd-cloudstack/ .. _libvirt: ../libvirt/