rbd

The rbd (RADOS Block Device) command-line tool is used to manage block devices in a Ceph storage cluster. RADOS Block Devices (RBD) are block storage volumes backed by the Ceph distributed object store. These can be used for cloud infrastructure, virtual machines, or other applications that need block storage.

Here’s a detailed overview of common rbd subcommands and their usage:

General Structure of rbd Command

rbd [options] <subcommand> [arguments]

• [options]: Global options such as --pool, --cluster, etc.

• <subcommand>: The specific operation or management action to be performed (e.g., create, info, rm).

• [arguments]: Additional arguments required by the subcommand.

Common rbd Subcommands

1. Image Management

rbd create

Creates a new RADOS Block Device (RBD) image.

rbd create <image-name> --size <size-in-MB|GB|TB> [options]

• Example:

  rbd create myimage --size 10G

This creates a new image named myimage of size 10 GB.

rbd rm

Removes a specified RBD image from a pool.

rbd rm <image-name> [--pool <pool-name>]

• Example:

  rbd rm myimage

This deletes the image myimage from the pool.

rbd info

Displays information about an RBD image, such as size, object size, features, etc.

rbd info <image-name>

• Example:

  rbd info myimage

This shows detailed information about the image myimage.

rbd resize

Resizes an existing RBD image.

rbd resize --size <new-size> <image-name>

• Example:

  rbd resize --size 20G myimage

This resizes the myimage to 20 GB.

2. Image Listing and Mapping

rbd ls

Lists all RBD images in a specified pool.

rbd ls [--pool <pool-name>]

• Example:

  rbd ls

This lists all RBD images in the default pool or the specified pool.

rbd map

Maps an RBD image to a block device on the local system.

rbd map <image-name> [--pool <pool-name>]

• Example:

  rbd map myimage

This maps the image myimage to a block device (e.g., /dev/rbd0) on the local system.

rbd unmap

Unmaps an RBD image from the local system.

rbd unmap /dev/rbd<x>

• Example:

  rbd unmap /dev/rbd0

This unmaps the block device /dev/rbd0.

3. Image Snapshot Management

rbd snap create

Creates a snapshot of an RBD image.

rbd snap create <image-name>@<snapshot-name>

• Example:

  rbd snap create myimage@mysnap

This creates a snapshot mysnap of the image myimage.

rbd snap ls

Lists all snapshots for a given image.

rbd snap ls <image-name>

• Example:

  rbd snap ls myimage

This lists all snapshots of the image myimage.

rbd snap rollback

Rolls back an image to a specific snapshot.

rbd snap rollback <image-name>@<snapshot-name>

• Example:

  rbd snap rollback myimage@mysnap

This rolls back the image myimage to the snapshot mysnap.

rbd snap rm

Removes a specific snapshot.

rbd snap rm <image-name>@<snapshot-name>

• Example:

  rbd snap rm myimage@mysnap

This removes the snapshot mysnap from the image myimage.

4. Image Cloning

rbd clone

Clones an RBD image from a snapshot to create a new image.

rbd clone <parent-image>@<snapshot-name> <new-image-name>

• Example:

  rbd clone myimage@mysnap myimage-clone

This creates a new image myimage-clone as a clone of the snapshot mysnap from the image myimage.

5. Layering and Caching

rbd feature disable

Disables specific features (e.g., layering) on an RBD image.

rbd feature disable <image-name> <feature>

• Example:

  rbd feature disable myimage layering

This disables the layering feature for the image myimage.

rbd feature enable

Enables specific features (e.g., layering) on an RBD image.

rbd feature enable <image-name> <feature>

• Example:

  rbd feature enable myimage layering

This enables the layering feature for the image myimage.

6. Export and Import

rbd export

Exports an RBD image to a file.

rbd export <image-name> <file-name>

• Example:

  rbd export myimage myimage_export.raw

This exports the myimage to a file named myimage_export.raw.

rbd import

Imports an image from a file into an RBD image.

rbd import <file-name> <image-name>

• Example:

  rbd import myimage_export.raw myimage

This imports the image from myimage_export.raw and stores it as a new image myimage in the pool.

rbd import-diff

Applies incremental differences to an RBD image from a file (created by rbd export-diff).

rbd import-diff <file-name> <image-name>

• Example:

  rbd import-diff myimage_diff myimage

This applies incremental changes to the myimage from the file myimage_diff.

7. Advanced Operations

rbd lock add

Adds a lock to an image, which prevents other clients from modifying the image.

rbd lock add <image-name>

• Example:

  rbd lock add myimage

This adds a lock on the myimage, preventing modification by other clients.

rbd lock remove

Removes a lock from an image.

rbd lock remove <image-name> <lock-id> <locker-id>

• Example:

  rbd lock remove myimage lockid lockerid

This removes the lock on the myimage with the specific lock ID and locker ID.

rbd du

Displays disk usage information for a specific RBD image.

rbd du <image-name>

• Example:

  rbd du myimage

This shows how much space is being used by the myimage.

rbd bench

Performs a benchmark test on an RBD image.

rbd bench <image-name> [options]

• Example:

  rbd bench myimage --io-type write --io-size 4M --io-total 1024M

This performs a benchmark test with 4 MB write I/O operations on myimage for a total of 1024 MB.

Conclusion

The rbd command provides extensive tools for managing block devices in Ceph, including creating, listing, resizing, and removing RBD images. You can also perform advanced operations like snapshots, cloning, exporting, and importing, as well as setting up features like layering. Mastering the rbd tool enables fine control over RADOS Block Devices, making it an essential skill for managing block storage in Ceph-based environments.