How to Increase the Size of an Image in Ceph

To increase the size of a Ceph RBD (RADOS Block Device) image, you use the rbd resize command, specifying the desired new total size in megabytes, gigabytes, or terabytes. This operation allows you to expand your virtual disks dynamically.

Understanding Ceph RBD Image Resizing

A Ceph RBD image functions as a virtual block device that can be attached to virtual machines or containers. Increasing its size involves updating the image's metadata within the Ceph cluster to reflect the new capacity. It's crucial to understand that you specify the new total size of the image, not the amount by which to extend it. For example, to change a 10GB image to 20GB, you specify 20GB as the new size.

Key Considerations Before Resizing:

• Online vs. Offline: RBD images can often be resized while in use (online), but the underlying filesystem within the guest operating system will still need to be expanded separately.
• Filesystem Expansion: Expanding the RBD image only makes more space available to the guest OS. You must then extend the filesystem (e.g., ext4, XFS, NTFS) inside the virtual machine or container to utilize this newly available space. Failing to do so will result in the guest OS not recognizing the increased capacity.
• Snapshots: While resizing is generally safe, having snapshots allows for easy rollback in case of issues.

Steps to Increase Ceph RBD Image Size

Follow these steps to successfully expand your Ceph RBD image:

1. Identify the Target RBD Image:
   First, you need to know the name of the Ceph pool and the specific RBD image you want to resize. You can list your images using:

   rbd ls <pool_name>

   To get detailed information about a specific image, including its current size:

   rbd info <pool_name>/<image_name>

2. Execute the rbd resize Command:
   Use the rbd resize command with the --size option to set the new total size for the RBD image. Specify the size using suffixes like M for megabytes, G for gigabytes, or T for terabytes.

   Syntax:

   rbd resize --image <pool_name>/<image_name> --size <new_total_size>

   Example:
   To increase an image named my_volume in the pool rbd_pool to a new total size of 50 Gigabytes:

   rbd resize --image rbd_pool/my_volume --size 50G

   Alternatively, you can specify the pool and image separately:

   rbd resize -p rbd_pool my_volume --size 50G

3. Verify the New RBD Image Size:
   After executing the command, verify that the RBD image size has been updated:

   rbd info <pool_name>/<image_name>

   The output should reflect the new total size you specified.

4. Expand the Filesystem within the Guest OS:
   This is a critical step. The operating system inside the virtual machine or container using this RBD image will not automatically recognize the expanded space. You must manually extend its filesystem. The exact commands vary depending on the guest OS and filesystem type.

   • For Linux Guests (e.g., ext4, xfs):
     1. Log in to the guest OS.
     2. Identify the block device associated with the RBD image (e.g., /dev/vda, /dev/sdb).
     3. Rescan the device to detect the new size (often automatic for virtio-blk, but might need echo 1 > /sys/class/block/<device>/device/rescan for SCSI).
     4. Expand the partition (if applicable): For a single partition covering the entire disk, use growpart or similar tools.

        sudo growpart /dev/vda 1 # Expands the first partition on /dev/vda

     5. Expand the filesystem:
        • For ext4:

          sudo resize2fs /dev/vda1 # If /dev/vda1 is your partition

        • For xfs:

          sudo xfs_growfs /mount/point # Replace with your mount point

   • For Windows Guests:
     1. Open Disk Management (diskmgmt.msc).
     2. Right-click the volume corresponding to the expanded RBD image.
     3. Select "Extend Volume..." and follow the wizard to utilize the unallocated space.

rbd resize Command Parameters

The following table summarizes key parameters used with the rbd resize command:

Option / Parameter	Description	Example Value
--image <pool>/<image>	Specifies the target RBD image by its pool and name. This is a common way to identify the image.	my_pool/my_image
--size <size_value>	Mandatory for increasing size. Defines the new, total size for the RBD image. You can use suffixes like K, M, G, T for kilobytes, megabytes, gigabytes, or terabytes. This will set the new size for the RBD image.	--size 100G or --size 102400M
--id <user_id>	Specifies the Ceph user ID (e.g., admin) for authentication, if not using the default client.	--id admin
--cluster <cluster_name>	Specifies the Ceph cluster name if you are interacting with a cluster other than the default ceph.	--cluster ceph-prod
--allow-shrink	Not for increasing size. This option is used when you intend to decrease the size of an image. Use with extreme caution as it can lead to data loss if the image contains data beyond the new, smaller size.

For more detailed information and advanced options, refer to the official Ceph RBD documentation.

By following these steps, you can effectively expand your Ceph RBD images to meet your growing storage requirements, ensuring your applications and virtual machines have the necessary capacity.