Connecting and configuring Ceph RBD using a Linux client

Ceph RBD (RADOS Block Device) provides users with a network block device that looks like a local disk on the system where it is connected. The block device is fully managed by the user. An user can create a file system there and use it according to his needs.

Advantages of RBD

Possibility to enlarge the image of the block device.
Import / export block device image.
Stripping and replication within the cluster.
Possibility to create read-only snapshots; restore snapshots (if you need snapshots on the RBD level you must contact us).
Possibility to connect using Linux or QEMU KVM client

Setup of RBD client (Linux)

⚠️

To connect RBD, it is recommended to have a newer kernel version on your system. In lower kernel versions are the appropriate RBD connection modules deprecated. So not all advanced features are supported. Developers even recommend a kernel version at least 5.0 or higher. However developers has backported some functionalities to CentOS 7 core.

Ceph client version For proper functioning it is highly desired to use the same version of Ceph tools as is the current version being operated on our clusters. Currently it is version 16 with the code name Pacific . So we will set up the appropriate repositories, see below.

CentOS setup

First, install the release.asc key for the Ceph repository.

sudo rpm --import 'https://download.ceph.com/keys/release.asc'

In the directory /etc/yum.repos.d/ create a text file ceph.repo and fill in the record for Ceph instruments.

[ceph]
    name=Ceph packages for $basearch
    baseurl=https://download.ceph.com/rpm-nautilus/el7/$basearch
    enabled=1
    priority=2
    gpgcheck=1
    gpgkey=https://download.ceph.com/keys/release.asc

Some packages from the Ceph repository also require third-party libraries for proper functioning, so add the EPEL repository.

CentOS 7

sudo yum install -y epel-release

CentOS 8

sudo dnf install -y epel-release

RedHat 7

sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

Finally, install the basic tools for Ceph which also include RBD support.

CentOS 7

sudo yum install ceph-common

On CentOS 8

sudo dnf install ceph-common

Ubuntu/Debian setup

Ubuntu/Ceph includes all necessary packages natively. So you can just run following command.

sudo apt install ceph

RBD configuration and its mapping

Use the credentials which you received from the system administrator to configure and connect the RBD. These are the following:

pool name: rbd_vo_poolname
image name: vo_name_username
keyring: [client.rbd_user] key = key_hash ==

In the directory /etc/ceph/ create the text file ceph.conf with the following content.

CL1 Data Storage

[global]
    fsid = 19f6785a-70e1-45e8-a23a-5cff0c39aa54
    mon_host = [v2:78.128.244.33:3300,v1:78.128.244.33:6789],[v2:78.128.244.37:3300,v1:78.128.244.37:6789],[v2:78.128.244.41:3300,v1:78.128.244.41:6789]
    auth_client_required = cephx

CL2 Data Storage

[global]
    fsid = 3ea58563-c8b9-4e63-84b0-a504a5c71f76
    mon_host = [v2:78.128.244.65:3300/0,v1:78.128.244.65:6789/0],[v2:78.128.244.69:3300/0,v1:78.128.244.69:6789/0],[v2:78.128.244.71:3300/0,v1:78.128.244.71:6789/0]
    auth_client_required = cephx

CL3 Data Storage

[global]
    fsid = b16aa2d2-fbe7-4f35-bc2f-3de29100e958
    mon_host = [v2:78.128.244.240:3300/0,v1:78.128.244.240:6789/0],[v2:78.128.244.241:3300/0,v1:78.128.244.241:6789/0],[v2:78.128.244.242:3300/0,v1:78.128.244.242:6789/0]
    auth_client_required = cephx

CL4 Data Storage

[global]
    fsid = c4ad8c6f-7ef3-4b0e-873c-b16b00b5aac4
    mon_host = [v2:78.128.245.29:3300/0,v1:78.128.245.29:6789/0] [v2:78.128.245.30:3300/0,v1:78.128.245.30:6789/0] [v2:78.128.245.31:3300/0,v1:78.128.245.31:6789/0]
    auth_client_required = cephx

Further in the directory /etc/ceph/ create the text file ceph.keyring. Then save in that file the keyring, see the example below.

[client.rbd_user]
        key = sdsaetdfrterp+sfsdM3iKY5teisfsdXoZ5==

⚠️

If the location of the files ceph.conf and username.keyring differs from the default directory /etc/ceph/, the corresponding paths must be specified during mapping. See below.

sudo rbd -c /home/username/ceph/ceph.conf -k /home/username/ceph/username.keyring --id rbd_user device map name_pool/name_image

If the location of the files ceph.conf and username.keyring differs from the default directory /etc/ceph/, the corresponding paths must be specified during mapping. See below.

sudo rbd -c /home/username/ceph/ceph.conf -k /home/username/ceph/username.keyring --id rbd_user device map name_pool/name_image

Then check the connection in kernel messages.

dmesg

Now check the status of RBD.

sudo rbd device list | grep "name_image"

Encrypting and creating a file system

The next step is to encrypt the mapped image. Use cryptsetup-luks for encryption.

sudo yum install cryptsetup-luks

Then it encrypts the device.

sudo cryptsetup -s 512 luksFormat --type luks2 /dev/rbdX

Finally, check the settings.

sudo cryptsetup luksDump /dev/rbdX

In order to perform further actions on an encrypted device, it must be decrypted first.

sudo cryptsetup luksOpen /dev/rbdX luks_rbdX

We recommend using XFS instead of EXT4 for larger images or those they will need to be enlarged to more than 200TB over time, because EXT4 has a limit on the number of inodes.

Now create file system on the device, here is an example xfs.

sudo mkfs.xfs -K /dev/mapper/luks_rbdX

⚠️

If you use XFS, do not use the nobarrier option while mounting, it could cause data loss!

Once the file system is ready, we can mount the device in a pre-created folder in /mnt/.

sudo mount /dev/mapper/luks_rbdX /mnt/rbd

Ending work with RBD

Unmount the volume.

sudo umount /mnt/rbd/

Close the encrypted volume.

sudo cryptsetup luksClose /dev/mapper/luks_rbdX

Volume unmapping.

sudo rbd --id rbd_user device unmap /dev/rbdX/

To get better performance choose appropriate size of read_ahead cache depends on your size of memory. Example for 8GB:

echo 8388608 > /sys/block/rbd0/queue/read_ahead_kb

Example for 512MB:

echo 524288 > /sys/block/rbd0/queue/read_ahead_kb

To apply changes you have to unmap image and map it again.

The approach described above is not persistent (won’t survive reboot). To do it persistent you have to add following line into “/etc/udev/rules.d/50-read-ahead-kb.rules” file.

You can set specific kernel parameters for a subset of block devices (Ceph RBD)

KERNEL=="rbd[0-9]*", ENV{DEVTYPE}=="disk", ACTION=="add|change", ATTR{bdi/read_ahead_kb}="524288"

Permanently mapping of RBD

Settings for automatic RBD connection, including LUKS encryption and mount filesystems. + proper disconnection (in reverse order) when the machine is switched off in a controlled manner.

RBD image

Edit configuration file in the path /etc/ceph/rbdmap by inserting following lines.

# RbdDevice             Parameters
#poolname/imagename     id=client,keyring=/etc/ceph/ceph.client.keyring
pool_name/image_name id=rbd_user,keyring=/etc/ceph/ceph.keyring

LUKS

Edit configuration file in the path /etc/crypttab by inserting following lines.

rbd_luks_pool /dev/rbd/pool_name/image_name  /etc/ceph/luks.keyfile luks,_netdev

where /etc/ceph/luks.keyfile is LUKS key.

Path to block device source device is generally /dev/rbd/$POOL/$IMAGE

fstab file

Edit configuration file in the path /etc/fstab by inserting following lines.

/dev/mapper/rbd_luks_pool /mnt/rbd_luks_pool btrfs defaults,noatime,auto,_netdev 0 0

path to LUKS container (file system) is generally /dev/mapper/$LUKS_NAME, where $LUKS_NAME is defined in /etc/crypttab (like taget name)

systemd unit

Edit configuration file in the path /etc/systemd/system/systemd-cryptsetup@rbd_luks_pool.service.d/10-deps.conf by inserting following lines.

[Unit]
    After=rbdmap.service
    Requires=rbdmap.service
    Before=mnt-rbd_luks_pool.mount

In one case, systemd units were used on Debian 10 for some reason ceph-rbdmap.service instead of rbdmap.service (must be adjusted to lines After= and Requires=)

Manual connection

If the dependencies of the systemd units are correct, it performs an RBD map, unlocks LUKS and mounts all the automatic fs dependent on the rbdmap that the specified .mount unit needs (⇒ mounts both images in the described configuration).

systemctl start mnt-rbd_luks_pool.mount

Manual disconnection

This command should execute if the dependencies are set correctly umount, LUKS close i RBD unmap.

systemctl stop rbdmap.service
(alternatively `systemctl stop ceph-rbdmap.service`)

Image resize

When resizing an encrypted image, you need to follow the order and the main one is the line with cryptsetup --verbose resize image_name.

rbd resize rbd_pool_name/image_name --size 200T
 
cryptsetup --verbose resize image_name
 
mount /storage/rbd/image_name
 
xfs_growfs /dev/mapper/image_name

Connecting and configuring Ceph RBD using a Linux client

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