How to do it...

1. Map the block device to the client-node1:

        # rbd map --image rbd1 --name client.rbd

1. With Ceph Jewel the new default format for RBD images is 2 and Ceph Jewel default configuration includes the following default Ceph Block Device features:
   • layering: layering support
   • exclusive-lock: exclusive locking support
   • object-map: object map support (requires exclusive-lock)
   • deep-flatten: snapshot flatten support
   • fast-diff: fast diff calculations (requires object-map)

     Using the krbd (kernel rbd) client on client-node1 we will be unable to map the block device image on CentOS kernel 3.10 as this kernel does not support object-map, deep-flatten and fast-diff (support was introduced in kernel 4.9). In order to work around this we will disable the unsupported features, there are several options to do this:

     • Disable the unsupported features dynamically (this is the option we will be using):

                               # rbd feature disable rbd1 
                                 exclusive-lock object-map 
                                 deep-flatten fast-diff

1. • • When creating the RBD image initially utilize the --image-feature layering option with the rbd create command which will only enable the layering feature:

                               # rbd create rbd1 --size 10240 
                                 --image-feature layering 
                                 --name client.rbd

1. • • Disable the feature in the Ceph configuration file:

                           rbd_default_features = 1

1. Retry mapping the block device with the unsupported features now disabled:

        # rbd map --image rbd1 --name client.rbd

1. Check the mapped block device:

        rbd showmapped --name client.rbd

1. To make use of this block device, we should create a filesystem on this and mount it:

      # fdisk -l /dev/rbd0
      # mkfs.xfs /dev/rbd0
      # mkdir /mnt/ceph-disk1
      # mount /dev/rbd0 /mnt/ceph-disk1
      # df -h /mnt/ceph-disk1

1. Test the block device by writing data to it:

        dd if=/dev/zero of=/mnt/ceph-disk1/file1 count=100 bs=1M

1. To map the block device across reboots, we will need to create and configure a services file:
   1. Create a new file in the /usr/local/bin directory for mounting and unmounting and include the following:

                    # cd /usr/local/bin
                    # vim rbd-mount

1. 1. Save the file and make it executable:

              # sudo chmod +x rbd-mount

This can be done automatically by grabbing the rbd-mount script from the Ceph-Designing-and-Implementing-Scalable-Storage-Systems repository and making it executable:

              # wget https://raw.githubusercontent.com/PacktPublishing/
                Ceph-Designing-and-Implementing-Scalable-Storage-Systems/Module_1/master/
                rbdmap -O /usr/local/bin/rbd-mount
              # chmod +x /usr/local/bin/rbd-mount

1. 1. Go to the systemd directory and create the service file, include the following in the file rbd-mount.service:

               # cd /etc/systemd/system/
               # vim rbd-mount.service

This can be done automatically by grabbing the service file from the Ceph-Designing-and-Implementing-Scalable-Storage-Systems/Chapter02 repository:

              # wget https://raw.githubusercontent.com/PacktPublishing/
                Ceph-Designing-and-Implementing-Scalable-Storage-Systems/Chapter02/rbd-mount.service

1. 1. After saving the file and exiting Vim, reload the systemd files and enable the rbd-mount.service to start at boot time:

              # systemctl daemon-reload
              # systemctl enable rbd-mount.service

1. Reboot client-node1 and verify that block device rbd0 is mounted to /mnt/ceph-disk1 after the reboot:

         root@client-node1 # reboot -f
         # df -h