Quintessential Ceph

Eric Gustafson


The following is my distillation of how to install Ceph in the most minimal way. This method of installation is NOT the {right, correct, best, …} way to install Ceph. It is what I could discern as the most minimal way to install the smallest set of Ceph components to have a minimally running system. I did this to better understand Ceph and hope it will help others as well.

Architecture & what’s needed

The Ceph Architecture page starts with both a good diagram and description of what makes up the cluster.

Minimal Steps to create a Ceph cluster

The architecture: 4 Hosts


Hosts: 4 – ok, this isn’t the minimum, but for this example it is.
OS: Ubuntu (4G root is sufficient) // I used Wily (15.10) Server
OSD Disk: A second “disk” for the OSD hosts. (sdb)
DNS: DNS should work to resolve the host names. (or /etc/hosts)

The above can be accomplished in visualized environments. Make sure your VM environment has networking such that all of the hosts can connect to each other on the same subnet.

Install Ceph Software

> sudo apt-get install ceph

Will install the Ceph components needed. Perform this on all hosts.

ceph.conf Configuration file

The following configuration file will be used on all hosts; it lives in /etc/ceph/ceph.conf:

fsid = d4b1b3b2-3bbb-4487-93ef-068d3ee73a22

mon_initial_members = a
mon_host =

auth_cluster_required = none
auth_service_required = none
auth_client_required = none

Two entries in this file should be changed:

  1. fsid = <uuid> ## use 'uuidgen' to generate a unique UUID
  2. mon_host = <ip-addr> ## place the actual IP of 'ceph-mon0' here.

Configure the Monitor host

  1. Place the ceph.conf file from the previous step in /etc/ceph. Owned by root and world readable (0644).

  2. Run ceph-mon to initialize the “mon data” directory (/var/lib/ceph/mon). This monitor, the only monitor in this example, will be monitor ‘a’. The -i a flag identifies the monitor as having id ‘a’. After running this command there will be a /var/lib/ceph/mon/ceph-a directory. The ‘ceph’ corresponds to the cluster name and the ‘-a’ corresponds to monitor id ‘a’.

> sudo ceph-mon --cluster=ceph -i a -f --mkfs
  1. Start ceph-mon as a daemon, (now that the data directory is initialized.
> sudo ceph-mon --cluster=ceph -i a
  1. Verify the monitor is running (from ceph-mon0)
> ceph health
HEALTH_ERR 64 pgs stuck inactive; 64 pgs stuck unclean; no osds

It is expected that the health is in error; there are no osd daemons running yet. If the call does not return, (hangs), that is the indicator of a problem.

Configure and join each OSD host

(optional: Running ceph -w on host ceph-mon0 will allow you to “tail” the activities of the cluster. You should see output after running each ceph-disk command. The ceph -w command can actually be run from any host configured to interact with the ceph cluster, but ceph-mon0 is convenient in this case.)

Repeat this process for each OSD host: mon-osd{0,1,2}

  1. Copy and install the ceph.conf file in /etc/ceph. Owned by root and world readable (0644). [do not change the mon_host IP address.]

  2. Ensure /dev/sdb is UNMOUNTED and available for both partitioning and formatting. All data on /dev/sdb will be DESTROYED

> sudo ceph-disk prepare \
    --cluster ceph \
    --cluster-uuid d4b1b3b2-3bbb-4487-93ef-068d3ee73a22 \
    --fs-type btrfs \
Creating new GPT entries.
Setting name!
partNum is 1
REALLY setting name!
The operation has completed successfully.
Setting name!
partNum is 0
REALLY setting name!
The operation has completed successfully.
WARNING: --leafsize is deprecated, use --nodesize
btrfs-progs v4.0
See http://btrfs.wiki.kernel.org for more information.

Turning ON incompat feature 'extref': increased hardlink limit per file to 65536
Turning ON incompat feature 'skinny-metadata': reduced-size metadata extent refs
fs created label (null) on /dev/sdb1
     nodesize 32768 leafsize 32768 sectorsize 4096 size 15.00GiB
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot or after you
run partprobe(8) or kpartx(8)
The operation has completed successfully.

> ceph df
    SIZE       AVAIL      RAW USED     %RAW USED
    15358M     15089M       33440k          0.21
    NAME     ID     USED     %USED     MAX AVAIL     OBJECTS
    rbd      0         0         0         5029M           0

> ps -ef | grep ceph-osd
root    2248   1  0 19:08 ?   00:00:00 /usr/bin/ceph-osd --id 0 --foreground --cluster ceph -c /etc/ceph/ceph.conf

In the above example the initial size of /dev/sdb was 20G and preparing the disk left approximately 155358M usable for storage. The remainder went to the partition used for the Ceph journal.

After applying the above command to the 2nd and 3rd (ceph-mon{1,2}) hosts you will see an appropriate increase in the available storage.

Observing the output of ps you see that the ceph-disk prepare command has started the OSD daemon as well. Upon reboot ceph-osd will be automatically started as a service.

Note: ceph-mon is not configured to autostart using this set of instructions.

Repeat for hosts mon-osd{1,2}


Exercise the Ceph cluster

The following commands will demonstrate basic cluster functionality; they can be performed from any host that has:

  1. Direct network connectivity to the cluster (no NAT - in case you used Virtualbox, et. al.)
  2. The ceph.conf file from above placed in /etc/ceph and readable.
  3. The ceph Ubuntu package installed (from step 1).

A convenient, but not necessary, host to use are any of the hosts you just installed.

List and create a pool for storage

Note: the following commands do not require the use of sudo. Authentication would be acomplished via the cephx protocol if it had not been disabled in the ceph.conf file, (the last three lines set the “auth” parameters to ‘none’).

> ceph osd pool ls

Ceph creates a default pool named ‘rbd’ when initialized.

> rados mkpool data
successfully created pool data

> rados df
pool name        KB  objects  clones  degraded  unfound   rd   rd KB   wr   wr KB
data              0        0       0         0        0    0       0    0       0
rbd               0        0       0         0        0    0       0    0       0
total used    102176       0
total avail 46354496
total space 47182788

Place a file directly in the ‘data’ pool

> touch test-file.out
> rados put -p data test-file.out test-file.out
> rados ls -p data

This concludes the quintessential Ceph installation and simple demonstration that the cluster is functioning.