ceph/doc/rados/operations/add-or-rm-osds.rst

======================
 Adding/Removing OSDs
======================

When you have a cluster up and running, you may add OSDs or remove OSDs
from the cluster at runtime. 

Adding OSDs
===========

When you want to expand a cluster, you may add an OSD at runtime. With Ceph, an
OSD is generally one Ceph ``ceph-osd`` daemon for one storage disk within a host
machine. If your host has multiple storage disks, you may map one ``ceph-osd``
daemon for each disk.

Generally, it's a good idea to check the capacity of your cluster to see if you
are reaching the upper end of its capacity. As your cluster reaches its ``near
full`` ratio, you should add one or more OSDs to expand your cluster's capacity.

.. warning:: Do not let your cluster reach its ``full ratio`` before
   adding an OSD. OSD failures that occur after the cluster reaches 
   its ``near full`` ratio may cause the cluster to exceed its
   ``full ratio``.

Deploy your Hardware
--------------------

If you are adding a new host when adding a new OSD, 
see `Hardware Recommendations`_ for details on minimum recommendations
for OSD hardware. To add a OSD host to your cluster, first make sure you have 
an up-to-date version of Linux installed (typically Ubuntu 12.04 precise), 
and you have made some initial preparations for your storage disks. 
See `Filesystem Recommendations`_ for details. 

Add your OSD host to a rack in your cluster, connect it to the network
and ensure that it has network connectivity.

.. _Hardware Recommendations: ../../install/hardware-recommendations
.. _Filesystem Recommendations: ../../config-cluster/file-system-recommendations

Install the Required Software
-----------------------------

For manually deployed clusters, you must install Ceph packages
manually. See `Installing Debian/Ubuntu Packages`_ for details.
You should configure SSH to a user with password-less authentication
and root permissions.

.. _Installing Debian/Ubuntu Packages: ../../install/debian

For clusters deployed with Chef, create a `chef user`_, `configure
SSH keys`_, `install Ruby`_ and `install the Chef client`_ on your host. See 
`Installing Chef`_ for details.

.. _chef user: ../../install/chef#createuser
.. _configure SSH keys: ../../install/chef#genkeys
.. _install the Chef client: ../../install/chef#installchef
.. _Installing Chef: ../../install/chef
.. _Install Ruby: ../../install/chef#installruby

Adding an OSD (Manual)
----------------------

This procedure sets up an ``ceph-osd`` daemon, configures it to use one disk,
and configures the cluster to distribute data to the OSD. If your host has
multiple disks,  you may add an OSD for each disk by repeating this procedure.

To add an OSD, create a data directory for it, mount a disk to that directory,
add the OSD to your configuration file, add the OSD to the cluster, and then
add it to the CRUSH map.

When you add the OSD to the CRUSH map, consider the weight you give to the new
OSD.  Hard disk capacity grows 40% per year, so newer OSD hosts may have larger
hard disks than older hosts in the cluster (i.e., they may have greater weight).

#. Create the default directory on your new OSD. :: 

	ssh {new-osd-host}
	sudo mkdir /var/lib/ceph/osd/ceph-{osd-number}
	

#. If the OSD is for a disk other than the OS disk, prepare it 
   for use with Ceph, and mount it to the directory you just created:: 

	ssh {new-osd-host}
	sudo mkfs -t {fstype} /dev/{disk}
	sudo mount -o user_xattr /dev/{hdd} /var/lib/ceph/osd/ceph-{osd-number}


#. Navigate to the host where you keep the master copy of the cluster's 
   ``ceph.conf`` file. :: 

	ssh {admin-host}
	cd /etc/ceph
	vim ceph.conf

#. Add the new OSD to your ``ceph.conf`` file.
	
   .. code-block:: ini

     [osd.123]
         host = {hostname}
 
#. From the host where you keep the master copy of the cluster's 
   ``ceph.conf`` file, copy the updated ``ceph.conf`` file to your 
   new OSD's ``/etc/ceph`` directory and to other hosts in your cluster. :: 

	ssh {new-osd} sudo tee /etc/ceph/ceph.conf < /etc/ceph/ceph.conf

#. Create the OSD. ::

	ceph osd create {osd-num}
	ceph osd create 123 #for example
	
#. Initialize the OSD data directory. :: 

	ssh {new-osd-host}
	ceph-osd -i {osd-num} --mkfs --mkkey
	
   The directory must be empty before you can run ``ceph-osd``.

#. Register the OSD authentication key. The value of ``ceph`` for 
   ``ceph-{osd-num}`` in the path is the ``$cluster-$id``.  If your 
   cluster name differs from ``ceph``, use your cluster name instead.::

	ceph auth add osd.{osd-num} osd 'allow *' mon 'allow rwx' -i /var/lib/ceph/osd/ceph-{osd-num}/keyring

#. Add the OSD to the CRUSH map so that it can begin receiving data. You may
   also decompile the CRUSH map, add the OSD to the device list, add the host as a
   bucket (if it's not already in the CRUSH map), add the device as an item in the
   host, assign it a weight, recompile it and set it. See	`Add/Move an OSD`_ for
   details. :: 

	ceph osd crush set {id} {name} {weight}  [{bucket-type}={bucket-name}, ...]


.. topic:: Argonaut (v0.48) Best Practices

 To limit impact on user I/O performance, add an OSD to the CRUSH map
 with an initial weight of ``0``. Then, ramp up the CRUSH weight a
 little bit at a time.  For example, to ramp by increments of ``0.2``,
 start with::

      ceph osd crush reweight {osd-id} .2

 and allow migration to complete before reweighting to ``0.4``,
 ``0.6``, and so on until the desired CRUSH weight is reached.

 To limit the impact of OSD failures, you can set::

      mon osd down out interval = 0

 which prevents down OSDs from automatically being marked out, and then
 ramp them down manually with::

      ceph osd reweight {osd-num} .8

 Again, wait for the cluster to finish migrating data, and then adjust
 the weight further until you reach a weight of 0.  Note that this
 problem prevents the cluster to automatically re-replicate data after
 a failure, so please ensure that sufficient monitoring is in place for
 an administrator to intervene promptly.

 Note that this practice will no longer be necessary in Bobtail and
 subsequent releases.


Adding an OSD (Chef)
--------------------

This procedure configures your OSD using ``chef-client``. If your host has
multiple disks, you may need to execute the procedure for preparing an OSD disk
for each data disk on your host.

When you add the OSD to the CRUSH map, consider the weight you give to the new
OSD.  Hard disk capacity grows 40% per year, so newer OSD hosts may have larger
hard disks than older hosts in the cluster.

#. Execute ``chef-client`` to register it with Chef as a Chef node.

#. Edit the node. See `Configure Nodes`_ for details.
   Change its environment to your Chef environment.
   Add ``"role[ceph-osd]"`` to the run list.

#. Execute `Prepare OSD Disks`_ for each disk.

#. Execute ``chef-client`` to invoke the run list.

#. Add the OSD to the CRUSH map so that it can begin receiving data. You may
   also decompile the CRUSH map edit the file, recompile it and set it. See
   `Add/Move an OSD`_ for details. :: 

	ceph osd crush set {id} {name} {weight} pool={pool-name}  [{bucket-type}={bucket-name}, ...]


Starting the OSD
----------------

After you add an OSD to Ceph, the OSD is in your configuration. However, 
it is not yet running. The OSD is ``down`` and ``out``. You must start 
your new OSD before it can begin receiving data. You may use
``service ceph`` from your admin host or start the OSD from its host
machine::

	service ceph -a start osd.{osd.num}
	#or alternatively
	ssh {new-osd-host}
	sudo /etc/init.d/ceph start osd.{osd-num}


Once you start your OSD, it is ``up``.

Put the OSD ``in`` the Cluster
------------------------------

After you start your OSD, it is ``up`` and ``out``.  You need to put it in to
the cluster so that Ceph can begin writing data to it. :: 

	ceph osd in {osd-num}


Observe the Data Migration
--------------------------

Once you have added your new OSD to the CRUSH map, Ceph  will begin rebalancing
the server by migrating placement groups to your new OSD. You can observe this
process with  the `ceph`_ tool. :: 

	ceph -w

You should see the placement group states change from ``active+clean`` to
``active, some degraded objects``, and finally ``active+clean`` when migration
completes. (Control-c to exit.)


.. _Add/Move an OSD: ../crush-map#addosd
.. _Configure Nodes: ../../config-cluster/chef#confignodes
.. _Prepare OSD Disks: ../../config-cluster/chef#prepdisks
.. _ceph: ../monitoring



Removing OSDs
=============

When you want to reduce the size of a cluster or replace hardware, you may
remove an OSD at runtime. With Ceph, an OSD is generally one Ceph ``ceph-osd``
daemon for one storage disk within a host machine. If your host has multiple
storage disks, you may need to remove one ``ceph-osd`` daemon for each disk.
Generally, it's a good idea to check the capacity of your cluster to see if you
are reaching the upper end of its capacity. Ensure that when you remove an OSD
that your cluster is not at its ``near full`` ratio.

.. warning:: Do not let your cluster reach its ``full ratio`` when
   removing an OSD. Removing OSDs could cause the cluster to reach 
   or exceed its ``full ratio``.
   

Take the OSD ``out`` of the Cluster
-----------------------------------

Before you remove an OSD, it is usually ``up`` and ``in``.  You need to take it
out of the cluster so that Ceph can begin rebalancing and copying its data to
other OSDs. :: 

	ceph osd out {osd-num}


Observe the Data Migration
--------------------------

Once you have taken your OSD ``out`` of the cluster, Ceph  will begin
rebalancing the cluster by migrating placement groups out of the OSD you
removed. You can observe  this process with  the `ceph`_ tool. :: 

	ceph -w

You should see the placement group states change from ``active+clean`` to
``active, some degraded objects``, and finally ``active+clean`` when migration
completes. (Control-c to exit.)


Stopping the OSD
----------------

After you take an OSD out of the cluster, it may still be running. 
That is, the OSD may be ``up`` and ``out``. You must stop 
your OSD before you remove it from the configuration. :: 

	ssh {new-osd-host}
	sudo /etc/init.d/ceph stop osd.{osd-num}

Once you stop your OSD, it is ``down``. 


Removing an OSD (Manual)
------------------------

This procedure removes an OSD from a cluster map, removes its authentication
key, removes the OSD from the OSD map, and removes the OSD from the
``ceph.conf`` file. If your host has multiple disks,  you may need to remove an
OSD for each disk by repeating this procedure.


#. Remove the OSD from the CRUSH map so that it no longer receives data. You may
   also decompile the CRUSH map, remove the OSD from the device list, remove the
   device as an item in the host bucket or remove the host  bucket (if it's in the
   CRUSH map and you intend to remove the host), recompile the map and set it. 
   See `Remove an OSD`_ for details. :: 

	ceph osd crush remove {name}
	
#. Remove the OSD authentication key. ::

	ceph auth del osd.{osd-num}
	
   The value of ``ceph`` for ``ceph-{osd-num}`` in the path is the ``$cluster-$id``. 
   If your cluster name differs from ``ceph``, use your cluster name instead.	
	
#. Remove the OSD. ::

	ceph osd rm {osd-num}
	#for example
	ceph osd rm 123
	
#. Navigate to the host where you keep the master copy of the cluster's 
   ``ceph.conf`` file. ::

	ssh {admin-host}
	cd /etc/chef
	vim ceph.conf

#. Remove the OSD entry from your ``ceph.conf`` file. ::

	[osd.123]
		host = {hostname}
 
#. From the host where you keep the master copy of the cluster's ``ceph.conf`` file, 
   copy the updated ``ceph.conf`` file to the ``/etc/ceph`` directory of other 
   hosts in your cluster. :: 

	ssh {osd} sudo tee /etc/ceph/ceph.conf < /etc/ceph/ceph.conf		
	
.. _Remove an OSD: ../crush-map#removeosd