=================================
 Mount CephFS using Kernel Driver
=================================

The CephFS kernel driver is part of the Linux kernel. It allows mounting
CephFS as a regular file system with native kernel performance. It is the
client of choice for most use-cases.

.. note:: CephFS mount device string now uses a new (v2) syntax. The mount
          helper (and the kernel) is backward compatible with the old syntax.
          This means that the old syntax can still be used for mounting with
          newer mount helpers and kernel. However, it is recommended to use
          the new syntax whenever possible.

Prerequisites
=============

Complete General Prerequisites
------------------------------
Go through the prerequisites required by both, kernel as well as FUSE mounts,
in `Mount CephFS: Prerequisites`_ page.

Is mount helper is present?
---------------------------
``mount.ceph`` helper is installed by Ceph packages. The helper passes the
monitor address(es) and CephX user keyrings automatically saving the Ceph
admin the effort to pass these details explicitly while mounting CephFS. In
case the helper is not present on the client machine, CephFS can still be
mounted using kernel but by passing these details explicitly to the ``mount``
command. To check whether it is present on your system, do::

    stat /sbin/mount.ceph

Which Kernel Version?
---------------------

Because the kernel client is distributed as part of the linux kernel (not
as part of packaged ceph releases), you will need to consider which kernel
version to use on your client nodes. Older kernels are known to include buggy
ceph clients, and may not support features that more recent Ceph clusters
support.

Remember that the "latest" kernel in a stable linux distribution is likely
to be years behind the latest upstream linux kernel where Ceph development
takes place (including bug fixes).

As a rough guide, as of Ceph 10.x (Jewel), you should be using a least a 4.x
kernel. If you absolutely have to use an older kernel, you should use the
fuse client instead of the kernel client.

This advice does not apply if you are using a linux distribution that
includes CephFS support, as in this case the distributor will be responsible
for backporting fixes to their stable kernel: check with your vendor.

Synopsis
========
In general, the command to mount CephFS via kernel driver looks like this::

  mount -t ceph {device-string}={path-to-mounted} {mount-point} -o {key-value-args} {other-args}

Mounting CephFS
===============
On Ceph clusters, CephX is enabled by default. Use ``mount`` command to
mount CephFS with the kernel driver::

  mkdir /mnt/mycephfs
  mount -t ceph <name>@<fsid>.<fs_name>=/ /mnt/mycephfs

``name`` is the username of the CephX user we are using to mount CephFS.
``fsid`` is the FSID of the ceph cluster which can be found using
``ceph fsid`` command. ``fs_name`` is the file system to mount. The kernel
driver requires MON's socket and the secret key for the CephX user, e.g.::

  mount -t ceph cephuser@b3acfc0d-575f-41d3-9c91-0e7ed3dbb3fa.cephfs=/ -o mon_addr=192.168.0.1:6789,secret=AQATSKdNGBnwLhAAnNDKnH65FmVKpXZJVasUeQ==

When using the mount helper, monitor hosts and FSID are optional. ``mount.ceph``
helper figures out these details automatically by finding and reading ceph conf
file, .e.g::

  mount -t ceph cephuser@.cephfs=/ -o secret=AQATSKdNGBnwLhAAnNDKnH65FmVKpXZJVasUeQ==

.. note:: Note that the dot (``.``) still needs to be a part of the device string.

A potential problem with the above command is that the secret key is left in your
shell's command history. To prevent that you can copy the secret key inside a file
and pass the file by using the option ``secretfile`` instead of ``secret``::

  mount -t ceph cephuser@.cephfs=/ /mnt/mycephfs -o secretfile=/etc/ceph/cephuser.secret

Ensure the permissions on the secret key file are appropriate (preferably, ``600``).

Multiple monitor hosts can be passed by separating each address with a ``/``::

  mount -t ceph cephuser@.cephfs=/ /mnt/mycephfs -o mon_addr=192.168.0.1:6789/192.168.0.2:6789,secretfile=/etc/ceph/cephuser.secret

In case CephX is disabled, you can omit any credential related options::

  mount -t ceph cephuser@.cephfs=/ /mnt/mycephfs

.. note:: The ceph user name still needs to be passed as part of the device string.

To mount a subtree of the CephFS root, append the path to the device string::

  mount -t ceph cephuser@.cephfs=/subvolume/dir1/dir2 /mnt/mycephfs -o secretfile=/etc/ceph/cephuser.secret

Backward Compatibility
======================
The old syntax is supported for backward compatibility.

To mount CephFS with the kernel driver::

    mkdir /mnt/mycephfs
    mount -t ceph :/ /mnt/mycephfs -o name=admin

The key-value argument right after option ``-o`` is CephX credential;
``name`` is the username of the CephX user we are using to mount CephFS.

To mount a non-default FS ``cephfs2``, in case the cluster has multiple FSs::

    mount -t ceph :/ /mnt/mycephfs -o name=admin,fs=cephfs2

    or

    mount -t ceph :/ /mnt/mycephfs -o name=admin,mds_namespace=cephfs2

.. note:: The option ``mds_namespace`` is deprecated. Use ``fs=`` instead when using the old syntax for mounting.

Unmounting CephFS
=================
To unmount the Ceph file system, use the ``umount`` command as usual::

    umount /mnt/mycephfs

.. tip:: Ensure that you are not within the file system directories before
   executing this command.

Persistent Mounts
==================

To mount CephFS in your file systems table as a kernel driver, add the
following to ``/etc/fstab``::

  {name}@.{fs_name}=/ {mount}/{mountpoint} ceph [mon_addr={ipaddress},secret=secretkey|secretfile=/path/to/secretfile],[{mount.options}]  {fs_freq}  {fs_passno}

For example::

  cephuser@.cephfs=/     /mnt/ceph    ceph    mon_addr=192.168.0.1:6789,noatime,_netdev    0       0

If the ``secret`` or ``secretfile`` options are not specified then the mount helper
will attempt to find a secret for the given ``name`` in one of the configured keyrings.

See `User Management`_ for details on CephX user management and mount.ceph_
manual for more options it can take. For troubleshooting, see
:ref:`kernel_mount_debugging`.

.. _fstab: ../fstab/#kernel-driver
.. _Mount CephFS\: Prerequisites: ../mount-prerequisites
.. _mount.ceph: ../../man/8/mount.ceph/
.. _User Management: ../../rados/operations/user-management/
