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.

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 mountng 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 :/ /mnt/mycephfs -o name=foo

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. The default value for name is guest.

The kernel driver also requires MON’s socket and the secret key for the CephX user. In case of the above command, mount.ceph helper figures out these details automatically by finding and reading Ceph conf file and keyring. In case you don’t have these files on the host where you’re running mount command, you can pass these details yourself too:

mount -t ceph 192.168.0.1:6789,192.168.0.2:6789:/ /mnt/mycephfs -o name=foo,secret=AQATSKdNGBnwLhAAnNDKnH65FmVKpXZJVasUeQ==

Passing a single MON socket in above command works too. A potential problem with the command above 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 :/ /mnt/mycephfs -o name=foo,secretfile=/etc/ceph/foo.secret

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

In case CephX is disabled, you can omit -o and the list of key-value arguments that follow it:

mount -t ceph :/ /mnt/mycephfs

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

mount -t ceph :/subvolume/dir1/dir2 /mnt/mycephfs -o name=fs

If you have more than one file system on your Ceph cluster, you can mount the non-default FS on your local FS as follows:

mount -t ceph :/ /mnt/mycephfs2 -o name=fs,mds_namespace=mycephfs2

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:

[{ipaddress}:{port}]:/ {mount}/{mountpoint} ceph [name=username,secret=secretkey|secretfile=/path/to/secretfile],[{mount.options}]

For example:

:/     /mnt/ceph    ceph    name=admin,noatime,_netdev    0       2

The default for the name= parameter is guest. 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 Kernel mount debugging.