Documentation / filesystems / ceph.rst

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.. SPDX-License-Identifier: GPL-2.0

Ceph Distributed File System

Ceph is a distributed network file system designed to provide good
performance, reliability, and scalability.

Basic features include:

 * POSIX semantics
 * Seamless scaling from 1 to many thousands of nodes
 * High availability and reliability.  No single point of failure.
 * N-way replication of data across storage nodes
 * Fast recovery from node failures
 * Automatic rebalancing of data on node addition/removal
 * Easy deployment: most FS components are userspace daemons


 * Flexible snapshots (on any directory)
 * Recursive accounting (nested files, directories, bytes)

In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
on symmetric access by all clients to shared block devices, Ceph
separates data and metadata management into independent server
clusters, similar to Lustre.  Unlike Lustre, however, metadata and
storage nodes run entirely as user space daemons.  File data is striped
across storage nodes in large chunks to distribute workload and
facilitate high throughputs.  When storage nodes fail, data is
re-replicated in a distributed fashion by the storage nodes themselves
(with some minimal coordination from a cluster monitor), making the
system extremely efficient and scalable.

Metadata servers effectively form a large, consistent, distributed
in-memory cache above the file namespace that is extremely scalable,
dynamically redistributes metadata in response to workload changes,
and can tolerate arbitrary (well, non-Byzantine) node failures.  The
metadata server takes a somewhat unconventional approach to metadata
storage to significantly improve performance for common workloads.  In
particular, inodes with only a single link are embedded in
directories, allowing entire directories of dentries and inodes to be
loaded into its cache with a single I/O operation.  The contents of
extremely large directories can be fragmented and managed by
independent metadata servers, allowing scalable concurrent access.

The system offers automatic data rebalancing/migration when scaling
from a small cluster of just a few nodes to many hundreds, without
requiring an administrator carve the data set into static volumes or
go through the tedious process of migrating data between servers.
When the file system approaches full, new nodes can be easily added
and things will "just work."

Ceph includes flexible snapshot mechanism that allows a user to create
a snapshot on any subdirectory (and its nested contents) in the
system.  Snapshot creation and deletion are as simple as 'mkdir
.snap/foo' and 'rmdir .snap/foo'.

Snapshot names have two limitations:

* They can not start with an underscore ('_'), as these names are reserved
  for internal usage by the MDS.
* They can not exceed 240 characters in size.  This is because the MDS makes
  use of long snapshot names internally, which follow the format:
  `_<SNAPSHOT-NAME>_<INODE-NUMBER>`.  Since filenames in general can't have
  more than 255 characters, and `<node-id>` takes 13 characters, the long
  snapshot names can take as much as 255 - 1 - 1 - 13 = 240.

Ceph also provides some recursive accounting on directories for nested
files and bytes.  That is, a 'getfattr -d foo' on any directory in the
system will reveal the total number of nested regular files and
subdirectories, and a summation of all nested file sizes.  This makes
the identification of large disk space consumers relatively quick, as
no 'du' or similar recursive scan of the file system is required.

Finally, Ceph also allows quotas to be set on any directory in the system.
The quota can restrict the number of bytes or the number of files stored
beneath that point in the directory hierarchy.  Quotas can be set using
extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg::

 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir
 getfattr -n ceph.quota.max_bytes /some/dir

A limitation of the current quotas implementation is that it relies on the
cooperation of the client mounting the file system to stop writers when a
limit is reached.  A modified or adversarial client cannot be prevented
from writing as much data as it needs.

Mount Syntax

The basic mount syntax is::

 # mount -t ceph user@fsid.fs_name=/[subdir] mnt -o mon_addr=monip1[:port][/monip2[:port]]

You only need to specify a single monitor, as the client will get the
full list when it connects.  (However, if the monitor you specify
happens to be down, the mount won't succeed.)  The port can be left
off if the monitor is using the default.  So if the monitor is at

 # mount -t ceph cephuser@07fe3187-00d9-42a3-814b-72a4d5e7d5be.cephfs=/ /mnt/ceph -o mon_addr=

is sufficient.  If /sbin/mount.ceph is installed, a hostname can be
used instead of an IP address and the cluster FSID can be left out
(as the mount helper will fill it in by reading the ceph configuration

  # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=mon-addr

Multiple monitor addresses can be passed by separating each address with a slash (`/`)::

  # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=

When using the mount helper, monitor address can be read from ceph
configuration file if available. Note that, the cluster FSID (passed as part
of the device string) is validated by checking it with the FSID reported by
the monitor.

Mount Options

	Monitor address to the cluster. This is used to bootstrap the
        connection to the cluster. Once connection is established, the
        monitor addresses in the monitor map are followed.

	FSID of the cluster (from `ceph fsid` command).

	Specify the IP and/or port the client should bind to locally.
	There is normally not much reason to do this.  If the IP is not
	specified, the client's IP address is determined by looking at the
	address its connection to the monitor originates from.

	Specify the maximum write size in bytes.  Default: 64 MB.

	Specify the maximum read size in bytes.  Default: 64 MB.

	Specify the maximum readahead size in bytes.  Default: 8 MB.

	Specify the timeout value for mount (in seconds), in the case
	of a non-responsive Ceph file system.  The default is 60

	Specify the maximum number of caps to hold. Unused caps are released
	when number of caps exceeds the limit. The default is 0 (no limit)

	When stat() is called on a directory, set st_size to 'rbytes',
	the summation of file sizes over all files nested beneath that
	directory.  This is the default.

	When stat() is called on a directory, set st_size to the
	number of entries in that directory.

	Disable CRC32C calculation for data writes.  If set, the storage node
	must rely on TCP's error correction to detect data corruption
	in the data payload.

        Use the dcache contents to perform negative lookups and
        readdir when the client has the entire directory contents in
        its cache.  (This does not change correctness; the client uses
        cached metadata only when a lease or capability ensures it is

        Do not use the dcache as above.  This avoids a significant amount of
        complex code, sacrificing performance without affecting correctness,
        and is useful for tracking down bugs.

	Do not use the dcache as above for readdir.

        Report overall filesystem usage in statfs instead of using the root
        directory quota.

        Don't use the RADOS 'copy-from' operation to perform remote object
        copies.  Currently, it's only used in copy_file_range, which will revert
        to the default VFS implementation if this option is used.

	Set auto reconnect mode in the case where the client is blocklisted. The
	available modes are "no" and "clean". The default is "no".

	* no: never attempt to reconnect when client detects that it has been
	  blocklisted. Operations will generally fail after being blocklisted.

	* clean: client reconnects to the ceph cluster automatically when it
	  detects that it has been blocklisted. During reconnect, client drops
	  dirty data/metadata, invalidates page caches and writable file handles.
	  After reconnect, file locks become stale because the MDS loses track
	  of them. If an inode contains any stale file locks, read/write on the
	  inode is not allowed until applications release all stale file locks.

More Information

For more information on Ceph, see the home page at

The Linux kernel client source tree is available at

and the source for the full system is at