.. 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
  
  Also,

   * 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'.
  
  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 monip[:port][,monip2[:port]...]:/[subdir] mnt
  
  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

  1.2.3.4::

  
   # mount -t ceph 1.2.3.4:/ /mnt/ceph
  
  is sufficient.  If /sbin/mount.ceph is installed, a hostname can be
  used instead of an IP address.
  
  
  
  Mount Options
  =============
  
    ip=A.B.C.D[:N]
  	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.

  
    wsize=X

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

  
    rsize=X

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

  
    rasize=X

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

  
    mount_timeout=X
  	Specify the timeout value for mount (in seconds), in the case

  	of a non-responsive Ceph file system.  The default is 60

  	seconds.

    caps_max=X
  	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)

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

  	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.

    dcache
          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
          valid.)
  
    nodcache
          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.

    noasyncreaddir
  	Do not use the dcache as above for readdir.

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

    nocopyfrom
          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.

    recover_session=<no|clean>

  	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

  	https://ceph.com/

  
  The Linux kernel client source tree is available at

  	- https://github.com/ceph/ceph-client.git
  	- git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git

  
  and the source for the full system is at

  	https://github.com/ceph/ceph.git