1.. SPDX-License-Identifier: GPL-2.0
2
3============================
4Ceph Distributed File System
5============================
6
7Ceph is a distributed network file system designed to provide good
8performance, reliability, and scalability.
9
10Basic features include:
11
12 * POSIX semantics
13 * Seamless scaling from 1 to many thousands of nodes
14 * High availability and reliability.  No single point of failure.
15 * N-way replication of data across storage nodes
16 * Fast recovery from node failures
17 * Automatic rebalancing of data on node addition/removal
18 * Easy deployment: most FS components are userspace daemons
19
20Also,
21
22 * Flexible snapshots (on any directory)
23 * Recursive accounting (nested files, directories, bytes)
24
25In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
26on symmetric access by all clients to shared block devices, Ceph
27separates data and metadata management into independent server
28clusters, similar to Lustre.  Unlike Lustre, however, metadata and
29storage nodes run entirely as user space daemons.  File data is striped
30across storage nodes in large chunks to distribute workload and
31facilitate high throughputs.  When storage nodes fail, data is
32re-replicated in a distributed fashion by the storage nodes themselves
33(with some minimal coordination from a cluster monitor), making the
34system extremely efficient and scalable.
35
36Metadata servers effectively form a large, consistent, distributed
37in-memory cache above the file namespace that is extremely scalable,
38dynamically redistributes metadata in response to workload changes,
39and can tolerate arbitrary (well, non-Byzantine) node failures.  The
40metadata server takes a somewhat unconventional approach to metadata
41storage to significantly improve performance for common workloads.  In
42particular, inodes with only a single link are embedded in
43directories, allowing entire directories of dentries and inodes to be
44loaded into its cache with a single I/O operation.  The contents of
45extremely large directories can be fragmented and managed by
46independent metadata servers, allowing scalable concurrent access.
47
48The system offers automatic data rebalancing/migration when scaling
49from a small cluster of just a few nodes to many hundreds, without
50requiring an administrator carve the data set into static volumes or
51go through the tedious process of migrating data between servers.
52When the file system approaches full, new nodes can be easily added
53and things will "just work."
54
55Ceph includes flexible snapshot mechanism that allows a user to create
56a snapshot on any subdirectory (and its nested contents) in the
57system.  Snapshot creation and deletion are as simple as 'mkdir
58.snap/foo' and 'rmdir .snap/foo'.
59
60Ceph also provides some recursive accounting on directories for nested
61files and bytes.  That is, a 'getfattr -d foo' on any directory in the
62system will reveal the total number of nested regular files and
63subdirectories, and a summation of all nested file sizes.  This makes
64the identification of large disk space consumers relatively quick, as
65no 'du' or similar recursive scan of the file system is required.
66
67Finally, Ceph also allows quotas to be set on any directory in the system.
68The quota can restrict the number of bytes or the number of files stored
69beneath that point in the directory hierarchy.  Quotas can be set using
70extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg::
71
72 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir
73 getfattr -n ceph.quota.max_bytes /some/dir
74
75A limitation of the current quotas implementation is that it relies on the
76cooperation of the client mounting the file system to stop writers when a
77limit is reached.  A modified or adversarial client cannot be prevented
78from writing as much data as it needs.
79
80Mount Syntax
81============
82
83The basic mount syntax is::
84
85 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt
86
87You only need to specify a single monitor, as the client will get the
88full list when it connects.  (However, if the monitor you specify
89happens to be down, the mount won't succeed.)  The port can be left
90off if the monitor is using the default.  So if the monitor is at
911.2.3.4::
92
93 # mount -t ceph 1.2.3.4:/ /mnt/ceph
94
95is sufficient.  If /sbin/mount.ceph is installed, a hostname can be
96used instead of an IP address.
97
98
99
100Mount Options
101=============
102
103  ip=A.B.C.D[:N]
104	Specify the IP and/or port the client should bind to locally.
105	There is normally not much reason to do this.  If the IP is not
106	specified, the client's IP address is determined by looking at the
107	address its connection to the monitor originates from.
108
109  wsize=X
110	Specify the maximum write size in bytes.  Default: 64 MB.
111
112  rsize=X
113	Specify the maximum read size in bytes.  Default: 64 MB.
114
115  rasize=X
116	Specify the maximum readahead size in bytes.  Default: 8 MB.
117
118  mount_timeout=X
119	Specify the timeout value for mount (in seconds), in the case
120	of a non-responsive Ceph file system.  The default is 60
121	seconds.
122
123  caps_max=X
124	Specify the maximum number of caps to hold. Unused caps are released
125	when number of caps exceeds the limit. The default is 0 (no limit)
126
127  rbytes
128	When stat() is called on a directory, set st_size to 'rbytes',
129	the summation of file sizes over all files nested beneath that
130	directory.  This is the default.
131
132  norbytes
133	When stat() is called on a directory, set st_size to the
134	number of entries in that directory.
135
136  nocrc
137	Disable CRC32C calculation for data writes.  If set, the storage node
138	must rely on TCP's error correction to detect data corruption
139	in the data payload.
140
141  dcache
142        Use the dcache contents to perform negative lookups and
143        readdir when the client has the entire directory contents in
144        its cache.  (This does not change correctness; the client uses
145        cached metadata only when a lease or capability ensures it is
146        valid.)
147
148  nodcache
149        Do not use the dcache as above.  This avoids a significant amount of
150        complex code, sacrificing performance without affecting correctness,
151        and is useful for tracking down bugs.
152
153  noasyncreaddir
154	Do not use the dcache as above for readdir.
155
156  noquotadf
157        Report overall filesystem usage in statfs instead of using the root
158        directory quota.
159
160  nocopyfrom
161        Don't use the RADOS 'copy-from' operation to perform remote object
162        copies.  Currently, it's only used in copy_file_range, which will revert
163        to the default VFS implementation if this option is used.
164
165  recover_session=<no|clean>
166	Set auto reconnect mode in the case where the client is blacklisted. The
167	available modes are "no" and "clean". The default is "no".
168
169	* no: never attempt to reconnect when client detects that it has been
170	  blacklisted. Operations will generally fail after being blacklisted.
171
172	* clean: client reconnects to the ceph cluster automatically when it
173	  detects that it has been blacklisted. During reconnect, client drops
174	  dirty data/metadata, invalidates page caches and writable file handles.
175	  After reconnect, file locks become stale because the MDS loses track
176	  of them. If an inode contains any stale file locks, read/write on the
177	  inode is not allowed until applications release all stale file locks.
178
179More Information
180================
181
182For more information on Ceph, see the home page at
183	https://ceph.com/
184
185The Linux kernel client source tree is available at
186	- https://github.com/ceph/ceph-client.git
187	- git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git
188
189and the source for the full system is at
190	https://github.com/ceph/ceph.git
191