1 .. SPDX-License-Identifier: GPL-2.0
3 ============================
4 Ceph Distributed File System
5 ============================
7 Ceph is a distributed network file system designed to provide good
8 performance, reliability, and scalability.
10 Basic features include:
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
22 * Flexible snapshots (on any directory)
23 * Recursive accounting (nested files, directories, bytes)
25 In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
26 on symmetric access by all clients to shared block devices, Ceph
27 separates data and metadata management into independent server
28 clusters, similar to Lustre. Unlike Lustre, however, metadata and
29 storage nodes run entirely as user space daemons. File data is striped
30 across storage nodes in large chunks to distribute workload and
31 facilitate high throughputs. When storage nodes fail, data is
32 re-replicated in a distributed fashion by the storage nodes themselves
33 (with some minimal coordination from a cluster monitor), making the
34 system extremely efficient and scalable.
36 Metadata servers effectively form a large, consistent, distributed
37 in-memory cache above the file namespace that is extremely scalable,
38 dynamically redistributes metadata in response to workload changes,
39 and can tolerate arbitrary (well, non-Byzantine) node failures. The
40 metadata server takes a somewhat unconventional approach to metadata
41 storage to significantly improve performance for common workloads. In
42 particular, inodes with only a single link are embedded in
43 directories, allowing entire directories of dentries and inodes to be
44 loaded into its cache with a single I/O operation. The contents of
45 extremely large directories can be fragmented and managed by
46 independent metadata servers, allowing scalable concurrent access.
48 The system offers automatic data rebalancing/migration when scaling
49 from a small cluster of just a few nodes to many hundreds, without
50 requiring an administrator carve the data set into static volumes or
51 go through the tedious process of migrating data between servers.
52 When the file system approaches full, new nodes can be easily added
53 and things will "just work."
55 Ceph includes flexible snapshot mechanism that allows a user to create
56 a snapshot on any subdirectory (and its nested contents) in the
57 system. Snapshot creation and deletion are as simple as 'mkdir
58 .snap/foo' and 'rmdir .snap/foo'.
60 Snapshot names have two limitations:
62 * They can not start with an underscore ('_'), as these names are reserved
63 for internal usage by the MDS.
64 * They can not exceed 240 characters in size. This is because the MDS makes
65 use of long snapshot names internally, which follow the format:
66 `_<SNAPSHOT-NAME>_<INODE-NUMBER>`. Since filenames in general can't have
67 more than 255 characters, and `<node-id>` takes 13 characters, the long
68 snapshot names can take as much as 255 - 1 - 1 - 13 = 240.
70 Ceph also provides some recursive accounting on directories for nested
71 files and bytes. That is, a 'getfattr -d foo' on any directory in the
72 system will reveal the total number of nested regular files and
73 subdirectories, and a summation of all nested file sizes. This makes
74 the identification of large disk space consumers relatively quick, as
75 no 'du' or similar recursive scan of the file system is required.
77 Finally, Ceph also allows quotas to be set on any directory in the system.
78 The quota can restrict the number of bytes or the number of files stored
79 beneath that point in the directory hierarchy. Quotas can be set using
80 extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg::
82 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir
83 getfattr -n ceph.quota.max_bytes /some/dir
85 A limitation of the current quotas implementation is that it relies on the
86 cooperation of the client mounting the file system to stop writers when a
87 limit is reached. A modified or adversarial client cannot be prevented
88 from writing as much data as it needs.
93 The basic mount syntax is::
95 # mount -t ceph user@fsid.fs_name=/[subdir] mnt -o mon_addr=monip1[:port][/monip2[:port]]
97 You only need to specify a single monitor, as the client will get the
98 full list when it connects. (However, if the monitor you specify
99 happens to be down, the mount won't succeed.) The port can be left
100 off if the monitor is using the default. So if the monitor is at
103 # mount -t ceph cephuser@07fe3187-00d9-42a3-814b-72a4d5e7d5be.cephfs=/ /mnt/ceph -o mon_addr=1.2.3.4
105 is sufficient. If /sbin/mount.ceph is installed, a hostname can be
106 used instead of an IP address and the cluster FSID can be left out
107 (as the mount helper will fill it in by reading the ceph configuration
110 # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=mon-addr
112 Multiple monitor addresses can be passed by separating each address with a slash (`/`)::
114 # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=192.168.1.100/192.168.1.101
116 When using the mount helper, monitor address can be read from ceph
117 configuration file if available. Note that, the cluster FSID (passed as part
118 of the device string) is validated by checking it with the FSID reported by
124 mon_addr=ip_address[:port][/ip_address[:port]]
125 Monitor address to the cluster. This is used to bootstrap the
126 connection to the cluster. Once connection is established, the
127 monitor addresses in the monitor map are followed.
130 FSID of the cluster (from `ceph fsid` command).
133 Specify the IP and/or port the client should bind to locally.
134 There is normally not much reason to do this. If the IP is not
135 specified, the client's IP address is determined by looking at the
136 address its connection to the monitor originates from.
139 Specify the maximum write size in bytes. Default: 64 MB.
142 Specify the maximum read size in bytes. Default: 64 MB.
145 Specify the maximum readahead size in bytes. Default: 8 MB.
148 Specify the timeout value for mount (in seconds), in the case
149 of a non-responsive Ceph file system. The default is 60
153 Specify the maximum number of caps to hold. Unused caps are released
154 when number of caps exceeds the limit. The default is 0 (no limit)
157 When stat() is called on a directory, set st_size to 'rbytes',
158 the summation of file sizes over all files nested beneath that
159 directory. This is the default.
162 When stat() is called on a directory, set st_size to the
163 number of entries in that directory.
166 Disable CRC32C calculation for data writes. If set, the storage node
167 must rely on TCP's error correction to detect data corruption
171 Use the dcache contents to perform negative lookups and
172 readdir when the client has the entire directory contents in
173 its cache. (This does not change correctness; the client uses
174 cached metadata only when a lease or capability ensures it is
178 Do not use the dcache as above. This avoids a significant amount of
179 complex code, sacrificing performance without affecting correctness,
180 and is useful for tracking down bugs.
183 Do not use the dcache as above for readdir.
186 Report overall filesystem usage in statfs instead of using the root
190 Don't use the RADOS 'copy-from' operation to perform remote object
191 copies. Currently, it's only used in copy_file_range, which will revert
192 to the default VFS implementation if this option is used.
194 recover_session=<no|clean>
195 Set auto reconnect mode in the case where the client is blocklisted. The
196 available modes are "no" and "clean". The default is "no".
198 * no: never attempt to reconnect when client detects that it has been
199 blocklisted. Operations will generally fail after being blocklisted.
201 * clean: client reconnects to the ceph cluster automatically when it
202 detects that it has been blocklisted. During reconnect, client drops
203 dirty data/metadata, invalidates page caches and writable file handles.
204 After reconnect, file locks become stale because the MDS loses track
205 of them. If an inode contains any stale file locks, read/write on the
206 inode is not allowed until applications release all stale file locks.
211 For more information on Ceph, see the home page at
214 The Linux kernel client source tree is available at
215 - https://github.com/ceph/ceph-client.git
217 and the source for the full system is at
218 https://github.com/ceph/ceph.git