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Documentation / filesystems / ceph.txt

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Based on kernel version 4.8. Page generated on 2016-10-06 23:15 EST.

1	Ceph Distributed File System
2	============================
4	Ceph is a distributed network file system designed to provide good
5	performance, reliability, and scalability.
7	Basic features include:
9	 * POSIX semantics
10	 * Seamless scaling from 1 to many thousands of nodes
11	 * High availability and reliability.  No single point of failure.
12	 * N-way replication of data across storage nodes
13	 * Fast recovery from node failures
14	 * Automatic rebalancing of data on node addition/removal
15	 * Easy deployment: most FS components are userspace daemons
17	Also,
18	 * Flexible snapshots (on any directory)
19	 * Recursive accounting (nested files, directories, bytes)
21	In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
22	on symmetric access by all clients to shared block devices, Ceph
23	separates data and metadata management into independent server
24	clusters, similar to Lustre.  Unlike Lustre, however, metadata and
25	storage nodes run entirely as user space daemons.  Storage nodes
26	utilize btrfs to store data objects, leveraging its advanced features
27	(checksumming, metadata replication, etc.).  File data is striped
28	across storage nodes in large chunks to distribute workload and
29	facilitate high throughputs.  When storage nodes fail, data is
30	re-replicated in a distributed fashion by the storage nodes themselves
31	(with some minimal coordination from a cluster monitor), making the
32	system extremely efficient and scalable.
34	Metadata servers effectively form a large, consistent, distributed
35	in-memory cache above the file namespace that is extremely scalable,
36	dynamically redistributes metadata in response to workload changes,
37	and can tolerate arbitrary (well, non-Byzantine) node failures.  The
38	metadata server takes a somewhat unconventional approach to metadata
39	storage to significantly improve performance for common workloads.  In
40	particular, inodes with only a single link are embedded in
41	directories, allowing entire directories of dentries and inodes to be
42	loaded into its cache with a single I/O operation.  The contents of
43	extremely large directories can be fragmented and managed by
44	independent metadata servers, allowing scalable concurrent access.
46	The system offers automatic data rebalancing/migration when scaling
47	from a small cluster of just a few nodes to many hundreds, without
48	requiring an administrator carve the data set into static volumes or
49	go through the tedious process of migrating data between servers.
50	When the file system approaches full, new nodes can be easily added
51	and things will "just work."
53	Ceph includes flexible snapshot mechanism that allows a user to create
54	a snapshot on any subdirectory (and its nested contents) in the
55	system.  Snapshot creation and deletion are as simple as 'mkdir
56	.snap/foo' and 'rmdir .snap/foo'.
58	Ceph also provides some recursive accounting on directories for nested
59	files and bytes.  That is, a 'getfattr -d foo' on any directory in the
60	system will reveal the total number of nested regular files and
61	subdirectories, and a summation of all nested file sizes.  This makes
62	the identification of large disk space consumers relatively quick, as
63	no 'du' or similar recursive scan of the file system is required.
66	Mount Syntax
67	============
69	The basic mount syntax is:
71	 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt
73	You only need to specify a single monitor, as the client will get the
74	full list when it connects.  (However, if the monitor you specify
75	happens to be down, the mount won't succeed.)  The port can be left
76	off if the monitor is using the default.  So if the monitor is at
79	 # mount -t ceph /mnt/ceph
81	is sufficient.  If /sbin/mount.ceph is installed, a hostname can be
82	used instead of an IP address.
86	Mount Options
87	=============
89	  ip=A.B.C.D[:N]
90		Specify the IP and/or port the client should bind to locally.
91		There is normally not much reason to do this.  If the IP is not
92		specified, the client's IP address is determined by looking at the
93		address its connection to the monitor originates from.
95	  wsize=X
96		Specify the maximum write size in bytes.  By default there is no
97		maximum.  Ceph will normally size writes based on the file stripe
98		size.
100	  rsize=X
101		Specify the maximum readahead.
103	  mount_timeout=X
104		Specify the timeout value for mount (in seconds), in the case
105		of a non-responsive Ceph file system.  The default is 30
106		seconds.
108	  rbytes
109		When stat() is called on a directory, set st_size to 'rbytes',
110		the summation of file sizes over all files nested beneath that
111		directory.  This is the default.
113	  norbytes
114		When stat() is called on a directory, set st_size to the
115		number of entries in that directory.
117	  nocrc
118		Disable CRC32C calculation for data writes.  If set, the storage node
119		must rely on TCP's error correction to detect data corruption
120		in the data payload.
122	  dcache
123	        Use the dcache contents to perform negative lookups and
124	        readdir when the client has the entire directory contents in
125	        its cache.  (This does not change correctness; the client uses
126	        cached metadata only when a lease or capability ensures it is
127	        valid.)
129	  nodcache
130	        Do not use the dcache as above.  This avoids a significant amount of
131	        complex code, sacrificing performance without affecting correctness,
132	        and is useful for tracking down bugs.
134	  noasyncreaddir
135		Do not use the dcache as above for readdir.
137	More Information
138	================
140	For more information on Ceph, see the home page at
141		http://ceph.newdream.net/
143	The Linux kernel client source tree is available at
144		git://ceph.newdream.net/git/ceph-client.git
145		git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git
147	and the source for the full system is at
148		git://ceph.newdream.net/git/ceph.git
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