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Sage Weil7ad920b2009-10-06 11:31:05 -07001Ceph Distributed File System
2============================
3
4Ceph is a distributed network file system designed to provide good
5performance, reliability, and scalability.
6
7Basic features include:
8
9 * POSIX semantics
10 * Seamless scaling from 1 to many thousands of nodes
Cheng Renquan8136b582010-03-29 19:05:57 +080011 * High availability and reliability. No single point of failure.
Sage Weil7ad920b2009-10-06 11:31:05 -070012 * 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
16
17Also,
18 * Flexible snapshots (on any directory)
19 * Recursive accounting (nested files, directories, bytes)
20
21In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
22on symmetric access by all clients to shared block devices, Ceph
23separates data and metadata management into independent server
24clusters, similar to Lustre. Unlike Lustre, however, metadata and
25storage nodes run entirely as user space daemons. Storage nodes
26utilize btrfs to store data objects, leveraging its advanced features
27(checksumming, metadata replication, etc.). File data is striped
28across storage nodes in large chunks to distribute workload and
29facilitate high throughputs. When storage nodes fail, data is
30re-replicated in a distributed fashion by the storage nodes themselves
31(with some minimal coordination from a cluster monitor), making the
32system extremely efficient and scalable.
33
34Metadata servers effectively form a large, consistent, distributed
35in-memory cache above the file namespace that is extremely scalable,
36dynamically redistributes metadata in response to workload changes,
37and can tolerate arbitrary (well, non-Byzantine) node failures. The
38metadata server takes a somewhat unconventional approach to metadata
39storage to significantly improve performance for common workloads. In
40particular, inodes with only a single link are embedded in
41directories, allowing entire directories of dentries and inodes to be
42loaded into its cache with a single I/O operation. The contents of
43extremely large directories can be fragmented and managed by
44independent metadata servers, allowing scalable concurrent access.
45
46The system offers automatic data rebalancing/migration when scaling
47from a small cluster of just a few nodes to many hundreds, without
48requiring an administrator carve the data set into static volumes or
49go through the tedious process of migrating data between servers.
50When the file system approaches full, new nodes can be easily added
51and things will "just work."
52
53Ceph includes flexible snapshot mechanism that allows a user to create
54a snapshot on any subdirectory (and its nested contents) in the
55system. Snapshot creation and deletion are as simple as 'mkdir
56.snap/foo' and 'rmdir .snap/foo'.
57
58Ceph also provides some recursive accounting on directories for nested
59files and bytes. That is, a 'getfattr -d foo' on any directory in the
60system will reveal the total number of nested regular files and
61subdirectories, and a summation of all nested file sizes. This makes
62the identification of large disk space consumers relatively quick, as
63no 'du' or similar recursive scan of the file system is required.
64
65
66Mount Syntax
67============
68
69The basic mount syntax is:
70
71 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt
72
73You only need to specify a single monitor, as the client will get the
74full list when it connects. (However, if the monitor you specify
75happens to be down, the mount won't succeed.) The port can be left
76off if the monitor is using the default. So if the monitor is at
771.2.3.4,
78
79 # mount -t ceph 1.2.3.4:/ /mnt/ceph
80
81is sufficient. If /sbin/mount.ceph is installed, a hostname can be
82used instead of an IP address.
83
84
85
86Mount Options
87=============
88
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
Francis Galieguea33f3222010-04-23 00:08:02 +020093 address its connection to the monitor originates from.
Sage Weil7ad920b2009-10-06 11:31:05 -070094
95 wsize=X
96 Specify the maximum write size in bytes. By default there is no
Cheng Renquan8136b582010-03-29 19:05:57 +080097 maximum. Ceph will normally size writes based on the file stripe
Sage Weil7ad920b2009-10-06 11:31:05 -070098 size.
99
100 rsize=X
101 Specify the maximum readahead.
102
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.
107
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.
112
113 norbytes
114 When stat() is called on a directory, set st_size to the
115 number of entries in that directory.
116
117 nocrc
Sage Weil23ab15a2010-03-22 09:37:14 -0700118 Disable CRC32C calculation for data writes. If set, the storage node
Sage Weil7ad920b2009-10-06 11:31:05 -0700119 must rely on TCP's error correction to detect data corruption
120 in the data payload.
121
Sage Weila40dc6c2012-01-10 09:12:55 -0800122 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.)
Sage Weil7ad920b2009-10-06 11:31:05 -0700128
Sage Weila40dc6c2012-01-10 09:12:55 -0800129 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.
133
134 noasyncreaddir
135 Do not use the dcache as above for readdir.
Sage Weil7ad920b2009-10-06 11:31:05 -0700136
137More Information
138================
139
140For more information on Ceph, see the home page at
141 http://ceph.newdream.net/
142
143The Linux kernel client source tree is available at
Cheng Renquan8136b582010-03-29 19:05:57 +0800144 git://ceph.newdream.net/git/ceph-client.git
145 git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git
Sage Weil7ad920b2009-10-06 11:31:05 -0700146
147and the source for the full system is at
Cheng Renquan8136b582010-03-29 19:05:57 +0800148 git://ceph.newdream.net/git/ceph.git