1Ceph 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
 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
 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
 93	address its connection to the monitor originates from.
 94
 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.
 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
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.
121
122  noasyncreaddir
123	Disable client's use its local cache to satisfy	readdir
124	requests.  (This does not change correctness; the client uses
125	cached metadata only when a lease or capability ensures it is
126	valid.)
127
128
129More Information
130================
131
132For more information on Ceph, see the home page at
133	http://ceph.newdream.net/
134
135The Linux kernel client source tree is available at
136	git://ceph.newdream.net/git/ceph-client.git
137	git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git
138
139and the source for the full system is at
140	git://ceph.newdream.net/git/ceph.git