ceph: documentation

Ceph Distributed File System

============================

Ceph is a distributed network file system designed to provide good

performance, reliability, and scalability.

Basic features include:

 * POSIX semantics

 * Seamless scaling from 1 to many thousands of nodes

 * High availability and reliability.  No single points of failure.

 * N-way replication of data across storage nodes

 * Fast recovery from node failures

 * Automatic rebalancing of data on node addition/removal

 * Easy deployment: most FS components are userspace daemons

Also,

 * Flexible snapshots (on any directory)

 * Recursive accounting (nested files, directories, bytes)

In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely

on symmetric access by all clients to shared block devices, Ceph

separates data and metadata management into independent server

clusters, similar to Lustre.  Unlike Lustre, however, metadata and

storage nodes run entirely as user space daemons.  Storage nodes

utilize btrfs to store data objects, leveraging its advanced features

(checksumming, metadata replication, etc.).  File data is striped

across storage nodes in large chunks to distribute workload and

facilitate high throughputs.  When storage nodes fail, data is

re-replicated in a distributed fashion by the storage nodes themselves

(with some minimal coordination from a cluster monitor), making the

system extremely efficient and scalable.

Metadata servers effectively form a large, consistent, distributed

in-memory cache above the file namespace that is extremely scalable,

dynamically redistributes metadata in response to workload changes,

and can tolerate arbitrary (well, non-Byzantine) node failures.  The

metadata server takes a somewhat unconventional approach to metadata

storage to significantly improve performance for common workloads.  In

particular, inodes with only a single link are embedded in

directories, allowing entire directories of dentries and inodes to be

loaded into its cache with a single I/O operation.  The contents of

extremely large directories can be fragmented and managed by

independent metadata servers, allowing scalable concurrent access.

The system offers automatic data rebalancing/migration when scaling

from a small cluster of just a few nodes to many hundreds, without

requiring an administrator carve the data set into static volumes or

go through the tedious process of migrating data between servers.

When the file system approaches full, new nodes can be easily added

and things will "just work."

Ceph includes flexible snapshot mechanism that allows a user to create

a snapshot on any subdirectory (and its nested contents) in the

system.  Snapshot creation and deletion are as simple as 'mkdir

.snap/foo' and 'rmdir .snap/foo'.

Ceph also provides some recursive accounting on directories for nested

files and bytes.  That is, a 'getfattr -d foo' on any directory in the

system will reveal the total number of nested regular files and

subdirectories, and a summation of all nested file sizes.  This makes

the identification of large disk space consumers relatively quick, as

no 'du' or similar recursive scan of the file system is required.

Mount Syntax

============

The basic mount syntax is:

 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt

You only need to specify a single monitor, as the client will get the

full list when it connects.  (However, if the monitor you specify

happens to be down, the mount won't succeed.)  The port can be left

off if the monitor is using the default.  So if the monitor is at

1.2.3.4,

 # mount -t ceph 1.2.3.4:/ /mnt/ceph

is sufficient.  If /sbin/mount.ceph is installed, a hostname can be

used instead of an IP address.

Mount Options

=============

  ip=A.B.C.D[:N]

	Specify the IP and/or port the client should bind to locally.

	There is normally not much reason to do this.  If the IP is not

	specified, the client's IP address is determined by looking at the

	address it's connection to the monitor originates from.

  wsize=X

	Specify the maximum write size in bytes.  By default there is no

	maximu.  Ceph will normally size writes based on the file stripe

	size.

  rsize=X

	Specify the maximum readahead.

  mount_timeout=X

	Specify the timeout value for mount (in seconds), in the case

	of a non-responsive Ceph file system.  The default is 30

	seconds.

  rbytes

	When stat() is called on a directory, set st_size to 'rbytes',

	the summation of file sizes over all files nested beneath that

	directory.  This is the default.

  norbytes

	When stat() is called on a directory, set st_size to the

	number of entries in that directory.

  nocrc

	Disable CRC32C calculation for data writes.  If set, the OSD

	must rely on TCP's error correction to detect data corruption

	in the data payload.

  noasyncreaddir

	Disable client's use its local cache to satisfy	readdir

	requests.  (This does not change correctness; the client uses

	cached metadata only when a lease or capability ensures it is

	valid.)

More Information

================

For more information on Ceph, see the home page at

	http://ceph.newdream.net/

The Linux kernel client source tree is available at

	git://ceph.newdream.net/linux-ceph-client.git

and the source for the full system is at

	git://ceph.newdream.net/ceph.git