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Signed-off-by: Venky Shankar <vshankar@redhat.com> Reviewed-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
210 lines
8.4 KiB
ReStructuredText
210 lines
8.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============================
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Ceph Distributed File System
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============================
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Ceph is a distributed network file system designed to provide good
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performance, reliability, and scalability.
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Basic features include:
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* POSIX semantics
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* Seamless scaling from 1 to many thousands of nodes
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* High availability and reliability. No single point of failure.
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* N-way replication of data across storage nodes
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* Fast recovery from node failures
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* Automatic rebalancing of data on node addition/removal
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* Easy deployment: most FS components are userspace daemons
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Also,
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* Flexible snapshots (on any directory)
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* Recursive accounting (nested files, directories, bytes)
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In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely
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on symmetric access by all clients to shared block devices, Ceph
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separates data and metadata management into independent server
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clusters, similar to Lustre. Unlike Lustre, however, metadata and
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storage nodes run entirely as user space daemons. File data is striped
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across storage nodes in large chunks to distribute workload and
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facilitate high throughputs. When storage nodes fail, data is
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re-replicated in a distributed fashion by the storage nodes themselves
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(with some minimal coordination from a cluster monitor), making the
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system extremely efficient and scalable.
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Metadata servers effectively form a large, consistent, distributed
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in-memory cache above the file namespace that is extremely scalable,
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dynamically redistributes metadata in response to workload changes,
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and can tolerate arbitrary (well, non-Byzantine) node failures. The
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metadata server takes a somewhat unconventional approach to metadata
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storage to significantly improve performance for common workloads. In
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particular, inodes with only a single link are embedded in
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directories, allowing entire directories of dentries and inodes to be
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loaded into its cache with a single I/O operation. The contents of
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extremely large directories can be fragmented and managed by
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independent metadata servers, allowing scalable concurrent access.
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The system offers automatic data rebalancing/migration when scaling
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from a small cluster of just a few nodes to many hundreds, without
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requiring an administrator carve the data set into static volumes or
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go through the tedious process of migrating data between servers.
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When the file system approaches full, new nodes can be easily added
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and things will "just work."
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Ceph includes flexible snapshot mechanism that allows a user to create
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a snapshot on any subdirectory (and its nested contents) in the
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system. Snapshot creation and deletion are as simple as 'mkdir
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.snap/foo' and 'rmdir .snap/foo'.
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Ceph also provides some recursive accounting on directories for nested
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files and bytes. That is, a 'getfattr -d foo' on any directory in the
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system will reveal the total number of nested regular files and
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subdirectories, and a summation of all nested file sizes. This makes
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the identification of large disk space consumers relatively quick, as
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no 'du' or similar recursive scan of the file system is required.
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Finally, Ceph also allows quotas to be set on any directory in the system.
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The quota can restrict the number of bytes or the number of files stored
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beneath that point in the directory hierarchy. Quotas can be set using
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extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg::
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setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir
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getfattr -n ceph.quota.max_bytes /some/dir
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A limitation of the current quotas implementation is that it relies on the
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cooperation of the client mounting the file system to stop writers when a
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limit is reached. A modified or adversarial client cannot be prevented
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from writing as much data as it needs.
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Mount Syntax
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============
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The basic mount syntax is::
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# mount -t ceph user@fsid.fs_name=/[subdir] mnt -o mon_addr=monip1[:port][/monip2[:port]]
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You only need to specify a single monitor, as the client will get the
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full list when it connects. (However, if the monitor you specify
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happens to be down, the mount won't succeed.) The port can be left
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off if the monitor is using the default. So if the monitor is at
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1.2.3.4::
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# mount -t ceph cephuser@07fe3187-00d9-42a3-814b-72a4d5e7d5be.cephfs=/ /mnt/ceph -o mon_addr=1.2.3.4
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is sufficient. If /sbin/mount.ceph is installed, a hostname can be
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used instead of an IP address and the cluster FSID can be left out
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(as the mount helper will fill it in by reading the ceph configuration
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file)::
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# mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=mon-addr
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Multiple monitor addresses can be passed by separating each address with a slash (`/`)::
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# mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=192.168.1.100/192.168.1.101
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When using the mount helper, monitor address can be read from ceph
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configuration file if available. Note that, the cluster FSID (passed as part
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of the device string) is validated by checking it with the FSID reported by
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the monitor.
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Mount Options
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=============
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mon_addr=ip_address[:port][/ip_address[:port]]
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Monitor address to the cluster. This is used to bootstrap the
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connection to the cluster. Once connection is established, the
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monitor addresses in the monitor map are followed.
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fsid=cluster-id
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FSID of the cluster (from `ceph fsid` command).
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ip=A.B.C.D[:N]
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Specify the IP and/or port the client should bind to locally.
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There is normally not much reason to do this. If the IP is not
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specified, the client's IP address is determined by looking at the
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address its connection to the monitor originates from.
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wsize=X
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Specify the maximum write size in bytes. Default: 64 MB.
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rsize=X
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Specify the maximum read size in bytes. Default: 64 MB.
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rasize=X
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Specify the maximum readahead size in bytes. Default: 8 MB.
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mount_timeout=X
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Specify the timeout value for mount (in seconds), in the case
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of a non-responsive Ceph file system. The default is 60
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seconds.
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caps_max=X
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Specify the maximum number of caps to hold. Unused caps are released
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when number of caps exceeds the limit. The default is 0 (no limit)
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rbytes
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When stat() is called on a directory, set st_size to 'rbytes',
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the summation of file sizes over all files nested beneath that
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directory. This is the default.
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norbytes
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When stat() is called on a directory, set st_size to the
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number of entries in that directory.
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nocrc
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Disable CRC32C calculation for data writes. If set, the storage node
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must rely on TCP's error correction to detect data corruption
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in the data payload.
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dcache
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Use the dcache contents to perform negative lookups and
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readdir when the client has the entire directory contents in
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its cache. (This does not change correctness; the client uses
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cached metadata only when a lease or capability ensures it is
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valid.)
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nodcache
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Do not use the dcache as above. This avoids a significant amount of
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complex code, sacrificing performance without affecting correctness,
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and is useful for tracking down bugs.
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noasyncreaddir
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Do not use the dcache as above for readdir.
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noquotadf
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Report overall filesystem usage in statfs instead of using the root
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directory quota.
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nocopyfrom
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Don't use the RADOS 'copy-from' operation to perform remote object
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copies. Currently, it's only used in copy_file_range, which will revert
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to the default VFS implementation if this option is used.
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recover_session=<no|clean>
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Set auto reconnect mode in the case where the client is blocklisted. The
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available modes are "no" and "clean". The default is "no".
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* no: never attempt to reconnect when client detects that it has been
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blocklisted. Operations will generally fail after being blocklisted.
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* clean: client reconnects to the ceph cluster automatically when it
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detects that it has been blocklisted. During reconnect, client drops
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dirty data/metadata, invalidates page caches and writable file handles.
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After reconnect, file locks become stale because the MDS loses track
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of them. If an inode contains any stale file locks, read/write on the
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inode is not allowed until applications release all stale file locks.
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More Information
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================
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For more information on Ceph, see the home page at
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https://ceph.com/
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The Linux kernel client source tree is available at
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- https://github.com/ceph/ceph-client.git
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- git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git
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and the source for the full system is at
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https://github.com/ceph/ceph.git
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