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171 lines
6.6 KiB
Plaintext
171 lines
6.6 KiB
Plaintext
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. Storage nodes
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utilize btrfs to store data objects, leveraging its advanced features
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(checksumming, metadata replication, etc.). 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 monip[:port][,monip2[:port]...]:/[subdir] mnt
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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 1.2.3.4:/ /mnt/ceph
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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.
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Mount Options
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=============
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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: 16 MB.
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rsize=X
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Specify the maximum read size in bytes. Default: 16 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 30
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seconds.
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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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More Information
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================
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For more information on Ceph, see the home page at
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http://ceph.newdream.net/
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The Linux kernel client source tree is available at
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git://ceph.newdream.net/git/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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git://ceph.newdream.net/git/ceph.git
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