Based on kernel version 3.19. Page generated on 2015-02-13 21:20 EST.
1 Ceph Distributed File System 2 ============================ 3 4 Ceph is a distributed network file system designed to provide good 5 performance, reliability, and scalability. 6 7 Basic 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 17 Also, 18 * Flexible snapshots (on any directory) 19 * Recursive accounting (nested files, directories, bytes) 20 21 In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely 22 on symmetric access by all clients to shared block devices, Ceph 23 separates data and metadata management into independent server 24 clusters, similar to Lustre. Unlike Lustre, however, metadata and 25 storage nodes run entirely as user space daemons. Storage nodes 26 utilize btrfs to store data objects, leveraging its advanced features 27 (checksumming, metadata replication, etc.). File data is striped 28 across storage nodes in large chunks to distribute workload and 29 facilitate high throughputs. When storage nodes fail, data is 30 re-replicated in a distributed fashion by the storage nodes themselves 31 (with some minimal coordination from a cluster monitor), making the 32 system extremely efficient and scalable. 33 34 Metadata servers effectively form a large, consistent, distributed 35 in-memory cache above the file namespace that is extremely scalable, 36 dynamically redistributes metadata in response to workload changes, 37 and can tolerate arbitrary (well, non-Byzantine) node failures. The 38 metadata server takes a somewhat unconventional approach to metadata 39 storage to significantly improve performance for common workloads. In 40 particular, inodes with only a single link are embedded in 41 directories, allowing entire directories of dentries and inodes to be 42 loaded into its cache with a single I/O operation. The contents of 43 extremely large directories can be fragmented and managed by 44 independent metadata servers, allowing scalable concurrent access. 45 46 The system offers automatic data rebalancing/migration when scaling 47 from a small cluster of just a few nodes to many hundreds, without 48 requiring an administrator carve the data set into static volumes or 49 go through the tedious process of migrating data between servers. 50 When the file system approaches full, new nodes can be easily added 51 and things will "just work." 52 53 Ceph includes flexible snapshot mechanism that allows a user to create 54 a snapshot on any subdirectory (and its nested contents) in the 55 system. Snapshot creation and deletion are as simple as 'mkdir 56 .snap/foo' and 'rmdir .snap/foo'. 57 58 Ceph also provides some recursive accounting on directories for nested 59 files and bytes. That is, a 'getfattr -d foo' on any directory in the 60 system will reveal the total number of nested regular files and 61 subdirectories, and a summation of all nested file sizes. This makes 62 the identification of large disk space consumers relatively quick, as 63 no 'du' or similar recursive scan of the file system is required. 64 65 66 Mount Syntax 67 ============ 68 69 The basic mount syntax is: 70 71 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt 72 73 You only need to specify a single monitor, as the client will get the 74 full list when it connects. (However, if the monitor you specify 75 happens to be down, the mount won't succeed.) The port can be left 76 off if the monitor is using the default. So if the monitor is at 77 22.214.171.124, 78 79 # mount -t ceph 126.96.36.199:/ /mnt/ceph 80 81 is sufficient. If /sbin/mount.ceph is installed, a hostname can be 82 used instead of an IP address. 83 84 85 86 Mount 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 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.) 128 129 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. 136 137 More Information 138 ================ 139 140 For more information on Ceph, see the home page at 141 http://ceph.newdream.net/ 142 143 The Linux kernel client source tree is available at 144 git://ceph.newdream.net/git/ceph-client.git 145 git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git 146 147 and the source for the full system is at 148 git://ceph.newdream.net/git/ceph.git