1.. SPDX-License-Identifier: GPL-2.0 2 3============================ 4Ceph Distributed File System 5============================ 6 7Ceph is a distributed network file system designed to provide good 8performance, reliability, and scalability. 9 10Basic features include: 11 12 * POSIX semantics 13 * Seamless scaling from 1 to many thousands of nodes 14 * High availability and reliability. No single point of failure. 15 * N-way replication of data across storage nodes 16 * Fast recovery from node failures 17 * Automatic rebalancing of data on node addition/removal 18 * Easy deployment: most FS components are userspace daemons 19 20Also, 21 22 * Flexible snapshots (on any directory) 23 * Recursive accounting (nested files, directories, bytes) 24 25In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely 26on symmetric access by all clients to shared block devices, Ceph 27separates data and metadata management into independent server 28clusters, similar to Lustre. Unlike Lustre, however, metadata and 29storage nodes run entirely as user space daemons. File data is striped 30across storage nodes in large chunks to distribute workload and 31facilitate high throughputs. When storage nodes fail, data is 32re-replicated in a distributed fashion by the storage nodes themselves 33(with some minimal coordination from a cluster monitor), making the 34system extremely efficient and scalable. 35 36Metadata servers effectively form a large, consistent, distributed 37in-memory cache above the file namespace that is extremely scalable, 38dynamically redistributes metadata in response to workload changes, 39and can tolerate arbitrary (well, non-Byzantine) node failures. The 40metadata server takes a somewhat unconventional approach to metadata 41storage to significantly improve performance for common workloads. In 42particular, inodes with only a single link are embedded in 43directories, allowing entire directories of dentries and inodes to be 44loaded into its cache with a single I/O operation. The contents of 45extremely large directories can be fragmented and managed by 46independent metadata servers, allowing scalable concurrent access. 47 48The system offers automatic data rebalancing/migration when scaling 49from a small cluster of just a few nodes to many hundreds, without 50requiring an administrator carve the data set into static volumes or 51go through the tedious process of migrating data between servers. 52When the file system approaches full, new nodes can be easily added 53and things will "just work." 54 55Ceph includes flexible snapshot mechanism that allows a user to create 56a snapshot on any subdirectory (and its nested contents) in the 57system. Snapshot creation and deletion are as simple as 'mkdir 58.snap/foo' and 'rmdir .snap/foo'. 59 60Ceph also provides some recursive accounting on directories for nested 61files and bytes. That is, a 'getfattr -d foo' on any directory in the 62system will reveal the total number of nested regular files and 63subdirectories, and a summation of all nested file sizes. This makes 64the identification of large disk space consumers relatively quick, as 65no 'du' or similar recursive scan of the file system is required. 66 67Finally, Ceph also allows quotas to be set on any directory in the system. 68The quota can restrict the number of bytes or the number of files stored 69beneath that point in the directory hierarchy. Quotas can be set using 70extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg:: 71 72 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir 73 getfattr -n ceph.quota.max_bytes /some/dir 74 75A limitation of the current quotas implementation is that it relies on the 76cooperation of the client mounting the file system to stop writers when a 77limit is reached. A modified or adversarial client cannot be prevented 78from writing as much data as it needs. 79 80Mount Syntax 81============ 82 83The basic mount syntax is:: 84 85 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt 86 87You only need to specify a single monitor, as the client will get the 88full list when it connects. (However, if the monitor you specify 89happens to be down, the mount won't succeed.) The port can be left 90off if the monitor is using the default. So if the monitor is at 911.2.3.4:: 92 93 # mount -t ceph 1.2.3.4:/ /mnt/ceph 94 95is sufficient. If /sbin/mount.ceph is installed, a hostname can be 96used instead of an IP address. 97 98 99 100Mount Options 101============= 102 103 ip=A.B.C.D[:N] 104 Specify the IP and/or port the client should bind to locally. 105 There is normally not much reason to do this. If the IP is not 106 specified, the client's IP address is determined by looking at the 107 address its connection to the monitor originates from. 108 109 wsize=X 110 Specify the maximum write size in bytes. Default: 64 MB. 111 112 rsize=X 113 Specify the maximum read size in bytes. Default: 64 MB. 114 115 rasize=X 116 Specify the maximum readahead size in bytes. Default: 8 MB. 117 118 mount_timeout=X 119 Specify the timeout value for mount (in seconds), in the case 120 of a non-responsive Ceph file system. The default is 60 121 seconds. 122 123 caps_max=X 124 Specify the maximum number of caps to hold. Unused caps are released 125 when number of caps exceeds the limit. The default is 0 (no limit) 126 127 rbytes 128 When stat() is called on a directory, set st_size to 'rbytes', 129 the summation of file sizes over all files nested beneath that 130 directory. This is the default. 131 132 norbytes 133 When stat() is called on a directory, set st_size to the 134 number of entries in that directory. 135 136 nocrc 137 Disable CRC32C calculation for data writes. If set, the storage node 138 must rely on TCP's error correction to detect data corruption 139 in the data payload. 140 141 dcache 142 Use the dcache contents to perform negative lookups and 143 readdir when the client has the entire directory contents in 144 its cache. (This does not change correctness; the client uses 145 cached metadata only when a lease or capability ensures it is 146 valid.) 147 148 nodcache 149 Do not use the dcache as above. This avoids a significant amount of 150 complex code, sacrificing performance without affecting correctness, 151 and is useful for tracking down bugs. 152 153 noasyncreaddir 154 Do not use the dcache as above for readdir. 155 156 noquotadf 157 Report overall filesystem usage in statfs instead of using the root 158 directory quota. 159 160 nocopyfrom 161 Don't use the RADOS 'copy-from' operation to perform remote object 162 copies. Currently, it's only used in copy_file_range, which will revert 163 to the default VFS implementation if this option is used. 164 165 recover_session=<no|clean> 166 Set auto reconnect mode in the case where the client is blocklisted. The 167 available modes are "no" and "clean". The default is "no". 168 169 * no: never attempt to reconnect when client detects that it has been 170 blocklisted. Operations will generally fail after being blocklisted. 171 172 * clean: client reconnects to the ceph cluster automatically when it 173 detects that it has been blocklisted. During reconnect, client drops 174 dirty data/metadata, invalidates page caches and writable file handles. 175 After reconnect, file locks become stale because the MDS loses track 176 of them. If an inode contains any stale file locks, read/write on the 177 inode is not allowed until applications release all stale file locks. 178 179More Information 180================ 181 182For more information on Ceph, see the home page at 183 https://ceph.com/ 184 185The Linux kernel client source tree is available at 186 - https://github.com/ceph/ceph-client.git 187 - git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git 188 189and the source for the full system is at 190 https://github.com/ceph/ceph.git 191