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 60Snapshot names have two limitations: 61 62* They can not start with an underscore ('_'), as these names are reserved 63 for internal usage by the MDS. 64* They can not exceed 240 characters in size. This is because the MDS makes 65 use of long snapshot names internally, which follow the format: 66 `_<SNAPSHOT-NAME>_<INODE-NUMBER>`. Since filenames in general can't have 67 more than 255 characters, and `<node-id>` takes 13 characters, the long 68 snapshot names can take as much as 255 - 1 - 1 - 13 = 240. 69 70Ceph also provides some recursive accounting on directories for nested files 71and bytes. You can run the commands:: 72 73 getfattr -n ceph.dir.rfiles /some/dir 74 getfattr -n ceph.dir.rbytes /some/dir 75 76to get the total number of nested files and their combined size, respectively. 77This makes the identification of large disk space consumers relatively quick, 78as no 'du' or similar recursive scan of the file system is required. 79 80Finally, Ceph also allows quotas to be set on any directory in the system. 81The quota can restrict the number of bytes or the number of files stored 82beneath that point in the directory hierarchy. Quotas can be set using 83extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg:: 84 85 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir 86 getfattr -n ceph.quota.max_bytes /some/dir 87 88A limitation of the current quotas implementation is that it relies on the 89cooperation of the client mounting the file system to stop writers when a 90limit is reached. A modified or adversarial client cannot be prevented 91from writing as much data as it needs. 92 93Mount Syntax 94============ 95 96The basic mount syntax is:: 97 98 # mount -t ceph user@fsid.fs_name=/[subdir] mnt -o mon_addr=monip1[:port][/monip2[:port]] 99 100You only need to specify a single monitor, as the client will get the 101full list when it connects. (However, if the monitor you specify 102happens to be down, the mount won't succeed.) The port can be left 103off if the monitor is using the default. So if the monitor is at 1041.2.3.4:: 105 106 # mount -t ceph cephuser@07fe3187-00d9-42a3-814b-72a4d5e7d5be.cephfs=/ /mnt/ceph -o mon_addr=1.2.3.4 107 108is sufficient. If /sbin/mount.ceph is installed, a hostname can be 109used instead of an IP address and the cluster FSID can be left out 110(as the mount helper will fill it in by reading the ceph configuration 111file):: 112 113 # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=mon-addr 114 115Multiple monitor addresses can be passed by separating each address with a slash (`/`):: 116 117 # mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=192.168.1.100/192.168.1.101 118 119When using the mount helper, monitor address can be read from ceph 120configuration file if available. Note that, the cluster FSID (passed as part 121of the device string) is validated by checking it with the FSID reported by 122the monitor. 123 124Mount Options 125============= 126 127 mon_addr=ip_address[:port][/ip_address[:port]] 128 Monitor address to the cluster. This is used to bootstrap the 129 connection to the cluster. Once connection is established, the 130 monitor addresses in the monitor map are followed. 131 132 fsid=cluster-id 133 FSID of the cluster (from `ceph fsid` command). 134 135 ip=A.B.C.D[:N] 136 Specify the IP and/or port the client should bind to locally. 137 There is normally not much reason to do this. If the IP is not 138 specified, the client's IP address is determined by looking at the 139 address its connection to the monitor originates from. 140 141 wsize=X 142 Specify the maximum write size in bytes. Default: 64 MB. 143 144 rsize=X 145 Specify the maximum read size in bytes. Default: 64 MB. 146 147 rasize=X 148 Specify the maximum readahead size in bytes. Default: 8 MB. 149 150 mount_timeout=X 151 Specify the timeout value for mount (in seconds), in the case 152 of a non-responsive Ceph file system. The default is 60 153 seconds. 154 155 caps_max=X 156 Specify the maximum number of caps to hold. Unused caps are released 157 when number of caps exceeds the limit. The default is 0 (no limit) 158 159 rbytes 160 When stat() is called on a directory, set st_size to 'rbytes', 161 the summation of file sizes over all files nested beneath that 162 directory. This is the default. 163 164 norbytes 165 When stat() is called on a directory, set st_size to the 166 number of entries in that directory. 167 168 nocrc 169 Disable CRC32C calculation for data writes. If set, the storage node 170 must rely on TCP's error correction to detect data corruption 171 in the data payload. 172 173 dcache 174 Use the dcache contents to perform negative lookups and 175 readdir when the client has the entire directory contents in 176 its cache. (This does not change correctness; the client uses 177 cached metadata only when a lease or capability ensures it is 178 valid.) 179 180 nodcache 181 Do not use the dcache as above. This avoids a significant amount of 182 complex code, sacrificing performance without affecting correctness, 183 and is useful for tracking down bugs. 184 185 noasyncreaddir 186 Do not use the dcache as above for readdir. 187 188 noquotadf 189 Report overall filesystem usage in statfs instead of using the root 190 directory quota. 191 192 nocopyfrom 193 Don't use the RADOS 'copy-from' operation to perform remote object 194 copies. Currently, it's only used in copy_file_range, which will revert 195 to the default VFS implementation if this option is used. 196 197 recover_session=<no|clean> 198 Set auto reconnect mode in the case where the client is blocklisted. The 199 available modes are "no" and "clean". The default is "no". 200 201 * no: never attempt to reconnect when client detects that it has been 202 blocklisted. Operations will generally fail after being blocklisted. 203 204 * clean: client reconnects to the ceph cluster automatically when it 205 detects that it has been blocklisted. During reconnect, client drops 206 dirty data/metadata, invalidates page caches and writable file handles. 207 After reconnect, file locks become stale because the MDS loses track 208 of them. If an inode contains any stale file locks, read/write on the 209 inode is not allowed until applications release all stale file locks. 210 211More Information 212================ 213 214For more information on Ceph, see the home page at 215 https://ceph.com/ 216 217The Linux kernel client source tree is available at 218 - https://github.com/ceph/ceph-client.git 219 220and the source for the full system is at 221 https://github.com/ceph/ceph.git 222