1Ceph Distributed File System 2============================ 3 4Ceph is a distributed network file system designed to provide good 5performance, reliability, and scalability. 6 7Basic 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 17Also, 18 * Flexible snapshots (on any directory) 19 * Recursive accounting (nested files, directories, bytes) 20 21In contrast to cluster filesystems like GFS, OCFS2, and GPFS that rely 22on symmetric access by all clients to shared block devices, Ceph 23separates data and metadata management into independent server 24clusters, similar to Lustre. Unlike Lustre, however, metadata and 25storage nodes run entirely as user space daemons. File data is striped 26across storage nodes in large chunks to distribute workload and 27facilitate high throughputs. When storage nodes fail, data is 28re-replicated in a distributed fashion by the storage nodes themselves 29(with some minimal coordination from a cluster monitor), making the 30system extremely efficient and scalable. 31 32Metadata servers effectively form a large, consistent, distributed 33in-memory cache above the file namespace that is extremely scalable, 34dynamically redistributes metadata in response to workload changes, 35and can tolerate arbitrary (well, non-Byzantine) node failures. The 36metadata server takes a somewhat unconventional approach to metadata 37storage to significantly improve performance for common workloads. In 38particular, inodes with only a single link are embedded in 39directories, allowing entire directories of dentries and inodes to be 40loaded into its cache with a single I/O operation. The contents of 41extremely large directories can be fragmented and managed by 42independent metadata servers, allowing scalable concurrent access. 43 44The system offers automatic data rebalancing/migration when scaling 45from a small cluster of just a few nodes to many hundreds, without 46requiring an administrator carve the data set into static volumes or 47go through the tedious process of migrating data between servers. 48When the file system approaches full, new nodes can be easily added 49and things will "just work." 50 51Ceph includes flexible snapshot mechanism that allows a user to create 52a snapshot on any subdirectory (and its nested contents) in the 53system. Snapshot creation and deletion are as simple as 'mkdir 54.snap/foo' and 'rmdir .snap/foo'. 55 56Ceph also provides some recursive accounting on directories for nested 57files and bytes. That is, a 'getfattr -d foo' on any directory in the 58system will reveal the total number of nested regular files and 59subdirectories, and a summation of all nested file sizes. This makes 60the identification of large disk space consumers relatively quick, as 61no 'du' or similar recursive scan of the file system is required. 62 63Finally, Ceph also allows quotas to be set on any directory in the system. 64The quota can restrict the number of bytes or the number of files stored 65beneath that point in the directory hierarchy. Quotas can be set using 66extended attributes 'ceph.quota.max_files' and 'ceph.quota.max_bytes', eg: 67 68 setfattr -n ceph.quota.max_bytes -v 100000000 /some/dir 69 getfattr -n ceph.quota.max_bytes /some/dir 70 71A limitation of the current quotas implementation is that it relies on the 72cooperation of the client mounting the file system to stop writers when a 73limit is reached. A modified or adversarial client cannot be prevented 74from writing as much data as it needs. 75 76Mount Syntax 77============ 78 79The basic mount syntax is: 80 81 # mount -t ceph monip[:port][,monip2[:port]...]:/[subdir] mnt 82 83You only need to specify a single monitor, as the client will get the 84full list when it connects. (However, if the monitor you specify 85happens to be down, the mount won't succeed.) The port can be left 86off if the monitor is using the default. So if the monitor is at 871.2.3.4, 88 89 # mount -t ceph 1.2.3.4:/ /mnt/ceph 90 91is sufficient. If /sbin/mount.ceph is installed, a hostname can be 92used instead of an IP address. 93 94 95 96Mount Options 97============= 98 99 ip=A.B.C.D[:N] 100 Specify the IP and/or port the client should bind to locally. 101 There is normally not much reason to do this. If the IP is not 102 specified, the client's IP address is determined by looking at the 103 address its connection to the monitor originates from. 104 105 wsize=X 106 Specify the maximum write size in bytes. Default: 16 MB. 107 108 rsize=X 109 Specify the maximum read size in bytes. Default: 16 MB. 110 111 rasize=X 112 Specify the maximum readahead size in bytes. Default: 8 MB. 113 114 mount_timeout=X 115 Specify the timeout value for mount (in seconds), in the case 116 of a non-responsive Ceph file system. The default is 30 117 seconds. 118 119 caps_max=X 120 Specify the maximum number of caps to hold. Unused caps are released 121 when number of caps exceeds the limit. The default is 0 (no limit) 122 123 rbytes 124 When stat() is called on a directory, set st_size to 'rbytes', 125 the summation of file sizes over all files nested beneath that 126 directory. This is the default. 127 128 norbytes 129 When stat() is called on a directory, set st_size to the 130 number of entries in that directory. 131 132 nocrc 133 Disable CRC32C calculation for data writes. If set, the storage node 134 must rely on TCP's error correction to detect data corruption 135 in the data payload. 136 137 dcache 138 Use the dcache contents to perform negative lookups and 139 readdir when the client has the entire directory contents in 140 its cache. (This does not change correctness; the client uses 141 cached metadata only when a lease or capability ensures it is 142 valid.) 143 144 nodcache 145 Do not use the dcache as above. This avoids a significant amount of 146 complex code, sacrificing performance without affecting correctness, 147 and is useful for tracking down bugs. 148 149 noasyncreaddir 150 Do not use the dcache as above for readdir. 151 152 noquotadf 153 Report overall filesystem usage in statfs instead of using the root 154 directory quota. 155 156 nocopyfrom 157 Don't use the RADOS 'copy-from' operation to perform remote object 158 copies. Currently, it's only used in copy_file_range, which will revert 159 to the default VFS implementation if this option is used. 160 161 recover_session=<no|clean> 162 Set auto reconnect mode in the case where the client is blacklisted. The 163 available modes are "no" and "clean". The default is "no". 164 165 * no: never attempt to reconnect when client detects that it has been 166 blacklisted. Operations will generally fail after being blacklisted. 167 168 * clean: client reconnects to the ceph cluster automatically when it 169 detects that it has been blacklisted. During reconnect, client drops 170 dirty data/metadata, invalidates page caches and writable file handles. 171 After reconnect, file locks become stale because the MDS loses track 172 of them. If an inode contains any stale file locks, read/write on the 173 inode is not allowed until applications release all stale file locks. 174 175More Information 176================ 177 178For more information on Ceph, see the home page at 179 https://ceph.com/ 180 181The Linux kernel client source tree is available at 182 https://github.com/ceph/ceph-client.git 183 git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git 184 185and the source for the full system is at 186 https://github.com/ceph/ceph.git 187