Ceph Distributed File System¶
Ceph is a distributed network file system designed to provide goodperformance, reliability, and scalability.
Basic features include:
POSIX semantics
Seamless scaling from 1 to many thousands of nodes
High availability and reliability. No single point of failure.
N-way replication of data across storage nodes
Fast recovery from node failures
Automatic rebalancing of data on node addition/removal
Easy deployment: most FS components are userspace daemons
Also,
Flexible snapshots (on any directory)
Recursive accounting (nested files, directories, bytes)
In contrast to cluster filesystems like GFS, OCFS2, and GPFS that relyon symmetric access by all clients to shared block devices, Cephseparates data and metadata management into independent serverclusters, similar to Lustre. Unlike Lustre, however, metadata andstorage nodes run entirely as user space daemons. File data is stripedacross storage nodes in large chunks to distribute workload andfacilitate high throughputs. When storage nodes fail, data isre-replicated in a distributed fashion by the storage nodes themselves(with some minimal coordination from a cluster monitor), making thesystem extremely efficient and scalable.
Metadata servers effectively form a large, consistent, distributedin-memory cache above the file namespace that is extremely scalable,dynamically redistributes metadata in response to workload changes,and can tolerate arbitrary (well, non-Byzantine) node failures. Themetadata server takes a somewhat unconventional approach to metadatastorage to significantly improve performance for common workloads. Inparticular, inodes with only a single link are embedded indirectories, allowing entire directories of dentries and inodes to beloaded into its cache with a single I/O operation. The contents ofextremely large directories can be fragmented and managed byindependent metadata servers, allowing scalable concurrent access.
The system offers automatic data rebalancing/migration when scalingfrom a small cluster of just a few nodes to many hundreds, withoutrequiring an administrator carve the data set into static volumes orgo through the tedious process of migrating data between servers.When the file system approaches full, new nodes can be easily addedand things will “just work.”
Ceph includes flexible snapshot mechanism that allows a user to createa snapshot on any subdirectory (and its nested contents) in thesystem. Snapshot creation and deletion are as simple as ‘mkdir.snap/foo’ and ‘rmdir .snap/foo’.
Snapshot names have two limitations:
They can not start with an underscore (‘_’), as these names are reservedfor internal usage by the MDS.
They can not exceed 240 characters in size. This is because the MDS makesuse of long snapshot names internally, which follow the format:_<SNAPSHOT-NAME>_<INODE-NUMBER>. Since filenames in general can’t havemore than 255 characters, and<node-id> takes 13 characters, the longsnapshot names can take as much as 255 - 1 - 1 - 13 = 240.
Ceph also provides some recursive accounting on directories for nested filesand bytes. You can run the commands:
getfattr -n ceph.dir.rfiles /some/dirgetfattr -n ceph.dir.rbytes /some/dir
to get the total number of nested files and their combined size, respectively.This makes the identification of large disk space consumers relatively quick,as no ‘du’ or similar recursive scan of the file system is required.
Finally, Ceph also allows quotas to be set on any directory in the system.The quota can restrict the number of bytes or the number of files storedbeneath that point in the directory hierarchy. Quotas can be set usingextended attributes ‘ceph.quota.max_files’ and ‘ceph.quota.max_bytes’, eg:
setfattr -n ceph.quota.max_bytes -v 100000000 /some/dirgetfattr -n ceph.quota.max_bytes /some/dir
A limitation of the current quotas implementation is that it relies on thecooperation of the client mounting the file system to stop writers when alimit is reached. A modified or adversarial client cannot be preventedfrom writing as much data as it needs.
Mount Syntax¶
The basic mount syntax is:
# mount -t ceph user@fsid.fs_name=/[subdir] mnt -o mon_addr=monip1[:port][/monip2[:port]]
You only need to specify a single monitor, as the client will get thefull list when it connects. (However, if the monitor you specifyhappens to be down, the mount won’t succeed.) The port can be leftoff if the monitor is using the default. So if the monitor is at1.2.3.4:
# mount -t ceph cephuser@07fe3187-00d9-42a3-814b-72a4d5e7d5be.cephfs=/ /mnt/ceph -o mon_addr=1.2.3.4
is sufficient. If /sbin/mount.ceph is installed, a hostname can beused instead of an IP address and the cluster FSID can be left out(as the mount helper will fill it in by reading the ceph configurationfile):
# mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=mon-addr
Multiple monitor addresses can be passed by separating each address with a slash (/):
# mount -t ceph cephuser@cephfs=/ /mnt/ceph -o mon_addr=192.168.1.100/192.168.1.101
When using the mount helper, monitor address can be read from cephconfiguration file if available. Note that, the cluster FSID (passed as partof the device string) is validated by checking it with the FSID reported bythe monitor.
Mount Options¶
- mon_addr=ip_address[:port][/ip_address[:port]]
Monitor address to the cluster. This is used to bootstrap theconnection to the cluster. Once connection is established, themonitor addresses in the monitor map are followed.
- fsid=cluster-id
FSID of the cluster (fromceph fsid command).
- ip=A.B.C.D[:N]
Specify the IP and/or port the client should bind to locally.There is normally not much reason to do this. If the IP is notspecified, the client’s IP address is determined by looking at theaddress its connection to the monitor originates from.
- wsize=X
Specify the maximum write size in bytes. Default: 64 MB.
- rsize=X
Specify the maximum read size in bytes. Default: 64 MB.
- rasize=X
Specify the maximum readahead size in bytes. Default: 8 MB.
- mount_timeout=X
Specify the timeout value for mount (in seconds), in the caseof a non-responsive Ceph file system. The default is 60seconds.
- caps_max=X
Specify the maximum number of caps to hold. Unused caps are releasedwhen number of caps exceeds the limit. The default is 0 (no limit)
- rbytes
When
stat()is called on a directory, set st_size to ‘rbytes’,the summation of file sizes over all files nested beneath thatdirectory. This is the default.- norbytes
When
stat()is called on a directory, set st_size to thenumber of entries in that directory.- nocrc
Disable CRC32C calculation for data writes. If set, the storage nodemust rely on TCP’s error correction to detect data corruptionin the data payload.
- dcache
Use the dcache contents to perform negative lookups andreaddir when the client has the entire directory contents inits cache. (This does not change correctness; the client usescached metadata only when a lease or capability ensures it isvalid.)
- nodcache
Do not use the dcache as above. This avoids a significant amount ofcomplex code, sacrificing performance without affecting correctness,and is useful for tracking down bugs.
- noasyncreaddir
Do not use the dcache as above for readdir.
- noquotadf
Report overall filesystem usage in statfs instead of using the rootdirectory quota.
- nocopyfrom
Don’t use the RADOS ‘copy-from’ operation to perform remote objectcopies. Currently, it’s only used in copy_file_range, which will revertto the default VFS implementation if this option is used.
- recover_session=<no|clean>
Set auto reconnect mode in the case where the client is blocklisted. Theavailable modes are “no” and “clean”. The default is “no”.
no: never attempt to reconnect when client detects that it has beenblocklisted. Operations will generally fail after being blocklisted.
clean: client reconnects to the ceph cluster automatically when itdetects that it has been blocklisted. During reconnect, client dropsdirty data/metadata, invalidates page caches and writable file handles.After reconnect, file locks become stale because the MDS loses trackof them. If an inode contains any stale file locks, read/write on theinode is not allowed until applications release all stale file locks.
More Information¶
- For more information on Ceph, see the home page at
- The Linux kernel client source tree is available at
- and the source for the full system is at