Differences from POSIX

CephFS aims to adhere to POSIX semantics wherever possible. For example, in contrast to many other common network file systems like NFS, CephFS maintains strong cache coherency across clients. The goal is for processes communicating via the file system to behave the same when they are on different hosts as when they are on the same host.

However, there are a few places where CephFS diverges from strict POSIX semantics for various reasons:

  • If a client is writing to a file and fails, its writes are not necessarily atomic. That is, the client may call write(2) on a file opened with O_SYNC with an 8 MB buffer and then crash and the write may be only partially applied. (Almost all file systems, even local file systems, have this behavior.)

  • In shared simultaneous writer situations, a write that crosses object boundaries is not necessarily atomic. This means that you could have writer A write “aa|aa” and writer B write “bb|bb” simultaneously (where | is the object boundary), and end up with “aa|bb” rather than the proper “aa|aa” or “bb|bb”.

  • POSIX includes the telldir(2) and seekdir(2) system calls that allow you to obtain the current directory offset and seek back to it. Because CephFS may refragment directories at any time, it is difficult to return a stable integer offset for a directory. As such, a seekdir to a non-zero offset may often work but is not guaranteed to do so. A seekdir to offset 0 will always work (and is equivalent to rewinddir(2)).

  • Sparse files propagate incorrectly to the stat(2) st_blocks field. Because CephFS does not explicitly track which parts of a file are allocated/written, the st_blocks field is always populated by the file size divided by the block size. This will cause tools like du(1) to overestimate consumed space. (The recursive size field, maintained by CephFS, also includes file “holes” in its count.)

  • When a file is mapped into memory via mmap(2) on multiple hosts, writes are not coherently propagated to other clients’ caches. That is, if a page is cached on host A, and then updated on host B, host A’s page is not coherently invalidated. (Shared writable mmap appears to be quite rare–we have yet to here any complaints about this behavior, and implementing cache coherency properly is complex.)

  • CephFS clients present a hidden .snap directory that is used to access, create, delete, and rename snapshots. Although the virtual directory is excluded from readdir(2), any process that tries to create a file or directory with the same name will get an error code. The name of this hidden directory can be changed at mount time with -o snapdirname=.somethingelse (Linux) or the config option client_snapdir (libcephfs, ceph-fuse).