mirror of
https://github.com/ceph/ceph
synced 2024-12-21 10:54:42 +00:00
50 lines
2.8 KiB
ReStructuredText
50 lines
2.8 KiB
ReStructuredText
========================
|
|
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).
|