mirror of
https://github.com/ceph/ceph
synced 2024-12-22 11:31:55 +00:00
d98b3e36a9
s/amount of times/number of times/ s/assosciated/associated/ s/availabe/available/ s/Commiting/Committing/ s/Containter/Container/ s/dependant/dependent/ s/developement/development/ s/filesytem/filesystem/ s/guarenteed/guaranteed/ s/hiearchy/hierarchy/ s/intance/instance/ s/Interger/Integer/ s/mutiple/multiple/ s/nubmer/number/ s/occured/occurred/ s/overriden/overridden/ s/reseted/reset/ s/sytem/system/ s/unkown/unknown/ Signed-off-by: Michael Prokop <mika@grml.org>
102 lines
4.9 KiB
ReStructuredText
102 lines
4.9 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".
|
|
- 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).
|
|
|
|
Perspective
|
|
-----------
|
|
|
|
People talk a lot about "POSIX compliance," but in reality most file
|
|
system implementations do not strictly adhere to the spec, including
|
|
local Linux file systems like ext4 and XFS. For example, for
|
|
performance reasons, the atomicity requirements for reads are relaxed:
|
|
processing reading from a file that is also being written may see torn
|
|
results.
|
|
|
|
Similarly, NFS has extremely weak consistency semantics when multiple
|
|
clients are interacting with the same files or directories, opting
|
|
instead for "close-to-open". In the world of network attached
|
|
storage, where most environments use NFS, whether or not the server's
|
|
file system is "fully POSIX" may not be relevant, and whether client
|
|
applications notice depends on whether data is being shared between
|
|
clients or not. NFS may also "tear" the results of concurrent writers
|
|
as client data may not even be flushed to the server until the file is
|
|
closed (and more generally writes will be significantly more
|
|
time-shifted than CephFS, leading to less predictable results).
|
|
|
|
However, all of there are very close to POSIX, and most of the time
|
|
applications don't notice too much. Many other storage systems (e.g.,
|
|
HDFS) claim to be "POSIX-like" but diverge significantly from the
|
|
standard by dropping support for things like in-place file
|
|
modifications, truncate, or directory renames.
|
|
|
|
|
|
Bottom line
|
|
-----------
|
|
|
|
CephFS relaxes more than local Linux kernel file systems (e.g., writes
|
|
spanning object boundaries may be torn). It relaxes strictly less
|
|
than NFS when it comes to multiclient consistency, and generally less
|
|
than NFS when it comes to write atomicity.
|
|
|
|
In other words, when it comes to POSIX, ::
|
|
|
|
HDFS < NFS < CephFS < {XFS, ext4}
|
|
|
|
|
|
fsync() and error reporting
|
|
---------------------------
|
|
|
|
POSIX is somewhat vague about the state of an inode after fsync reports
|
|
an error. In general, CephFS uses the standard error-reporting
|
|
mechanisms in the client's kernel, and therefore follows the same
|
|
conventions as other file systems.
|
|
|
|
In modern Linux kernels (v4.17 or later), writeback errors are reported
|
|
once to every file description that is open at the time of the error. In
|
|
addition, unreported errors that occurred before the file description was
|
|
opened will also be returned on fsync.
|
|
|
|
See `PostgreSQL's summary of fsync() error reporting across operating systems
|
|
<https://wiki.postgresql.org/wiki/Fsync_Errors>`_ and `Matthew Wilcox's
|
|
presentation on Linux IO error handling
|
|
<https://www.youtube.com/watch?v=74c19hwY2oE>`_ for more information.
|