[Python-Dev] small PATCH to fix os.fsync on OS X (original) (raw)

Richard Boulton richard at tartarus.org
Thu Aug 7 07🔞53 CEST 2008

James Y Knight wrote:

On Aug 7, 2008, at 12:36 AM, Ian Charnas wrote:

While some people might suggest that fullfsync should be exposed as a method on the 'os' module for platforms that support it, I believe this is not the best approach. Because the docstring for os.fsync says that it forces the file to be written to disk, I think it's up to python to ensure that functionality is really there on all platforms. Currently on OS X, fullfsync is needed to provide this functionality, so I think python's os.fsync should use fullfsync on OS X. Please don't make that change. fsync on OSX does the same thing as fsync traditionally does on other OSes. That is: it flushes the data to "disk", but doesn't make any effort to ensure that the disk drive actually flushes the data from its own write cache to the platter. Other OSes just don't expose that functionality at all. See e.g. this thread: http://lists.apple.com/archives/Darwin-dev/2005/Feb/msg00072.html You should instead propose a patch to add FFULLSYNC to the fcntl module when present.

I can't think of a situation where it would be useful to do an fsync() in which you don't want the data to be flushed as far as possible. If you're bothering to call fsync(), it's presumably because you need to guarantee that the data will be in a consistent state in the event of a failure (like a power cut). So I think it would be helpful for an fsync call in a high-level language to handle the details of this.

However, my perspective is largely tied to databases, so there may be other situations that I can't think of where it does make sense to want to push changes as far as the on-disk buffers, but no further.

If the change suggested isn't to be made to os.fsync(), I think it would at least be helpful to add a note to the os.fsync docstring mentioning this limitation, and pointing to the F_FULLSYNC in the fcntl module when this is implemented. And perhaps add an implementation of fsync which does also do F_FULLSYNC elsewhere (shutil.fsync()?), to save all those users of fsync() who also want F_FULLSYNC from having to implement code which calls F_FULLSYNC when available and os.fsync() otherwise.

-- Richard

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