fsync() doesn't work as advertised?

Brian D briandenzer at gmail.com
Wed Jan 6 14:10:56 CET 2010

On Jan 5, 1:08 pm, Nobody <nob... at nowhere.com> wrote:
> On Mon, 04 Jan 2010 08:09:56 -0800, Brian D wrote:
> > If I'm running a process in a loop that runs for a long time, I
> > occasionally would like to look at a log to see how it's going.
> > I know about the logging module, and may yet decide to use that.
> > Still, I'm troubled by howfsync() doesn't seem to work as advertised:
> >http://docs.python.org/library/os.html
> > "If you’re starting with a Python file object f, first do f.flush(),
> > and then do os.fsync(f.fileno())"
> The .flush() method (and the C fflush() function) causes the
> contents of application buffers to be sent to the OS, which basically
> copies the data into the OS-level buffers.
> fsync() causes the OS-level buffers to be written to the physical drive.
> File operations normally use the OS-level buffers; e.g. if one process
> write()s to a file and another process read()s it, the latter will see
> what the former has written regardless of whether the data has been
> written to the drive.
> The main reason for usingfsync() is to prevent important data from being
> lost in the event of an unexpected reboot or power-cycle (an expected
> reboot via the "shutdown" or "halt" commands will flush all OS-level
> buffers to the drive first). Other than that,fsync() is almost invisible
> (I say "almost", as there are mechanisms to bypass the OS-level buffers,
> e.g. the O_DIRECT open() flag).

An excellent explanation of the process. I've seen this in other
programming environments, so I could visualize something to that
effect, but couldn't have verbalized it. I certainly have a better
idea what's happening. Thanks for the contribution.

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