fsync(2) manual and hdd write caching

[Garrett Cooper]

On Tue, Oct 26, 2010 at 4:40 PM, Alexander Best <arundel at freebsd.org> wrote:
> On Wed Oct 27 10, Bruce Cran wrote:
>> On Tue, 26 Oct 2010 21:36:18 +0000
>> Alexander Best <arundel at freebsd.org> wrote:
>>
>> > since there's a thread on freebsd-questions@ concerning fsync(2) and
>> > the fact that hdd write caching can cause this syscall to basically
>> > be a no op, could somebody please copy the BUGS section from sync(2)
>> > to fsync(2)?
>>
>> Shouldn't the BUGS section of sync(2) be removed?
>>
>> "The sync() system call may return before the buffers are completely
>>      flushed."
>>
>> But from
>> http://www.opengroup.org/onlinepubs/009695399/functions/sync.html :
>>
>> "The writing, although scheduled, is not necessarily complete upon
>> return from sync()."
>>
>> That would suggest it's not actually a bug.
>
> well...you are right on the one hand. but still this should be documented imo.
> how about turning BUGS into a CAVEATS section and then adding that section to
> fsync(2)?
>
> the reason posix mentions this sync/fsync behavior is probably the fact that
> they know that this cannot be avoided. so that statement seems itself to be a
> caveat rather than a feature. ;)

    Just a sidenote, but that's POSIX 2004[.6?] spec, not POSIX 2008.1
(which is the most current spec -- http://www.unix.org/2008edition/ ).
I double checked and the wording didn't differ for the fsync(2) system
interface, but it could differ in others.
HTH,
-Garrett