Re: open(.. O_DIRECT ..) difference in between Linux and FreeBSD ..

[Rob van Nieuwkerk]

Arjan van de Ven wrote:
> On Fri, Jun 13, 2003 at 10:27:52AM +0200, Rob van Nieuwkerk wrote:
> > 
> > Arjan van de Ven wrote:
> > > On Fri, Jun 13, 2003 at 01:12:57AM +0200, Rob van Nieuwkerk wrote:
> > > > FYI:
> > > > It appears that somewhere between RH kernels 2.4.18-27.7.x and 2.4.20-18.9
> > > > something has changed so that my application needs a O_SYNC too besides
> > > > the O_DIRECT to make sure that writes will be synchronous.  If I leave
> > > > the O_SYNC out with 2.4.20-18.9 the write will happen physically 35
> > > > seconds after the write() was done.
> > > 
> > > O_DIRECT is nothing but a hint and the 2.4.20-18.9 kernel decides to not
> > > honor it
> > 
> > Hi Arjan,
> > 
> > Do you mean that the 2.4.20-18.9 kernel always ignores the O_DIRECT flag ?
> 
> yes.

Hi Arjan,

OK, that would explain why I see an old problem (*) re-appear in my 
application that was solved/worked-around by using O_DIRECT when using
2.4.20-18.9.

Just to make sure I understand it correctly, is it like this: ?
   "Kernel 2.4.20-18.9 completely ignores the O_DIRECT flag.  Not only the
    "synchronous writes part" but also you will get read-ahead despite
    using O_DIRECT.  The 2.4.20-18.9 with O_DIRECT behaviour is similar to
    the 2.4.18-27.7.x without O_DIRECT (concerning synchronity of write()
    and the number of physical media reads & writes)."

Just curious: what is the reason for ignoring O_DIRECT in 2.4.20-18.9 ?
Interactivity behaviour ?

	Greetings,
	Rob van Nieuwkerk

(*) I have an application that runs from CompactFlash that uses a Philips
    webcam (pwc driver).  It turned out that too much CompactFlash access
    (in PIO mode) causes the camera(driver?) to stall and never wake up
     again :-(  I only log 2048 byte records to a raw partition.  With
    O_DIRECT and proper data aligning I could reduce the CF-access to
    exactly 4 512 byte sector writes.  This was enough to never trigger
    the problem.