Re: O_DIRECT, ext3fs, kernel 2.4.32... again

[DervishD <lkml@dervishd.net>]

    Hi Nathan :)

 * Nathan Scott <nathans@sgi.com> dixit:
> > > Nothing else really make sense due to fcntl...
> > > 	fcntl(fd, F_SETFL, O_DIRECT);
> > > ...can happen at any time, to enable/disable direct I/O.
> > 
> >     I know, but that fcntl call should fail just like the open() one.
> > I mean, I don't find this very different, it's just another point
> > where the flag can be activated and so it should fail if the
> > underlying filesystem doesn't support it (and doesn't ignore it
> > in read()/write()).
> 
> Problem is there is no way to know whether the underlying fs
> supports direct IO or not here (fcntl is implemented outside the
> filesystem, entirely).

    I thought that it was implemented per filesystem.

> Which is not unfixable in itself (could use a superblock flag or
> something similar) but it's way out of scope for the sort of change
> going into 2.4 these days.

    Which approach does 2.6 kernel use? O_DIRECT is correctly handled
for ext3 there, AFAIK :? Are the differences too large?

    I know that this change would be intrusive and probably large,
but IMHO is a quite important bug, because it prevents apps to
selectively disable O_DIRECT (the flag is accepted by open(), so
there's no reason the app should bother about which caused the
read()/write() failures. In fact, is very difficult to know that
those failures are caused by partial/buggy support of O_DIRECT flag).

    Thanks for the information! :)

    Raúl Núñez de Arenas Coronado

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