Re: dd PATCH: add conv=direct

[Andy Isaacson <adi@hexapodia.org>]

On Wed, Apr 07, 2004 at 01:46:31PM -0700, Paul Eggert wrote:
> Andrew Morton <akpm@osdl.org> writes:
> > In 2.6 we do the check at open() and fcntl() time.  In 2.4 we don't
> > fail until the actual I/O attempt.
> 
> This raises the issue of what "dd conv=direct" should do in 2.4
> kernels.  I propose that it should report an error and exit, when the
> write fails, since conv=direct can't be implemented.  The basic idea
> is that on systems that lack direct I/O, conv=direct should fail.

I agree, this is the appropriate behavior.  When "dd conv=direct" is
used on a fd that cannot support direct I/O, and the kernel doesn't tell
us until it's too late, erroring on the I/O is the only sane action.

Fortuitously, that's what happens with the patches I posted.

(Note that it's a function of the kernel and the underlying FS whether
direct I/O is going to work.  So some kind of heuristic on kernel
version is a bad idea.)

> Another issue with this patch: in Solaris, direct I/O is done by
> invoking directio(DIRECTIO_ON); see
> <http://docs.sun.com/db/doc/816-0213/6m6ne37so?q=directio&a=view>.
> Is Solaris direct I/O a direct analog to Linux direct I/O, or are
> there subtle differences in semantics that should be made visible to
> the users of GNU "dd"?

The Solaris semantics are more sane than the Linux ones, to be sure --
it always does the I/O, falling back to buffered I/O if conditions are
not right for direct.

I don't believe I have a test system for the Solaris case (when was
directio(3C) added?) so someone else will have to add that support.

-andy