Re: dd PATCH: add conv=direct

[Andrew Morton <akpm@osdl.org>]

Andy Isaacson <adi@hexapodia.org> wrote:
>
> On Tue, Apr 06, 2004 at 05:33:26PM -0700, Andrew Morton wrote:
> > Andy Isaacson <adi@hexapodia.org> wrote:
> > >          dd(1) is convenient for this purpose, but is lacking a method
> > > to force O_DIRECT.  The enclosed patch adds a "conv=direct" flag to
> > > enable this usage.
> > 
> > This would be rather nice to have.  You'll need to ensure that the data
> > is page-aligned in memory.
> 
> So, some confusion on my part about O_DIRECT:  I can't get O_DIRECT to
> work on ext3, at all, on 2.4.25

ext3 doesn't support O_DIRECT in 2.4 kernels.  I did a patch once and I
think it's in 2.4-aa kernels.

ext3 supports O_DIRECT in 2.6 kernels.  Quite a number of filesystems do.

> -- open(O_DIRECT) succeeds, but the write
> returns EINVAL.

Yup that's a bit silly.  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.

>  Same code works fine when writing to a block device.
> If the problem is that ext3 can't support O_DIRECT, why does the open
> succeed?

We have been insufficiently assiduous in merging externally-supported
patches into the mainline 2.4 tree.

> > While you're there, please add an fsync-before-closing option.
> 
> Easy enough.  How does this look?  Note that C_TWOBUFS ensures the
> output buffer is getpagesize()-aligned.

Looks nice and simple.  You'll need an ext2 filesystem to test it under 2.4.

Be aware that it's rather a challenge to actually get the O_DIRECT #define
in scope under some glibc versions.  I think you need to define _GNU_SOURCE
or something like that.