Messages in this thread | | | Date | Wed, 7 Apr 2004 11:18:41 -0700 | From | Andrew Morton <> | Subject | Re: dd PATCH: add conv=direct |
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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.
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