Messages in this thread | | | Date | Wed, 7 Apr 2004 16:09:50 -0500 | From | Andy Isaacson <> | Subject | Re: dd PATCH: add conv=direct |
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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.
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