Messages in this thread | | | Date | Wed, 7 Apr 2004 14:06:28 -0700 | From | Andrew Morton <> | Subject | Re: dd PATCH: add conv=direct |
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Paul Eggert <eggert@CS.UCLA.EDU> 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,
And when the read 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 think that's best. It's a bit user-unfriendly, but a silent fallback is misleading.
It might be acceptable to print a warning, then fall back.
> 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"?
solaris directio(DIRECTIO_ON) is a hint only. If the system cannot perform the uncached zerocopy then it will fall back to buffered IO. Solaris will perform direct-IO "when the application's buffer is aligned on a two-byte (short) boundary, the offset into the file is on a device sector boundary, and the size of the operation is a multiple of device sectors."
On Linux you set direct-io with open(O_DIRECT) or fcntl(F_SETFL, O_DIRECT). If the filesystem doesn't support direct-io we will fail the open/fcntl attempt up-front in 2.6 only.
If O_DIRECT was successfully set and the IO is not correctly aligned Linux will fail the relevant I/O attempt. Alignment requirements are:
2.4: page aligned on-disk and in-memory
2.6: device sector aligned on-disk and in-memory.
I'd recomment that dd use getpagesize() alignment on-disk and in-memory. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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