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Re: dd PATCH: add conv=direct
From: |
Andrew Morton |
Subject: |
Re: dd PATCH: add conv=direct |
Date: |
Wed, 7 Apr 2004 14:06:28 -0700 |
Paul Eggert <address@hidden> wrote:
>
> Andrew Morton <address@hidden> 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.
- Re: dd PATCH: add conv=direct, (continued)
- Message not available
- Message not available
- Message not available
- Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/13
- Re: dd PATCH: add conv=direct, Bryce, 2004/04/14
- Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/14
- Re: dd PATCH: add conv=direct, Jim Meyering, 2004/04/14
- Re: dd PATCH: add conv=direct, Bryce, 2004/04/15
- Re: dd PATCH: add conv=direct, Bryce, 2004/04/15
- Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/07
- Re: dd PATCH: add conv=direct,
Andrew Morton <=
- Re: dd PATCH: add conv=direct, Andy Isaacson, 2004/04/08
Re: dd PATCH: add conv=direct, Nathan Straz, 2004/04/08
Re: Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/08