Re: [dm-devel] [Lsf] Preliminary Agenda and Activities for LSF

[Joel Becker <jlbec evilplan org>]

On Fri, Apr 01, 2011 at 11:19:07AM -0400, Ted Ts'o wrote:
> The closest place that we have to any official documentation about
> O_DIRECT semantics is the open(2) man page in the Linux manpages, and
> it doesn't say anything about this.  It does give a recommendation
> against not mixing buffered and O_DIRECT accesses to the same file,
> but it does promise that things will work in that case.  (Even if it
> does, do we really want to make the promise that it will always work?)

	No, we do not.  Some OSes will silently turn buffered I/O into
direct I/O if another file already has it opened O_DIRECT.  Some OSes
will fail the write, or the open, or both, if it doesn't match the mode
of an existing fd.  Some just leave O_DIRECT and buffered access
inconsistent.
	I think that Linux should strive to make the mixed
buffered/direct case work; it's the nicest thing we can do.  But we
should not promise it.

Joel

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