Re: [PATCH] O_STREAMING - flag for optimal streaming I/O

From: Chris Wedgwood (
Date: Tue Oct 08 2002 - 14:52:23 EST

On Tue, Oct 08, 2002 at 03:17:16PM -0400, Robert Love wrote:

> Yep. Linux treats most "hints" (e.g. madvise) as a requirement - it
> fails if it cannot do it. That is against the spec most of the
> time, but oh well...

There is no spec for O_DIRECT... SGI 'invented' this in '93 or perhaps
earlier (but the idea wasn't new) for IRIX.

O_DIRECT is a very special thing, you shouldn't ask for this unless
yoy know you want it and how to deal with it --- treating it as
anything less that a requirement is bogus IMO.

> Shrug. I do not have much experience with O_DIRECT. I suspect the
> synchronous nature and the requirement of aligned buffers is not
> ideal.

I'm not sure how being synchornous matters if you use a different
thread (perhaps it's a pain), this also allows your own user-space
code to implement read-ahead? Buffer alignment issues in practice
really aren't that bad.

If someone can think of a meaningful benchmark that would be cool; if
not, then I'll hack up the code I wrote to stream DVD vobs about and
see how that compares.

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