linux-ext4 - Re: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens for mode where DIO doesn't work

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Date:	Tue, 26 Apr 2016 11:07:40 -0400
From:	Mike Marshall <hubcap@...ibond.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dmitry Monakhov <dmonlist@...il.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-api@...r.kernel.org
Subject: Re: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens
 for mode where DIO doesn't work

We have Orangefs users who have applications they
can't run because the applications open with
O_DIRECT. They are happy when I show them
how to "pretend" to support O_DIRECT - the
way they did it in NFS back in the 2.6 era...

I was thinking of adding it to the upstream version,
maybe as a mount option... so I like this "hint" idea...

-Mike

On Tue, Apr 26, 2016 at 4:14 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Tue, Apr 26, 2016 at 09:49:46AM +1000, Dave Chinner wrote:
>> Why not just transparently fall back to buffered IO if direct IO
>> cannot be done? Saves people from wondering why applications fail
>> on one ext4 filesystem and not another....
>
> I've been doing an audit of our direct I/O implementations, and most
> of them does some form of transparent fallback, including some that
> only pretend to support O_DIRECT, but do anything special for it at all,
> while at the same time we go through greast efforts to check a file
> system actualy supports direct I/O, leading to nasty no-op ->direct_IO
> implementations as we even got that abstraction wrong.
>
> At this point I wonder if we should simply treat O_DIRECT as a hint
> and always allow it, and just let the file system optimize for it
> (skip buffering, require alignment, relaxed Posix atomicy requirements)
> if it is set.
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