Re: [Moosefs-users] o_direct for chunkserver

[Michał B. <mic...@co...>]

Hi!

We already thought about O_DIRECT some time ago but even Linus Torvalds
advises against using it (citation below) and probably it would decrease the
chunkserver performance. 

On the other hand it would quite easy to implement O_DIRECT, but we would
need some strong arguments for doing this. Can you share some with us?


Citations:
On Thu, 11 Jan 2007, Aubrey wrote:
>
> Now, my question is, is there a existing way to mount a filesystem 
> with O_DIRECT flag? so that I don't need to change anything in my 
> system. If there is no option so far, What is the right way to achieve 
> my purpose?

The right way to do it is to just not use O_DIRECT. 

The whole notion of "direct IO" is totally braindamaged. Just say no.

	This is your brain: O
	This is your brain on O_DIRECT: .

	Any questions?

I should have fought back harder. There really is no valid reason for EVER
using O_DIRECT. You need a buffer whatever IO you do, and it might as well
be the page cache. There are better ways to control the page cache than play
games and think that a page cache isn't necessary.

So don't use O_DIRECT. Use things like madvise() and posix_fadvise()
instead. 

		Linus



From: Linus Torvalds [email blocked]
Subject: Re: O_DIRECT question
Date:	Wed, 10 Jan 2007 19:15:48 -0800 (PST)



On Wed, 10 Jan 2007, Linus Torvalds wrote:
> 
> So don't use O_DIRECT. Use things like madvise() and posix_fadvise() 
> instead.

Side note: the only reason O_DIRECT exists is because database people are
too used to it, because other OS's haven't had enough taste to tell them to
do it right, so they've historically hacked their OS to get out of the way.

As a result, our madvise and/or posix_fadvise interfaces may not be all that
strong, because people sadly don't use them that much. It's a sad example of
a totally broken interface (O_DIRECT) resulting in better interfaces not
getting used, and then not getting as much development effort put into them.

So O_DIRECT not only is a total disaster from a design standpoint (just look
at all the crap it results in), it also indirectly has hurt better
interfaces. For example, POSIX_FADV_NOREUSE (which _could_ be a useful and
clean interface to make sure we don't pollute memory unnecessarily with
cached pages after they are all done) ends up being a no-op ;/

Sad. And it's one of those self-fulfilling prophecies. Still, I hope some
day we can just rip the damn disaster out.

			Linus



Kind regards
Michał Borychowski 
MooseFS Support Manager


-----Original Message-----
From: Corin Langosch [mailto:in...@co...] 
Sent: Monday, March 26, 2012 5:23 PM
To: moo...@li...
Subject: [Moosefs-users] o_direct for chunkserver

Hi,

is it already possible to make chunkservers (latest version of moosefs) use
o_direct for all file operations? I couldn't find any configuration option
for this in the man pages.

Otherwise I'd like to suggest to support it in a future version :)

Corin


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