linux-kernel - Re: O_DIRECT question

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Date:	Fri, 12 Jan 2007 16:17:38 -0500 (EST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Michael Tokarev <mjt@....msk.ru>
cc:	Chris Mason <chris.mason@...cle.com>,
	dean gaudet <dean@...tic.org>, Viktor <vvp01@...ox.ru>,
	Aubrey <aubreylee@...il.com>, Hua Zhong <hzhong@...il.com>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	hch@...radead.org, kenneth.w.chen@...el.com, akpm@...l.org
Subject: Re: O_DIRECT question

On Sat, 13 Jan 2007, Michael Tokarev wrote:
> 
> (No, really - this load isn't entirely synthetic.  It's a typical database
> workload - random I/O all over, on a large file.  If it can, it combines
> several I/Os into one, by requesting more than a single block at a time,
> but overall it is random.)

My point is that you can get basically ALL THE SAME GOOD BEHAVIOUR without 
having all the BAD behaviour that O_DIRECT adds.

For example, just the requirement that O_DIRECT can never create a file 
mapping, and can never interact with ftruncate would actually make 
O_DIRECT a lot more palatable to me. Together with just the requirement 
that an O_DIRECT open would literally disallow any non-O_DIRECT accesses, 
and flush the page cache entirely, would make all the aliases go away.

At that point, O_DIRECT would be a way of saying "we're going to do 
uncached accesses to this pre-allocated file". Which is a half-way 
sensible thing to do.

But what O_DIRECT does right now is _not_ really sensible, and the 
O_DIRECT propeller-heads seem to have some problem even admitting that 
there _is_ a problem, because they don't care. 

A lot of DB people seem to simply not care about security or anything 
else.anything else. I'm trying to tell you that quoting numbers is 
pointless, when simply the CORRECTNESS of O_DIRECT is very much in doubt.

I can calculate PI to a billion decimal places in my head in .1 seconds. 
If you don't care about the CORRECTNESS of the result, that is.

See? It's not about performance. It's about O_DIRECT being fundamentally 
broken as it behaves right now.

		Linus
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