linux-kernel - Re: O_DIRECT question

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Date:	Sat, 27 Jan 2007 15:14:31 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	7eggert@....de
Cc:	Bill Davidsen <davidsen@....com>, Michael Tokarev <mjt@....msk.ru>,
	Phillip Susi <psusi@....rr.com>,
	Linus Torvalds <torvalds@...l.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@in
Subject: Re: O_DIRECT question

On Saturday 27 January 2007 15:01, Bodo Eggert wrote:
> Denis Vlasenko <vda.linux@...glemail.com> wrote:
> > On Friday 26 January 2007 19:23, Bill Davidsen wrote:
> >> Denis Vlasenko wrote:
> >> > On Thursday 25 January 2007 21:45, Michael Tokarev wrote:
> 
> >> >> But even single-threaded I/O but in large quantities benefits from
> >> >> O_DIRECT significantly, and I pointed this out before.
> >> > 
> >> > Which shouldn't be true. There is no fundamental reason why
> >> > ordinary writes should be slower than O_DIRECT.
> >> > 
> >> Other than the copy to buffer taking CPU and memory resources.
> > 
> > It is not required by any standard that I know. Kernel can be smarter
> > and avoid that if it can.
> 
> The kernel can also solve the halting problem if it can.
> 
> Do you really think an entropy estamination code on all access patterns in the
> system will be free as in beer,

Actually I think we need this heuristic:

if (opened_with_O_STREAM && buffer_is_aligned
		&& io_size_is_a_multiple_of_sectorsize)
	do_IO_directly_to_user_buffer_without_memcpy

is not *that* compilcated.

I think that we can get rid of O_DIRECT peculiar requirements
"you *must* not cache me" + "you *must* write me directly to bare metal"
by replacing it with O_STREAM ("*advice* to not cache me") + O_SYNC
("write() should return only when data is written to storage, not sooner").

Why?

Because these O_DIRECT "musts" are rather unusual and overkill. Apps
should not have that much control over what kernel does internally;
and also O_DIRECT was mixing shampoo and conditioner on one bottle
(no-cache and sync writes) - bad API.
--
vda
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