linux-kernel - Re: O_DIRECT question

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Message-ID: <45B281BB.50607@tls.msk.ru>
Date:	Sat, 20 Jan 2007 23:55:23 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Denis Vlasenko <vda.linux@...glemail.com>
CC:	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@...el.com, akpm@...l.org
Subject: Re: O_DIRECT question

Denis Vlasenko wrote:
> On Thursday 11 January 2007 18:13, Michael Tokarev wrote:
>> example, which isn't quite possible now from userspace.  But as long as
>> O_DIRECT actually writes data before returning from write() call (as it
>> seems to be the case at least with a normal filesystem on a real block
>> device - I don't touch corner cases like nfs here), it's pretty much
>> THE ideal solution, at least from the application (developer) standpoint.
> 
> Why do you want to wait while 100 megs of data are being written?
> You _have to_ have threaded db code in order to not waste
> gobs of CPU time on UP + even with that you eat context switch
> penalty anyway.

Usually it's done using aio ;)

It's not that simple really.

For reads, you have to wait for the data anyway before doing something
with it.  Omiting reads for now.

For writes, it's not that problematic - even 10-15 threads is nothing
compared with the I/O (O in this case) itself -- that context switch
penalty.

> I hope you agree that threaded code is not ideal performance-wise
> - async IO is better. O_DIRECT is strictly sync IO.

Hmm.. Now I'm confused.

For example, oracle uses aio + O_DIRECT.  It seems to be working... ;)
As an alternative, there are multiple single-threaded db_writer processes.
Why do you say O_DIRECT is strictly sync?

In either case - I provided some real numbers in this thread before.
Yes, O_DIRECT has its problems, even security problems.  But the thing
is - it is working, and working WAY better - from the performance point
of view - than "indirect" I/O, and currently there's no alternative that
works as good as O_DIRECT.

Thanks.

/mjt
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