linux-kernel - Re: [PATCH 5/7] vfs: Add wbcflush sysfs knob to disable storage device writeback cache flushes

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Date:	Mon, 30 Mar 2009 18:01:27 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	chris.mason@...cle.com, david@...morbit.com, tj@...nel.org
Subject: Re: [PATCH 5/7] vfs: Add  wbcflush sysfs knob to disable storage
 device writeback cache flushes

Bartlomiej Zolnierkiewicz wrote:
> calling fsync() after every mail fetched.  Adding mandatory cache flush
> to it only makes things worse (again looking from performance POV).
> 
> BTW in Linux world we never made any guarantees for fsync() on devices
> using write caching:

Quite true, but I've always thought that was trading away correctness 
for performance...  at a critical juncture where a consistency 
checkpoint was explicitly requested by the app.

My ideal would probably be blkdev cache flushing by default on fsync(2), 
with a block layer "desktop mode" knob to turn it off if you don't want it.

The current alternatives -- mount sync or disable blkdev writeback cache 
-- are far, far slower and punish the entire system just to provide a 
consistency checkpoint for a handful of fsync-needful apps.

	Jeff


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