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 19:59:53 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Bartlomiej Zolnierkiewicz <bzolnier@...il.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

On Mon, Mar 30 2009, Jeff Garzik wrote:
> Jens Axboe wrote:
>> On Mon, Mar 30 2009, Bartlomiej Zolnierkiewicz wrote:
>>> On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
>>>> Add a sysfs knob to disable storage device writeback cache flushes.
>>> The horde of casual desktop users (with me included) would probably prefer
>>> having two settings -- one for filesystem barriers and one for fsync().
>>>
>>> IOW I prefer higher performance at the cost of risking losing few last
>>> seconds/minutes of work in case of crash / powerfailure but I would still
>>> like to have the filesystem in the consistent state after such accident.
>>
>> The knob is meant to control whether we really need to send a flush to
>> the device or not, so it's an orthogonal issue to what you are talking
>> about. For battery backed caches, we never need to flush. This knob is
>> useful IFF we have devices with write back caches that STILL do a cache
>> flush.
>
> How do installers and/or kernels detect a battery-backed cache that does
> not need flush?

They obviously can't, otherwise it would not be an issue at all. And
whether it's an issue is up for debate, until someone can point at such
a device. You could add a white/blacklist.

So either that knob has to be turned by an administrator (yeah...), or
the in-kernel info would have to be updated. Or a udev rule.

-- 
Jens Axboe

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