linux-kernel - Re: [PATCH 1/7] block: Add block_flush

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Message-ID: <49D26E28.1080203@rtr.ca>
Date:	Tue, 31 Mar 2009 15:25:28 -0400
From:	Mark Lord <lkml@....ca>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Jeff Garzik <jeff@...zik.org>,
	Christoph Hellwig <hch@...radead.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 1/7] block: Add block_flush_device()

Ric Wheeler wrote:
..
> As Mark pointed out, most S-ATA/SAS drives will flush the write cache 
> when they see a bus reset so even without barriers, the cache will be 
> preserved (or flushed) after a reboot or panic.  Power outages are the 
> problem barriers/flushes are meant to help with.
..

I still see barriers as a separate issue from flushes.
Flushes are there for power failures and hot-removable devices.

Barriers are there for that, but also for better odds of data integrity
in the even of a filesystem or kernel crash.

Even if I don't want the kernel needlessly flushing my battery-backed
write caches, I still do want the barrier ordering that improves the
odds of filesystem consistency in the event of a kernel crash.

Cheers
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