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

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Date:	Tue, 31 Mar 2009 10:49:48 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Mark Lord <lkml@....ca>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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()

Mark Lord wrote:
> Jens Axboe wrote:
>> On Mon, Mar 30 2009, Linus Torvalds wrote:
>>> On Mon, 30 Mar 2009, Jens Axboe wrote:
>>>> Sorry, I just don't see much point to doing it this way instead. So now
>>>> the fs will have to check a queue bit after it has issued the flush, how
>>>> is that any better than having the 'error' returned directly?
>>> No.
>>>
>>> Now the fs SHOULD NEVER CHECK AT ALL.
>>>
>>> Either it did the ordering, or the FS cannot do anything about it. 
>>>
>>> That's the point. EOPNOTSUPP is n ot a useful error message. You can't 
>>> _do_ anything about it.
>> My point is that some file systems may or may not have different paths
>> or optimizations depending on whether barriers are enabled and working
>> or not. Apparently that's just reiserfs and Chris says we can remove it,
>> so it is probably a moot point.
> ..
> 
> XFS appears to have something along those lines.
> I believe it tries to disable the drive write caches
> if it discovers that it cannot do cache flushes.

No, it just stops issuing barriers if the initial mount-time test finds
that they're not supported.  ext3/4/reiserfs do similar.

> I'll check next time my MythTV box boots up.
> It has a RAID0 under XFS, and the md raid0 code doesn't
> appear to pass the cache flushes to libata for raid0,
> so XFS complains and tries to turn off the write caches.

doesn't touch write caches; just complains and stops issuing barriers.

> And I have a script to damn well turn them back ON again
> after it does so.  Stupid thing tries to override user policy again.

It does not do this.

-Eric
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