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

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Message-ID: <49D13123.7040007@rtr.ca>
Date:	Mon, 30 Mar 2009 16:52:51 -0400
From:	Mark Lord <lkml@....ca>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	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()

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.

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.

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.

Cheers
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