linux-kernel - Re: [PATCH 2/7] ext3: call blkdev_issue

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Date:	Wed, 01 Apr 2009 10:03:21 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Garzik <jeff@...zik.org>
CC:	Jens Axboe <jens.axboe@...cle.com>, Theodore Tso <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	david@...morbit.com
Subject: Re: [PATCH 2/7] ext3: call blkdev_issue_flush() on fsync()

Jeff Garzik wrote:
> Jens Axboe wrote:
>> On Tue, Mar 31 2009, Tejun Heo wrote:
>>> Hello,
>>>
>>> Theodore Tso wrote:
>>>> On Mon, Mar 30, 2009 at 10:15:51AM -0400, Chris Mason wrote:
>>>>> I'm not sure we want to stick Fernando with changing how barriers are
>>>>> done in individual filesystems, his patch is just changing the
>>>>> existing
>>>>> call points.
>>>> Well, his patch actually added some calls to block_issue_flush().  But
>>>> yes, it's probably better if he just changes the existing call points,
>>>> and we can have the relevant filesystem maintainers double check to
>>>> make sure that there aren't any new call points which are needed.
>>> How about having something like blk_ensure_cache_flushed() which
>>> issues flush iff there hasn't been any write since the last flush?
>>> It'll be easy to implement and will filter out duplicate flushes in
>>> most cases.
>>
>> My original ide implementation of flushes actually did this. My memory
>> is a little hazy on why it was dropped, I'm guessing because it
>> basically never triggered anyway.
> 
> Yeah, and it probably wouldn't trigger today unless we add new code that
> starts generating enough duplicate cache flushes for this to be
> significant...

Well, the thread was about adding such a call, so...

> And since duplicate cache flushes are harmless to the drive, you're only
> talking about no-op ATA command overhead.  Which is only mildly notable
> on legacy IDE (eight or so inb/outb operations).
> 
> I would put duplicate cache flush filtering way, way down on the
> priority list, IMO.

Yeap, unless FS guys need it, there's no reason to push it.
Although having dup flush detection Theodore described (w/ callstack
saving at issue time) would be nice for debugging.

Thanks.

-- 
tejun
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