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 16:35:39 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
Cc:	Jeff Garzik <jeff@...zik.org>,
	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, Fernando Luis Vázquez Cao wrote:
> Jens Axboe wrote:
>> On Mon, Mar 30 2009, Fernando Luis Vázquez Cao wrote:
>>> Add a sysfs knob to disable storage device writeback cache flushes.
>>>
>>> Signed-off-by: Fernando Luis Vazquez Cao <fernando@....ntt.co.jp>
>>> ---
>>>
>>> diff -urNp linux-2.6.29-orig/block/blk-barrier.c linux-2.6.29/block/blk-barrier.c
>>> --- linux-2.6.29-orig/block/blk-barrier.c	2009-03-24 08:12:14.000000000 +0900
>>> +++ linux-2.6.29/block/blk-barrier.c	2009-03-30 17:08:28.000000000 +0900
>>> @@ -318,6 +318,9 @@ int blkdev_issue_flush(struct block_devi
>>>  	if (!q)
>>>  		return -ENXIO;
>>>
>>> +	if (blk_queue_nowbcflush(q))
>>> +		return -EOPNOTSUPP;
>>> +
>>>  	bio = bio_alloc(GFP_KERNEL, 0);
>>>  	if (!bio)
>>>  		return -ENOMEM;
>>> diff -urNp linux-2.6.29-orig/block/blk-core.c linux-2.6.29/block/blk-core.c
>>> --- linux-2.6.29-orig/block/blk-core.c	2009-03-24 08:12:14.000000000 +0900
>>> +++ linux-2.6.29/block/blk-core.c	2009-03-30 17:08:28.000000000 +0900
>>> @@ -1452,7 +1452,8 @@ static inline void __generic_make_reques
>>>  			goto end_io;
>>>  		}
>>>  		if (bio_barrier(bio) && bio_has_data(bio) &&
>>> -		    (q->next_ordered == QUEUE_ORDERED_NONE)) {
>>> +		    (blk_queue_nowbcflush(q) ||
>>> +		     q->next_ordered == QUEUE_ORDERED_NONE)) {
>>>  			err = -EOPNOTSUPP;
>>>  			goto end_io;
>>>  		}
>>
>> This (and the above hunk) should be changed. -EOPNOTSUPP means the
>> target does not support barriers, that is a different thing to flushes
>> not being needed. A file system issuing a barrier and getting
>> -EOPNOTSUPP back will disable barriers, since it now thinks that
>> ordering cannot be guaranteed.
>
> The reason I decided to use -EOPNOTSUPP was that I wanted to keep
> barriers and device flushes from entering the block layer when
> they are not needed. I feared that if we pass them down the block
> stack (knowing in advance they will not be actually submitted to
> disk) we may end up slowing things down unnecessarily.

But that's just wrong, you need to make sure that the block layer / io
scheduler doesn't reorder as well. It's a lot more complex than just the
device end. So just returning -EOPNOTSUPP and pretending that you need
not use barriers at the fs end is just wrong.

> As you mentioned, filesystems such as ext3/4 will disable
> barriers if they get -EOPNOTSUPP when issuing one. I was planning
> to add a notifier mechanism so that we can notify filesystems has
> been a change in the barrier settings. This might be
> over-engineering, though. Especially considering that "-o
> remount,barrier=1" will bring us the barriers back.

I think that is over-engineering.

>> A more appropriate change would be to successfully complete a flush
>> without actually sending it down to the device if blk_queue_nowbcflush()
>> is true. Then blkdev_issue_flush() would just work as well. It also
>> needs to take stacking into account, or stacked drivers will have to
>> propagate the settings up the stack. If you allow simply the barrier to
>> be passed down, you get that for free.
>
> Aren't we risking slowing things down? Does the small optimization above
> make sense (especially taking the remount trick into account)?

It's not, I think you are missing the bigger picture.

-- 
Jens Axboe

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