linux-ext4 - Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

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Date:	Tue, 25 Jan 2011 12:39:24 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	"Darrick J. Wong" <djwong@...ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>, tytso@....edu,
	shli@...nel.org, neilb@...e.de, adilger.kernel@...ger.ca,
	jack@...e.cz, snitzer@...hat.com, linux-kernel@...r.kernel.org,
	kmannth@...ibm.com, cmm@...ibm.com, linux-ext4@...r.kernel.org,
	rwheeler@...hat.com, hch@....de, josef@...hat.com
Subject: Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

On 2011-01-25 11:21, Tejun Heo wrote:
> Hello, Darrick.
> 
> On Mon, Jan 24, 2011 at 12:31:55PM -0800, Darrick J. Wong wrote:
>>> So, I think it's better to start with something simple and improve it
>>> with actual testing.  If the current simple implementation can match
>>> Darrick's previous numbers, let's first settle the mechanisms.  We can
>>
>> Yep, the fsync-happy numbers more or less match... at least for 2.6.37:
>> http://tinyurl.com/4q2xeao
> 
> Good to hear.  Thanks for the detailed testing.
> 
>> I'll give 2.6.38-rc2 a try later, though -rc1 didn't boot for me, so these
>> numbers are based on a backport to .37. :(
> 
> Well, there hasn' been any change in the area during the merge window
> anyway, so I think testing on 2.6.37 should be fine.
> 
>>> I don't really think we should design the whole thing around broken
>>> devices which incorrectly report writeback cache when it need not.
>>> The correct place to work around that is during device identification
>>> not in the flush logic.
>>
>> elm3a4_sas and elm3c71_extsas advertise writeback cache yet the
>> flush completion times are suspiciously low.  I suppose it could be
>> useful to disable flushes to squeeze out that last bit of
>> performance, though I don't know how one goes about querying the
>> disk array to learn if there's a battery behind the cache.  I guess
>> the current mechanism (admin knob that picks a safe default) is good
>> enough.
> 
> Yeap, that or a blacklist of devices which lie.
> 
> Jens, what do you think?  If you don't object, let's put this through
> linux-next.

I like the approach, I'll queue it up for 2.6.39.

-- 
Jens Axboe

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