linux-kernel - Re: [PATCHSET] Refactor barrier=/nobarrier flags from fs to block layer

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Message-ID: <4D404EBF.8020508@redhat.com>
Date:	Wed, 26 Jan 2011 10:41:35 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	djwong@...ibm.com, Tejun Heo <tj@...nel.org>,
	Vivek Goyal <vgoyal@...hat.com>, axboe@...nel.dk,
	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, hch@....de, josef@...hat.com
Subject: Re: [PATCHSET] Refactor barrier=/nobarrier flags from fs to block
 layer

On 1/26/11 5:49 AM, Ric Wheeler wrote:
> On 01/26/2011 02:12 AM, Darrick J. Wong wrote:
>> Hello,
>>
>>  From what I can tell, most of the filesystems that know how to issue commands
>> to flush the write cache also have some mechanism for the user to override
>> whether or not the filesystem actually issues those flushes.  Unfortunately,
>> the term "barrier" is obsolete having been changed into flushes in 2.6.36, and
>> many of the filesystems implement the mount options with slightly different
>> syntaxes (barrier=[0|1|none|flush], nobarrier, etc).
>>
>> This patchset adds to the block layer a sysfs knob that an administrator can
>> use to disable flushes, and removes the mount options from the filesystem code.
>> As a starting point, I'm removing the mount options and flush toggle from
>> jbd2/ext4.
>>
>> Anyway, I'm looking for some feedback about refactoring the barrier/flush
>> control knob into the block layer.  It sounds like we want a knob that picks
>> the safest option (issue flushes when supported) unless the administrator
>> decides that it is appropriate to do otherwise.  I suspect that there are good
>> arguments for not having a knob at all, and good arguments for a safe knob.
>> However, since I don't see the barrier options being removed en masse, I'm
>> assuming that we still want a knob somewhere.  Do we need the ignore_fua knob
>> too?  Is this the proper way to deprecate mount options out of filesystems?
>>
>> --D
> 
> Just to be clear, I strongly object to remove the mount options.

Agreed, we are just finally, barely starting to win the education battle here.
Removing or changing the option now will just set us back.  It should at
LEAST remain as a deprecated option, with the deprecation message pointing
to crystal-clear documentation.

-Eric

> "Barrier" and "poke a control knob in the block" layer are both equally mysterious and meaningless to real users, so I do not see this as a gain,
> 
> Ric
> 
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