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

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Message-ID: <4D400A44.6030502@redhat.com>
Date:	Wed, 26 Jan 2011 06:49:24 -0500
From:	Ric Wheeler <rwheeler@...hat.com>
To:	djwong@...ibm.com
CC:	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 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 the term "barrier" is obsolete having been changed into flushes in 2.6.36, and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.

"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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