linux-ext4 - Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes

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Date:	Tue, 30 Oct 2012 08:19:41 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	NeilBrown <neilb@...e.de>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes

>>>>> "Neil" == NeilBrown  <neilb@...e.de> writes:

Neil,

>> Might be nice to make the sysfs knob tweakable. Also, don't forget to
>> add a suitable blurb to Documentation/ABI/.

Neil>  It isn't at all clear to me that having the sysfs knob
Neil>  'tweakable' is a good idea.  From the md/raid5 perspective, I
Neil>  would want to know for certain whether the pages in a give bio
Neil>  are guaranteed not to change, or if they might.  I could set the
Neil>  BDI_CAP_STABLE_WRITES and believe they will never change, or test
Neil>  the BDI_CAP_STABLE_WRITES and let that tell me if they might
Neil>  change or not.  But if the bit can be changed at any moment, then
Neil>  it can never be trusted and so becomes worthless to me.

I was mostly interested in being able to turn it on for devices that
haven't explicitly done so. I agree that turning it off can be
problematic.

-- 
Martin K. Petersen	Oracle Linux Engineering
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