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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Message-ID: <20121030154844.1898f068@notabene.brown>
Date:	Tue, 30 Oct 2012 15:48:44 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	"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

On Tue, 30 Oct 2012 00:10:22 -0400 "Martin K. Petersen"
<martin.petersen@...cle.com> wrote:

> >>>>> "Darrick" == Darrick J Wong <darrick.wong@...cle.com> writes:
> 
> Darrick> This creates BDI_CAP_STABLE_WRITES, which indicates that a
> Darrick> device requires stable page writes.  It also plumbs in a sysfs
> Darrick> attribute so that admins can check the device status.
> 
> Might be nice to make the sysfs knob tweakable. Also, don't forget to
> add a suitable blurb to Documentation/ABI/.

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

 At the very least it should only be possible to change it when there is no
 IO in flight.

NeilBrown

> 
> Feel free to add my Acked-by:
> 

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