linux-ext4 - Re: [PATCH 1/4] bdi: Track users that require stable page writes

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Date:	Thu, 22 Nov 2012 09:06:43 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
	lucho@...kov.net, jack@...e.cz, ericvh@...il.com, tytso@....edu,
	rminnich@...dia.gov, viro@...iv.linux.org.uk,
	martin.petersen@...cle.com, david@...morbit.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
	v9fs-developer@...ts.sourceforge.net, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/4] bdi: Track users that require stable page writes

On Wed, 21 Nov 2012 13:52:07 -0800 "Darrick J. Wong"
<darrick.wong@...cle.com> wrote:

> > Can you add a rationale on why we'd want to allow users to change the
> > value?  I can't really think of any.
> 
> I dislike the idea that if a program is dirtying pages that are being written
> out, then I don't really know whether the disk will write the before or after
> version.  If the power goes out before the inevitable second write, how do you
> know which version you get?  Sure would be nice if I could force on stable
> writes if I'm feeling paranoid.

I don't think this fear is at all rational (but then you did suggest
paranoia).

If the power goes out, then any write that has been requested, but for which
an 'fsync' hasn't completed, may - or may not - have been written.   Setting
this flag doesn't really change that.

The filesystem should provide some degree of certainty - i.e. either old
data or new data and I believe they mostly do - though ext3 with
journal=writeback explicitly doesn't promise very much.  Beyond that, if you
want any certainty then the app must provide that by using fsync.

So I'm with Christoph here: I don't think the flag should be user-settable.

NeilBrown

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