linux-ext4 - Re: BLKZEROOUT + pread should return zeroes, right?

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Message-ID: <20141016200415.GA16637@birch.djwong.org>
Date:	Thu, 16 Oct 2014 13:04:15 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Dave Chinner <david@...morbit.com>,
	Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: BLKZEROOUT + pread should return zeroes, right?

On Tue, Oct 14, 2014 at 09:32:25PM -0400, Martin K. Petersen wrote:
> >>>>> "Darrick" == Darrick J Wong <darrick.wong@...cle.com> writes:
> 
> Darrick> Well then let's change BLKZEROOUT to require O_DIRECT instead
> Darrick> of hiding the coherency problem, 
> 
> That would break existing users, though.

<shrug> If they're using the existing BLKZEROOUT without O_DIRECT they're
already broken.

But I might as well just fix them quietly.

--D

> Darrick> A further optimization to mke2fs would be to detect that we've
> Darrick> run discard-with-zeroes and therefore can skip issuing
> Darrick> subsequent zeroouts on the same ranges, but I'm wary that
> Darrick> discard-zeroes-data does what it purports to do.
> 
> It's dubious. I'm working on making sure we only set discard_zeroes_data
> when the device guarantees it for 3.19.
> 
> -- 
> Martin K. Petersen	Oracle Linux Engineering
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