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

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Message-ID: <yq1iojm6rti.fsf@sermon.lab.mkp.net>
Date:	Tue, 14 Oct 2014 21:32:25 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Dave Chinner <david@...morbit.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-fsdevel@...r.kernel.org,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: BLKZEROOUT + pread should return zeroes, right?

>>>>> "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.

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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