linux-ext4 - Re: discard_zeroes

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Date:	Mon, 9 Nov 2015 15:38:45 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	"Martin K. Petersen" <mkp@....net>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: discard_zeroes_data questions

On Mon, Nov 09, 2015 at 05:08:38PM -0600, Eric Sandeen wrote:
> Hi Martin (and linux-ext4 list...) 
> 
> tl;dr:
> 
> mke2fs today thinks that if discard_zeroes_data == 1 and a BLKDISCARD ioctl
> of the whole device succeeds, then we have guarantees that any blocks read
> back will be full of zero, and we don't need to initialize them to zero.  Is
> this ok?  (barring crappy hardware, that is).
> 
> slightly longer:
> 
> Does discard_zeroes_data == 1 mean that a discard *request* will guarantee
> zeroes on a read, or does it mean that a
> discard-request-which-actually-was-executed-and-not-ignored-as-just-a-hint
> will give us back zeroes on a read?  (because UNMAP is a hint, right?  I
> don't know about SATA trim ...)
> 
> I did see 7985090 sd: disable discard_zeroes_data for UNMAP - so I think that
> for v3.19+, on *scsi*, what e2fsprogs is doing is ok (now).
> 
> But I'm wondering about dm-thin and SATA, too, so trying to figure out what
> discard_zeroes_data really implies.  That after a BLKDISCARD, a read *will*
> return zeros, or that it'll return zeros *iff* the hint is taken?

Last winter I sent in a patch to invalidate the page cache after a discard:
https://marc.info/?l=linux-kernel&m=142249686225748&w=2

...because e2fsck gets confused it discards part of a d_z_d=1 device and gets
non-zeroed buffers back (from the page cache!) immediately after.

But it never went in.  Should I resend it?  Again?  Jens never acted on it.

--D

> I hope that made sense, but I've been communicating badly today.  ;)
> 
> Thanks,
> -Eric
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