From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Darrick J. Wong" Subject: Re: discard_zeroes_data questions Date: Mon, 9 Nov 2015 15:38:45 -0800 Message-ID: <20151109233845.GA31524@birch.djwong.org> References: <56412776.6080604@sandeen.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Martin K. Petersen" , "linux-ext4@vger.kernel.org" To: Eric Sandeen Return-path: Received: from aserp1040.oracle.com ([141.146.126.69]:41763 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751058AbbKIXjA (ORCPT ); Mon, 9 Nov 2015 18:39:00 -0500 Content-Disposition: inline In-Reply-To: <56412776.6080604@sandeen.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html