From: Andreas Dilger Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks. Date: Fri, 22 Oct 2010 12:17:31 -0600 Message-ID: <6C34898A-508C-4140-A494-B279C04EDD50@dilger.ca> References: <1287670556-23460-1-git-send-email-lczerner@redhat.com> <6388FD2D-50A8-42B9-A955-3824451ACBF4@dilger.ca> <4CC175E6.5000700@gmail.com> <4CC19BC2.9010503@gmail.com> <4CC1A3AA.6040004@gmail.com> <386E61B0-BF4D-4F96-9541-A614F63DE808@dilger.ca> Mime-Version: 1.0 (Apple Message framework v1081) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8BIT Cc: Ric Wheeler , linux-ext4@vger.kernel.org, tytso@mit.edu, sandeen@redhat.com To: Lukas Czerner Return-path: Received: from idcmail-mo2no.shaw.ca ([64.59.134.9]:34905 "EHLO idcmail-mo2no.shaw.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755842Ab0JVSRd convert rfc822-to-8bit (ORCPT ); Fri, 22 Oct 2010 14:17:33 -0400 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On 2010-10-22, at 12:01, Lukas Czerner wrote: >> That patch also checks for the zeroing feature. When this patch was first under discussion, I proposed that we validate that the device is actually zeroed by doing a write a non-zero block to the disk and then calling discard+zero for that region, and reading back the block and verifying it. >> >> Eric wasn't convinced that was necessary, maybe you can convince him more... > > One of the counter arguments was, that some devices does not preserve > this behavior through power cycles. I think Ted was the one talking > about that. Sure, I don't think we can handle every pathology, but doing a write/discard/read of a few blocks (when it has the potential to avoid many GB of writes for zeroing) is surely easy and worthwhile? In any case, I thought that discussion was about a device that didn't report BLKDISCARDSZEROES=1, but only that a normal DISCARD would read back zero until the next restart? That prevents optimizations like "read until we see non-zero data, then start writing zeroes", which would still be faster for many RAID devices (or older kernels that don't have DISCARD/ZERO support at all). Cheers, Andreas