From: Lukas Czerner Subject: Re: Feature request: e2fsck -z Date: Wed, 10 Aug 2011 11:35:34 +0200 (CEST) Message-ID: References: <4E4173D4.9010104@zytor.com> <4E422686.8080207@gmail.com> <4420E0D8-5E16-4FD0-A7A9-B462F02D742D@dilger.ca> <4E423FD0.40108@gmail.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Andreas Dilger , "H. Peter Anvin" , linux-ext4@vger.kernel.org To: Ric Wheeler Return-path: Received: from mx1.redhat.com ([209.132.183.28]:21872 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752878Ab1HJJfj (ORCPT ); Wed, 10 Aug 2011 05:35:39 -0400 In-Reply-To: <4E423FD0.40108@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, 10 Aug 2011, Ric Wheeler wrote: > On 08/10/2011 09:16 AM, Andreas Dilger wrote: > > On 2011-08-10, at 12:34 AM, Ric Wheeler wrote: > > > On 08/09/2011 06:52 PM, H. Peter Anvin wrote: > > > > Hi all, > > > > > > > > This is something I've wanted to see for a very long time, and it > > > > finally occurred to me that perhaps I should say something about it! > > > > > > > > It would be a very nice thing to have a flag to e2fsck, presumably -z, > > > > to zero out any unused data blocks, inodes and so on. The goal is to > > > > minimize the amount of space required after compressing a virtual disk > > > > image or similar, and to make sure any non-data isn't lying around. > > > Do you need it to be in the fsck tool? > > > > > > If you have a sparsely allocated block map under your file system, doing a > > > zero of all blocks could add hours for a big, slow S-ATA drives (2-3 hours > > > for a 1TB drive). > > I think Ted has a tool that does this already. It should be relatively > > simple oo do, like "dd if=/dev/zero of=/mountpoint/temp_zero_file&& rm > > /mountpoint/temp_zero_file. > > This will work but will be a potential multi-hour long process (and cause out > of space errors at some point for other applications for a very brief window > :)) > > > > > > An alternative for SSD's and devices that do TRIM/UNMAP would be to use > > > one of the batched discard tools (that would make discarded data read back > > > as zeroed). > > In fact, I thought Lukas has already made a tool for sending BLKDISCARD for > > all unused parts of the filesystem? > > > > Cheers, Andreas > > Right, I think he has. That tool (not part of fsck) would do the job much > quicker for enabled devices, You're right. It is a part of e2fsck and you can enable it with -E discard. Thanks! -Lukas