From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755433AbYHVRIx (ORCPT ); Fri, 22 Aug 2008 13:08:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752486AbYHVRIo (ORCPT ); Fri, 22 Aug 2008 13:08:44 -0400 Received: from anchor-post-31.mail.demon.net ([194.217.242.89]:2582 "EHLO anchor-post-31.mail.demon.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751682AbYHVRIo (ORCPT ); Fri, 22 Aug 2008 13:08:44 -0400 Message-ID: <48AEF2A3.7020905@lougher.demon.co.uk> Date: Fri, 22 Aug 2008 18:08:51 +0100 From: Phillip Lougher User-Agent: Thunderbird 2.0.0.6 (X11/20071008) MIME-Version: 1.0 To: Arnd Bergmann CC: jaredeh@gmail.com, Linux-kernel@vger.kernel.org, linux-embedded@vger.kernel.org, linux-mtd , =?windows-1252?Q?J=F6rn_Engel?= , tim.bird@am.sony.com, cotte@de.ibm.com, nickpiggin@yahoo.com.au Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c References: <48AD00F0.5030403@gmail.com> <200808211417.14425.arnd@arndb.de> <48AE22F5.3000309@lougher.demon.co.uk> <200808221200.26052.arnd@arndb.de> In-Reply-To: <200808221200.26052.arnd@arndb.de> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Arnd Bergmann wrote: > On Friday 22 August 2008, Phillip Lougher wrote: >>> This looks very nice, but could use some comments about how the data is >>> actually stored on disk. It took me some time to figure out that it actually >>> allows to do tail merging into compressed blocks, which I was about to suggest >>> you implement ;-). Cramfs doesn't have them, and I found that they are the >>> main reason why squashfs compresses better than cramfs, besides the default >>> block size, which you can change on either one. >> Squashfs has much larger block sizes than cramfs (last time I looked it >> was limited to 4K blocks), and it compresses the metadata which helps to >> get better compression. But tail merging (fragments in Squashfs >> terminology) is obviously a major reason why Squashfs gets good compression. > > The *default* block size in cramfs is smaller than in squashfs, but they both > have user selectable block sizes. I found the impact of compressed metadata > to be almost zero. Squashfs stores significantly more metadata than cramfs. Remember cramfs has no support for filesystems > ~ 16Mbytes, no inode timestamps, truncates uid/gids, no hard-links, no nlink counts, no hashed directories, no unique inode numbers. If Squashfs didn't compress the metadata it would be significantly larger than cramfs. Cheers Phillip