From: Amir Goldstein Subject: Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor Date: Fri, 16 Sep 2011 09:45:48 +0300 Message-ID: References: <20110915131606.GG15782@thunk.org> <20110915215759.GG28181@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Andreas Dilger , "Darrick J. Wong" , Sunil Mushran , Andi Kleen , Mingming Cao , Joel Becker , linux-ext4@vger.kernel.org, Coly Li , Yongqiang Yang To: "Ted Ts'o" Return-path: Received: from mail-ww0-f42.google.com ([74.125.82.42]:48141 "EHLO mail-ww0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752644Ab1IPGpt convert rfc822-to-8bit (ORCPT ); Fri, 16 Sep 2011 02:45:49 -0400 Received: by wwn22 with SMTP id 22so184192wwn.1 for ; Thu, 15 Sep 2011 23:45:48 -0700 (PDT) In-Reply-To: <20110915215759.GG28181@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Sep 16, 2011 at 12:57 AM, Ted Ts'o wrote: > On Thu, Sep 15, 2011 at 01:08:34PM -0600, Andreas Dilger wrote: >> In that light, why not continue to use an inode to map the exclude b= itmap >> blocks, where the bitmap offset is (group * blocksize), instead of >> explicitly listing all of the blocks in the group descriptor? =A0Thi= s is >> how the buddy bitmap works in memory only, but it could be done for = the >> exclude bitmap on disk. > And this exactly is how the exclude inode works, except only the DIND block is used for mapping, just like the resize inode. > I seem to recall the use of an inode to map the exclude bitmap added = a > huge amount of complexity to the snapshot patches. =A0Amir, am I > remembering this correctly? > No, I am not sure this is accurate. I think after we over viewed the e2fsprogs snapshots patch set, you has 2 observations: 1. the largest part (in lines of code) of the e2fsprogs snapshot patch = set is related to the exclude inode/bitmap code. 2. it reminded you of resize inode too much and you didn't like that 3. There was also the issue of whether or not to allow the removal of the exclude inode/bitmap and then re-allocation would not be in optimal layout In retrospect, after Yongqiang has implemented the alternative exclude bitmap patch set for e2fsprogs, I can say: 1. The alternative patch set is not smaller 2. It is a lot more elegant and deals with correct layout of exclude bitmap (next to block bitmap) 3. It will be able to deal with 64bit fs (unlike exclude/resize inode) and 64bit resize Yongqiang, do you have anything else to add to the exclude inode vs. group desc issue? Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html