From: Yongqiang Yang Subject: Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor Date: Fri, 16 Sep 2011 19:43:19 +0800 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: "Ted Ts'o" , Andreas Dilger , "Darrick J. Wong" , Sunil Mushran , Andi Kleen , Mingming Cao , Joel Becker , linux-ext4@vger.kernel.org, Coly Li To: Amir Goldstein Return-path: Received: from mail-gy0-f174.google.com ([209.85.160.174]:38037 "EHLO mail-gy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751073Ab1IPLnV convert rfc822-to-8bit (ORCPT ); Fri, 16 Sep 2011 07:43:21 -0400 Received: by gyg10 with SMTP id 10so2813575gyg.19 for ; Fri, 16 Sep 2011 04:43:21 -0700 (PDT) In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Sep 16, 2011 at 2:45 PM, Amir Goldstein wr= ote: > 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 = bitmap >>> blocks, where the bitmap offset is (group * blocksize), instead of >>> explicitly listing all of the blocks in the group descriptor? =A0Th= is 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 patc= h set > =A0 =A0is 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 > =A0 =A0and then re-allocation would not be in optimal layout > > In retrospect, after Yongqiang has implemented the alternative exclud= e > 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? Nope, regarding resize group desc is better than exclude inode. For meta_bg, group desc is much more welcome. Yongqiang. > > Amir. > --=20 Best Wishes Yongqiang Yang -- 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