From: Amir Goldstein Subject: Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor Date: Fri, 16 Sep 2011 09:55:03 +0300 Message-ID: References: <20110915131606.GG15782@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Ted Ts'o" , "Darrick J. Wong" , Sunil Mushran , Andi Kleen , Mingming Cao , Joel Becker , linux-ext4@vger.kernel.org, Coly Li , Yongqiang Yang To: Andreas Dilger Return-path: Received: from mail-wy0-f180.google.com ([74.125.82.180]:36792 "EHLO mail-wy0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750828Ab1IPGzG convert rfc822-to-8bit (ORCPT ); Fri, 16 Sep 2011 02:55:06 -0400 Received: by wyj26 with SMTP id 26so4350253wyj.11 for ; Thu, 15 Sep 2011 23:55:04 -0700 (PDT) In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, Sep 15, 2011 at 10:08 PM, Andreas Dilger wrote: > On 2011-09-15, at 7:47 AM, Amir Goldstein wrote: >> On Thu, Sep 15, 2011 at 4:16 PM, Ted Ts'o wrote: >>> On Thu, Sep 15, 2011 at 09:50:20AM +0300, Amir Goldstein wrote: >>>> -#define EXT2_FEATURE_COMPAT_EXCLUDE_INODE =A0 =A0 =A00x0080 >>>> +/* #define EXT2_FEATURE_COMPAT_EXCLUDE_INODE =A0 0x0080 not used = */ >>>> +#define EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP =A0 =A0 0x0100 >>> >>> Why this change? =A0Is it because you're already using 0x0100 in >>> shipping systems? >> >> I am using 0x80 in shipping systems and it signifies something a bit >> different then the proposed 0x100. >> >> EXCLUDE_INODE means that special inode 9 is used to reference exclud= e >> bitmap blocks. =A0EXCLUDE_BITMAP means that exclude bitmap blocks ar= e >> referenced from group descriptors. >> With this distinction it will be easier for me to make the migration= =2E > > In that light, why not continue to use an inode to map the exclude bi= tmap > blocks, where the bitmap offset is (group * blocksize), instead of > explicitly listing all of the blocks in the group descriptor? =A0This= is > how the buddy bitmap works in memory only, but it could be done for t= he > exclude bitmap on disk. > > The advantage of this is that it would allow the 32-bit bitmap checks= ums > to both fit into the group descriptor. =A0The disadvantage is that th= ere > is a chance this inode would become corrupted and the location of the > exclude bitmaps is lost. =A0I don't know how serious that is (e.g. if= e2fsck > could fix it by regenerating the bitmaps, or just deleting the snapsh= ot). > > Cheers, Andreas > fsck can fix it. it just marks all blocks used by snapshot inodes. 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