From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qa0-f46.google.com ([209.85.216.46]:54451 "EHLO mail-qa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751219AbaG0C5T (ORCPT ); Sat, 26 Jul 2014 22:57:19 -0400 Received: by mail-qa0-f46.google.com with SMTP id v10so6311311qac.33 for ; Sat, 26 Jul 2014 19:57:18 -0700 (PDT) Message-ID: <53D46A8B.2030002@gmail.com> Date: Sat, 26 Jul 2014 22:57:15 -0400 From: Austin S Hemmelgarn MIME-Version: 1.0 To: Chris Mason , Qu Wenruo , linux-btrfs@vger.kernel.org Subject: Re: [PATCH RFC] btrfs: Use backup superblocks if and only if the first superblock is valid but corrupted. References: <1403841234-10393-1-git-send-email-quwenruo@cn.fujitsu.com> <53D17A88.5090905@fb.com> In-Reply-To: <53D17A88.5090905@fb.com> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms020300060301060205080306" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms020300060301060205080306 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 07/24/2014 05:28 PM, Chris Mason wrote: >=20 >=20 > On 06/26/2014 11:53 PM, Qu Wenruo wrote: >> Current btrfs will only use the first superblock, making the backup >> superblocks only useful for 'btrfs rescue super' command. >> >> The old problem is that if we use backup superblocks when the first >> superblock is not valid, we will be able to mount a none btrfs >> filesystem, which used to contains btrfs but other fs is made on it. >> >> The old problem can be solved related easily by checking the first >> superblock in a special way: >> 1) If the magic number in the first superblock does not match: >> This filesystem is not btrfs anymore, just exit. >> If end-user consider it's really btrfs, then old 'btrfs rescue supe= r' >> method is still available. >> >> 2) If the magic number in the first superblock matches but checksum do= es >> not match: >> This filesystem is btrfs but first superblock is corrupted, use >> backup roots. Just continue searching remaining superblocks. >=20 > I do agree that in these cases we can trust that the backup superblock > comes from the same filesystem. >=20 > But, for right now I'd prefer the admin get involved in using the backu= p > supers. I think silently using the backups is going to lead to surpris= es. Maybe there could be a mount non-default mount-option to use backup superblocks iff the first one is corrupted, and then log a warning whenever this actually happens? 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