From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mailout-de.gmx.net ([213.165.64.23]:32805 "HELO mailout-de.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1750737Ab2HBLSJ (ORCPT ); Thu, 2 Aug 2012 07:18:09 -0400 Message-ID: <501A61ED.3000908@gmx.net> Date: Thu, 02 Aug 2012 13:18:05 +0200 From: Arne Jansen MIME-Version: 1.0 To: Liu Bo CC: Stefan Behrens , Jan Schmidt , linux-btrfs@vger.kernel.org Subject: Re: [PATCH v2] Btrfs: remove superblock writing after fatal error References: <1343821552-28726-1-git-send-email-sbehrens@giantdisaster.de> <50191AD3.9080401@gmail.com> <50192A11.3060109@jan-o-sch.net> <50192FCE.30006@gmail.com> <50193DDA.20504@giantdisaster.de> <501A56E3.3080401@giantdisaster.de> <501A583D.8030600@gmail.com> In-Reply-To: <501A583D.8030600@gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 02.08.2012 12:36, Liu Bo wrote: > On 08/02/2012 06:30 PM, Stefan Behrens wrote: >> On Wed, 01 Aug 2012 16:31:54 +0200, Stefan Behrens wrote: >>> On Wed, 01 Aug 2012 21:31:58 +0800, Liu Bo wrote: >>>> On 08/01/2012 09:07 PM, Jan Schmidt wrote: >>>>> On Wed, August 01, 2012 at 14:02 (+0200), Liu Bo wrote: >>>>>> On 08/01/2012 07:45 PM, Stefan Behrens wrote: >>>>>>> With commit acce952b0, btrfs was changed to flag the filesystem with >>>>>>> BTRFS_SUPER_FLAG_ERROR and switch to read-only mode after a fatal >>>>>>> error happened like a write I/O errors of all mirrors. >>>>>>> In such situations, on unmount, the superblock is written in >>>>>>> btrfs_error_commit_super(). This is done with the intention to be able >>>>>>> to evaluate the error flag on the next mount. A warning is printed >>>>>>> in this case during the next mount and the log tree is ignored. >>>>>>> >>>>>>> The issue is that it is possible that the superblock points to a root >>>>>>> that was not written (due to write I/O errors). >>>>>>> The result is that the filesystem cannot be mounted. btrfsck also does >>>>>>> not start and all the other btrfs-progs tools fail to start as well. >>>>>>> However, mount -o recovery is working well and does the right things >>>>>>> to recover the filesystem (i.e., don't use the log root, clear the >>>>>>> free space cache and use the next mountable root that is stored in the >>>>>>> root backup array). >>>>>>> >>>>>>> This patch removes the writing of the superblock when >>>>>>> BTRFS_SUPER_FLAG_ERROR is set, and removes the handling of the error >>>>>>> flag in the mount function. >>>>>>> >>>>>> >>>>>> Yes, I have to admit that this can be a serious problem. >>>>>> >>>>>> But we'll need to send the error flag stored in the super block into >>>>>> disk in the future so that the next mount can find it unstable and do >>>>>> fsck by itself maybe. >>>>> >>>>> Hum, that's possible. However, I neither see >>>>> >>>>> a) a safe way to get that flag to disk >>>>> >>>>> nor >>>>> >>>>> b) a situation where this flag would help. When we abort a transaction, we just >>>>> roll everything back to the last commit, i.e. a consistent state. So if we stop >>>>> writing a potentially corrupt super block, we should be fine anyway. Or am I >>>>> missing something? >>>>> >>>> >>>> I'm just wondering if we can roll everything back well, why do we need fsck? >>> >>> If the disks support barriers, we roll everything back very well. The >>> most recent superblock on the disks always defines a consistent >>> filesystem state. There are only two remaining filesystem consistency >>> issues left that can cause inconsistent states, one is the one that the >>> patch in this email addresses, and the second one is that the error >>> result from barrier_all_devices() is ignored (which I want to change next). >> >> Hi Liu Bo, >> >> Do you have any remaining objections to that patch? >> > > Hi Stefan, > > Still I have another question: > > Our metadata can be flushed into disk if we reach the limit, 32k, so we > can end up with updated metadata and the latest superblock if we do not > write the current super block. The old metadata stays valid until the new superblock is written, so no problem here, or maybe I don't understand your question :) > > Any ideas? > > thanks, > liubo > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html