linux-kernel - Re: Tux3 Report: Initial fsck has landed

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Message-ID: <20130129002011.GA4757@blackbox.djwong.org>
Date:	Mon, 28 Jan 2013 16:20:11 -0800 (PST)
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	David Lang <david@...g.hm>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Daniel Phillips <daniel.raymond.phillips@...il.com>,
	linux-kernel@...r.kernel.org, tux3@...3.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Tux3 Report: Initial fsck has landed

On Mon, Jan 28, 2013 at 03:27:38PM -0800, David Lang wrote:
> On Mon, 28 Jan 2013, Theodore Ts'o wrote:
> 
> >On Sun, Jan 27, 2013 at 10:13:37PM -0800, Daniel Phillips wrote:
> >>>The thing that jumps out at me with this is the question of how you will
> >>>avoid the 'filesystem image in a file' disaster that reiserfs had (where
> >>>it's fsck could mix up metadata chunks from the main filesystem with
> >>>metadata chunks from any filesystem images that it happened to stumble
> >>>across when scanning the disk)

Did that ever get fixed in reiserfs?

> >>>
> >>Only superficially. Deep thoughts are in order. First, there needs to be a
> >>hole in the filesystem structure, before we would even consider trying to
> >>plug something in there. Once we know there is a hole, we want to
> >>narrow down the list of candidates to fill it. If a candidate already lies
> >>within a perfectly viable file, obviously we would not want to interpret
> >>that as lost metadata. Unless the filesystem is really mess up...
> >>
> >>That is about as far as I have got with the analysis. Clearly, much more
> >>is required. Suggestions welcome.
> >
> >The obvious answer is what resierfs4 ultimately ended up using.  Drop
> >a file system UUID in the superblock; mix the UUID into a checksum
> >which protects each of the your metadata blocks.  We're mixing in the
> >inode number as well as the fs uuid in in ext4's new metadata checksum
> >feature to protect against an inode table block getting written to the
> >wrong location on disk.  It will also mean that e2fsck won't mistake
> >an inode table from an earlier mkfs with the current file system.
> >This will allow us to avoid needing to zero the inode table for newly
> >initialized file systems.
> 
> The situation I'm thinking of is when dealing with VMs, you make a
> filesystem image once and clone it multiple times. Won't that end up
> with the same UUID in the superblock?

Yes, but one ought to be able to change the UUID a la tune2fs -U.  Even
still... so long as the VM images have a different UUID than the fs that they
live on, it ought to be fine.

--D
> 
> David Lang
> --
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