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

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Date:	Mon, 28 Jan 2013 15:27:38 -0800 (PST)
From:	David Lang <david@...g.hm>
To:	"Theodore Ts'o" <tytso@....edu>
cc:	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, 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)
>>>
>> 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?

David Lang
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