Re: Tux3 Report: Initial fsck has landed

[David Lang]

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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