[svlug] ext3 and dd disk cloning

Karl F. Larsen k5di at zianet.com
Sat Jan 26 15:30:01 PST 2002

Yes your right and the author covers the serious *system* problem by 
saying "a few bytes of data may be lost". 

The main thing different is that with a power down you come right back up 
without the old ext2 10 minute dance with fixing the file system. That 
something might not work anymore is sure true.

On Sat, 26 Jan 2002, Jeffrey Siegal wrote:

> Karl F. Larsen wrote:
> > Read http://linux.lexilog.org.uk/fs.html which is a very clear explanation 
> > of this subject.
> That article has at least one serious error:
> "A journaling filesystem avoids this problem by keeping track of all 
> changes to the metadata (and to some extent to the data itself) as they 
> happen, in such a way that the system can always be returned to a 
> consistent state if a disk update is abnormally interrupted. A small 
> amount of data may well be lost in such an event, but the consequences 
> are not catastrophic: the last second or so's activity before the power 
> suddenly went off may have gone, but the state of the disks when the 
> machine is brought up again will be what it would have been if the 
> system had been shut down in an orderly way that second or so prior to 
> the actual abrupt switch-off."
> This is not correct.  A journaling filesystem ensures that the 
> filesystem itself can be returned to a consistent state, but not the 
> *system*.  Data within files (and also an expected relationship between 
> different files) may still be rendered inconsistent and unusable, unless 
> the applications themselves are also written in a transactional manner.

Yours Truly,

  	 - Karl F. Larsen, k5di at arrl.net  (505) 524-3303  -

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