[svlug] ext3 and dd disk cloning

Jeffrey Siegal jbs at quiotix.com
Sat Jan 26 15:03:02 PST 2002


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.






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