[svlug] ext3 and dd disk cloning
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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