Re: definiing deduplication
>> Or are you referring to the data being altered while a backup is in
>> progress?
> Yes. Data of different files or at different places in the same file
> may have relations which may become inconsistent during change operations
> until the overall change is complete.
Arguably this can be considered as a bug in the application (because
a failure in the middle could thus result in an inconsistent state).
> If you are unlucky you can even catch a plain text file that is only half
> stored.
Indeed, many such files are written an a non-atomic way.
> The risk for this is not 0 with filesystem snapshots, but it grows further
> if there is a time interval during which changes may or may not be copied
> into the backup, depending on filesystem internals and bad luck.
With snapshots, such problems can be considered application bugs, but if
you don't use snapshots, then your backup will not see "the state at time
T" but instead will see the state of different files at different times,
and in the case you can very easily see an inconsistent state even
without any bug in an application: the bug is in the backup
process itself.
If some part of your filesystem is frequently/constantly being modified,
then such inconsistent backups can be very common.
Stefan
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