Bill Szkotnicki wrote:
> In the manual there are the following two definitions:
>
> ** **
>
> **Differential**
>
> A backup that includes all files changed since the last Full save
> started. Note, other backup programs may define this differently.
>
> **Incremental**
>
> A backup that includes all files changed since the last Full,
> Differential, or Incremental backup started. It is normally specified on
> the *Level* directive within the Job resource definition, or in a
> Schedule resource.
>
> Why do we need both?
>
> It seems that a full followed by a number of incrementals would get all
> the files?
>
> Would it be that a differential in between adds some redundancy?
True, you could do just a full backup, then incrementals forever. But
each incremental records only changes since the last backup of *any
level*. So, if you do a full backup, do daily incrementals for 18
months, and then the disk crashes and you have to do a restore, you have
to restore something on the order of 540 jobs to restore the machine.
The value of differentials is that they roll up ALL changes since the
last Full backup into a single job. So suppose, instead, you do as I
do, and run a weekly Differential and incrementals every other night of
the week. And let's suppose the disk still croaks after 18 months.
With weekly differentials, you can always fully restore that machine by
restoring AT MOST eight jobs, and on average you'll only need five.
(In practice, you'd probably want to run a new Full backup in rather
less than 18 months. I try to run a Full backup every 4-6 months.)
--
Phil Stracchino [EMAIL PROTECTED]
Renaissance Man, Unix generalist, Perl hacker
Mobile: 603-216-7037 Landline: 603-886-3518
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