differential vs incremental

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differential vs incremental

Bill Szkotnicki

 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?

 

 

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Re: differential vs incremental

Phil Stracchino
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       [hidden email]
    Renaissance Man, Unix generalist, Perl hacker
 Mobile: 603-216-7037         Landline: 603-886-3518


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Re: differential vs incremental

Mike Hemstock
In reply to this post by Bill Szkotnicki
On Wednesday 07 September 2005 16:54, 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?

Incremental backups are fast to backup as they only backup the files that have
changed since they were last backed up.  They are also cheap as the use the
minimal amount of tape.  However, as they only take a single copy of the file
as they change so if you get a suspect saveset, chewed up tape, etc, you are
likely to lose date.  The other drawback to incrementals is that if you have
a full backup and seven incrementals, then you will have to restore from 8
tapes if you have to do a disater recovery.  This is very time consuming and
not what you need if you need your server back up in a hurry.

Differential backups take longer to backup as they have a lot more files to
back up, but this does give you redundancy.  They are also more expensive as
you will be using more tapes.  However, they offer more redundancy and they
are quicker to restore as they only require a recovery of the full backup and
the latest differential.

Mike.


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