ADSM-L

Re: AW: Incremental Forever

2000-05-02 15:13:25
Subject: Re: AW: Incremental Forever
From: "Allen S. Rout" <asr AT NERSP.NERDC.UFL DOT EDU>
Date: Tue, 2 May 2000 15:13:25 -0400
=> On Tue, 2 May 2000 07:35:43 +0200, sal Salak Juraj <sal AT KEBA.CO DOT AT> 
said:

> Allen,

>> There is simply no technical reason to prefer a full/incremental scheme ...

> There is one reason for not to go with incremental backup: the restore
> speed.  Due to its very nature, the restore speed will be slower comparing
> to restore from full backups. (when both use same tape HW) Depending on your
> requirements an environmenmt this limitation does not necessarilly have to,
> but do can apply.

> best regards


I'm sorry, but I must respectfully disagree with you.  In fact, a
well-managed incrementals-forever scheme in a system like ADSM can be faster
than a full/incremental scheme.  Consider:

On any day except the day the full was taken, you must mount a minimum of two
tapes to complete a restore.

A TSM stgpool with colocation active (and adequate media levels) will only
need a single mount, unless there is more than one tape's worth of data.

There are of course variables in either scheme:

If you're using "true incrementals", i.e. only backing up files changed since
the last _backup_ instead of since the last _full_, you can require a whole
weeks' tapes.

You could have your data on a mostly-reclaimable tape, in which case you might
need a second mount.

etc. etc. etc.

But in any case, a well-implemented storage managment system will be better
than the average case for a full/incremental scheme, and will be much better
than the worst case.


Allen S. Rout
NERDC TSM admin
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