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Re: Q's on backup to CD-R



on Tue, Apr 23, 2002, Ross Boylan (RossBoylan@stanfordalumni.org) wrote:
> I'm trying to do a fairly comprehensive system backup to CD-R.  The
> amount I have to backup is much larger than the free space on the
> system, which in turn is larger than a CD-R.

CD-Rs are too small to be practical for any reasonably sized drive these
days -- 20 - 120+ GiB.  It's like backing up to floppies on a 100 MiB
system.  Tedious.

For spot archival, it's a decent idea, but real backups call for:

  - Tape.
  - Networked backup.
  - Tape.
  - Removable "fixed" backup (e.g.:  drive caddies).
  - Tape.

Tape's a bitch, but the cost, reliability, standardization, and
capacity, weigh strongly in its favor.  Still crazy after all these
years.

DVD-R loses on several counts including size (2-4 GiB still isn't big
enough), and in particular, lack of standardization.

<...>

> Also, though I've always used tar, I understand it's a very fragile
> backup scheme and that afio (or maybe cpio--couldn't quite tell) is
> safer for archiving.  Any comments?

Internal structures are such that if your media or files are damaged,
you've got a better chance for recovery with afio/cpio.  That said, I
find the interfaces to both far more awkward than tar.  I'd bet on a set
of _differential_ backups from a known good comprehensive backup.
Preferably one or more cycles back:

Scheduled something like:

    week 0 day 0:  comprehensive backup
      w 0 d 1-7:  differential since w 0 d 0 (you're playing catch-up)

    week 1 day 0:  comprehensive backup
      w 1 d 1-7:  differential since w 0 d 0 (you're playing catch-up)

    week 2 day 0:  comprehensive backup
      w 2 d 1-7:  differential since w 0 d 0 (you've finally caught up)

    week 3 day 0:  comprehensive backup
      w 2 d 1-7:  differential since w 1 d 0 (you're moving forward)

From here forward, you're doing differentials since two prior
comprehensives.  Why?  Because you now have your pick of any of three
comprehensive backups, and (with diminishing recency), *** THREE WEEKS
*** of daily differentials which will get you at least _some_ of what
you lost.

Tower of Hanoi schemes and such can minimize tape use and archival time,
but the recovery process is twisted, and if _any_ backup is lost from
the set, you're hosed.  This enlightenment comes by way of a friend who
held backup tapes in his hand while wading through the server room,
explaining to PHB what their ToH backup scheme did and didn't optimize
for.

My focus is this:  backups are a data recovery mechanism.  As such,
optimize for data recovery, and make a realistic assessment of your
costs elsewhere.  You know the drill:

   - DDS-4 40 GiB tape library:  $7000
   - Media cost:  $40
   - Nightly backups:  $2.50 for Starbucks.
   - Recovering lost data:  Priceless.


> By the way, here are two helpful sites I've culled from previous
> messages:
> http://kmself.home.netcom.com/Linux/FAQs/backups.html
> http://www.linux-backup.net/app.gwif.html
> 
> The problem is a lot of the stuff doesn't address the peculiarities of
> CD-R.
> 
> Final question, about what to back up.  Karsten Self's nice page
> (first reference) says some parts of /var are good to backup.  Which
> ones?  

The ones you need ;-)

If you'll drop lower on that page my script includes an environment
variable $backupdirs, including: 

     /var/backups /var/lib /var/log /var/www

> Also, I have a nagging feeling that some of /lib or /usr/lib
> has some customizations (vague memories of editing or adding files
> there, maybe for browser add-ons...).  

Mozilla adds its plug-ins there.  If they can be installed at all.
Broken IMVAO.

Peace.

-- 
Karsten M. Self <kmself@ix.netcom.com>        http://kmself.home.netcom.com/
 What Part of "Gestalt" don't you understand?
   GNU/Linux & BSD:  We *are* the way out.
     http://www.wehadthewayout.com/

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