Subject: General means for backups (was: Re: Backup tapes recommendation)

From: Omer Zak <[email-masked]>
Date: Thu, 7 Nov 2002 00:57:17 +0200 (IST)

<META_RULES_OF_DISCOURSE>
In spite of Aviram's request to keep the subject off list, I would like to
generalize the discussion.  Since the discussion is not specific to Linux,
I am cross-posting it to the Hackers-IL mailing list and urge people who
want to contribute to it - to post only to Hackers-IL, and people who want
to make Linux-specific recommendations - to send personal E-mail to
Aviram.
</META_RULES_OF_DISCOURSE>

In the past, tapes (and later cartridges) were the way to backup
computers.
However, since the advent of cheap giant hard disks, CD-RW and even DVD/W
drives, it may be a good idea to reappraise the economics of various means
of backup.

1. If you need to backup less than 650MB (compressed) a day, and you don't
   need to destroy/recycle media, then CD-ROMs may be the best way.
2. If you want to recycle media (to wipe out older information), you may
   want to consider CD-R media.
3. Tape cartridges have capacities of the order of 10GB or so.  Hard disks
   now hold as much as 120GB, and they can be recycled (especially if you
   install them by means of a bay which allows them to be easily removed).
4. If you don't like the idea of hot swapping of hard disks, then connect
   a second cheap computer by LAN to your main server, and have the main
   server copy files to the backup computer for backing up to the other
   computer's removable hard disk.  Then you can turn the second computer
   off without disturbing your main server.
5. And finally, don't forget to consider paper tape :-).  It is immune
   against EMP, which is a risk everywhere nuclear bomb programs exist
   (such as in Iraq).

On Wed, 6 Nov 2002, Aviram Jenik wrote to Linux-IL mailing list:

> We're looking for recommendations for a backup tape for daily Linux
> backups using Amanda. I would like to hear recommendations (off list
> please; I will send a summary of the responses I receive) on
> Linux-friendly backup tapes and especially good/bad experiences, things
> to avoid, potential pitfalls, etc.

[... snipped ...]
(Aviram mentions that their total backup needs are less than 10GB and
low cost is more important than high quality/availability.)

                                             --- Omer
WARNING TO SPAMMERS:  see at http://www.zak.co.il/spamwarning.html

---

From: Gilad Ben-Yossef <[email-masked]>
Date: 07 Nov 2002 10:42:16 +0200

>    want to consider CD-R media.

Do yourself a big favor and don't.

The act of reading information from a CDROM is a small miracle of
engineering. The little "holes" on the CDROM that reflect the laser beam
are actually smaller then the beam's "width" and hence for various
reasons having to do with advanced physics there is a certain
statistical chance for a misread built right into the very process of
reading CDs. 

To overcome this, CDs contain a big amount of redundancy, which
guarantees that the actual error levels will remain generally acceptable
over time[1] for *normal usage*, but ever since I had the misfortune
once to try to debug a problem that was caused by the fact that the very
small percentage of actual read errors still gets to be very significant
given high enough numbers of CDROM/reads when used in high scale
production environment (and we're talking about factory burned CDs here,
NOT CD-R!) I don't trust CDROMs for anything important since then.

[1] Because of this you can theoretical drill a 1 cm hole on a used
surface of a CD and still read the information - the information is
recorded in several copies in several places on the CD. Theoretical,
because drilling a hole in the CD will make it not balanced and then you
won't be able to read it in normal equipment - but the data will still
be there.

Gilad.

-- 
 Gilad Ben-Yossef <[email-masked]> 
 http://benyossef.com 
 "Denial really is a river in Eygept."

---

From: "Nadav Har'El" <[email-masked]>
Date: Sun, 10 Nov 2002 12:40:54 +0200

On Thu, Nov 07, 2002, Gilad Ben-Yossef wrote about "[hackers-il] Re: General means for backups (was: Re: Backup tapes recommendation)":
>...

The first backup system I encountered, around 1985, was based on optical
disks. These were larger than CD-ROMs, and could hold 3 GB of data (at that
time, when the typical hard disk was 80 MB, this was considered a huge amount
of data). The system was called "WORM", for "Write-once-read-many", and much
like current CD-Rs, it was possible to write only once to each track and
later read it as many times as you want.

Why was that system chosen over the customary backup tapes? I am not sure,
but I can offer a few guesses:

 1. Contrary to what Gilad is saying, this WORM system was supposedly very
    reliable; An expected lifetime of 30 years was claimed for these disks.
    Tapes have a much shorter lifetime, especially if you use them
    intensively.

 2. 3 GB was a huge amount of data and the disks took relatively little
    space; I don't think tape technology was as advanced then.

 3. This was not a personal backup system, it was a departmental backup
    system (in AT&T's Bell Labs), intended to serve dozens (or even
    hundreds) of people. It was expected that very often, probably even
    several times a day, people would want to recover files they accidentally 
    rm'ed. People might even want to compare a file as it was two days ago
    to a file as it was yesterday, to see what was ruined. A random-access
    device helps a lot in making this a viable service: Waiting 2 hours
    or even 10 minutes for an old copy of your file is a completely different
    experience from waiting 30 seconds for it.

So backing up on optical disks is not as useless as it might appear.
The most serious problem with it nowadays is that optical disks grew "small"
relative to current hard drives. Writable DVDs can help, but they are still
not "huge" compared to hard drives.

-- 
Nadav Har'El                        |       Sunday, Nov 10 2002, 5 Kislev 5763
[email-masked]             |-----------------------------------------
Phone: +972-53-245868, ICQ 13349191 |Unix is user friendly - it's just picky
http://nadav.harel.org.il           |about it's friends.