[H-GEN] CDs vs tapes [was: Assistance Required]

[Robert Brockway]

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On Wed, 27 Mar 2002, David Jericho wrote:

> > Hi Arjen.  My problem with CDs as backups is recovery time.  How much time
> > does it take to restore 30Gb off 660Mb CDs?  How messy is it?  How much
> > fun it is while the entire company is sitting around waiting for you to
> > finish because someone trounced the fileserver?  Fun fun fun :)
> 
> About the same time as tape, if not slightly quicker, assuming you've
> done your backups right.

Hi David.  I disagree :)  But then I probably didn't make myself clear
either :)

If a tape backup was done properly then only a small number of tapes need
to be hit to restore the system.  You can start the restore off tape & put
your head down to sleep while the box restores (this is occuring at 3am
isn't it?)

Swapping 45 CDs (roughly equivalent to 30Gb) sounds like fun.  Oh, and if
you have incrementals across CDs watch out for disks missing in the
sequence (I know you mentioned about people throwing unmarked CDs out...).

To each their own but to my mind CDs don't have the capacity or 
flexability for a really robust restoration procedure.

As someone (Ray?) mentioned in another post, they're fine for backing up a
certain amount of data (say financial records) but I prefer to look at the
worst case - the need for a full restore[1]

[1] Actually the worst case is that the office & all servers are gone and
all you have is insurance money to buy new gear and some tapes to 
restore data from. [2]

[2] Come to think of it, the worst case scenario is _not_ having insurance
money to buy new servers, but that is a different topic.

> Tape on the other hand has a slight advantage on the social side,
> people not recoginising them will often come ask. And a lot is known
> about the longevity of tape media, having been in use for as long as
> digital computers have been around. So long as you don't keep hitting
> the same peice of tape over and over, and the tape is kept in a
> healthy environment, it can keep backups solid for 10 years or more,
> assuming your tape drive isn't sick.

DLT's should last indefinately I believe.  DAT & Exabyte (and probably
others) have manufacturer stated lifetimes of a few years even under good
conditions (cool dry place yada yada...).  I seem to recall something
about 12 months for DAT and 2 years for Exabyte.  They might work past
that.  This all came up in a SAGE-AU talk by Jason Andrade some time
back.  Was a useful talk indeed.

The longevity (or rather lack of longevity) of just about every storage
medium we currently use is raising questions about how much of our media
will survive 100 years, 500 years, etc.  A few years ago it was a widely
held belief that they era of historical information being lost was over as
so much data would be left.  The problem is it is looks like the data
might not last as well as hoped[2] as well as issues of readability that
formed a seperate thread a few weeks back.

[2] Some very old movie prints are already lost.  Many are being copied to
more modern tape technology.  Perhaps the key to data survival is to carry
the data with you by updating it to modern media every few decades.  This
hopefully clears up readability issues as well as media deterioration.
Eventually the entire sum of human knowledge will be in the computer core
of each ship in Star Fleet of course.

> Then again, if your backup media goes wandering, I'd address the issue
> of where it's kept, and why people have access to it.

Indeed.  Physical security, off site backups.  The Humbug archives would
include good discussions of these I suspect :)

> Tape, CD, they all restore at roughly the same speed, unless you've

This is true.  I was really meaning the delays due to humans having to
change CDs, but was more concerned at how annoying that is when an urgent
restore is needed. I wasn't really clear on my last post.

> got some uber l33t0 DLT tape unit (in which case, you already know all
> of this). Then it comes down to how long it actually takes you to get
> at the data stored on the media.
> 
> A SAGE-AU magazine a few months back had a short article discussing
> backup solutions and software, and time taken to actually rebuild the
> machine back to the point where the data on the media is useful. In a 
> previous life, I had a beta level system where I could whack in a
> CD, rip the old partition information off the backup server and fdisk
> the machine up, and then just start a command to zap that data across
> the network to the newly created partitions. Took 5 minutes plus the
> time to actually transfer the data to restore a machine in its
> entirety. Of course it took a few minutes more and some hardware
> shuffling if the backup server had imploded.

This is a great system (I really like these) but in this scenario the
image file containing the working OS was presumably on a harddrive
somwwhere on the LAN which isn't viable for the scenarios we were talking
about (since it isn't an off site backup).  This is my take anyway :) 

> Ease of restoration comes down partly to how well you've documented
> your procedures, and what dependancies you have. If you were as the

Definately.

> sys admin to be (God forbid) be hit by a bus on the way to work, could

Truck number = 1

> the receptionist restore a system that blew up the night before? Or
> did it take someone a tad more competent like a fellow HUMBUGer or a
> consultant to do so? Would they waste half a day merely figuring out
> how your potentially obfuscated and convuluted system worked? Will
> they need to use only one backup, or a series of backups to get the
> system to a current state?

All important questions.  As readers of my previous posts on topics like
this would have likely garnered I am a fan of a well documented straight
forward backup/restoration procedure.  Sounds obvious doesn't it.  How
could anyone argue with such logic you might ask?  How indeed, but I have
seem far more obfiscated, undocumented backup procedure than I care to
mention.  Often they started out straight forward but have had bits tacked
on & removed for years until they are an utter mess.  Then oneday someone
End-Of-Life's a server sitting in a corner and the backup procedure fails. 
No one even knew the old server was involved.  It really happens as I'm
sure you know David. 

Anyway I'm probably preaching to the converted but I have trouble seeing
how people manage to turn something which is essentially a very simple
procedure (at least in many cases) into something so obfiscated.  And then
I am further amazed at how they never get around to documenting it :)

> There are dozens more questions that can be asked, but I look at
> backups from the perspective of worst case scenarios. 

Same :)
Cheers,
	-Rob

-- Robert Brockway B.Sc. email: robert at timetraveller.org  ICQ: 104781119
   Linux counter project ID #16440 (http://counter.li.org)
   blake: up 78 days, 22:24,  1 user,  load average: 2.12, 1.92, 1.90
   "The earth is but one country and mankind its citizens" -Baha'u'llah


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