The Perils of Backing Up
by Jerry Stern
ASPects Editor
### Visit Jerry web sites: graphcat.com and filetiger.com ###
Lately, I've been dealing with the restoration of a corporate database for a customer with
a small network. The file server was stolen, containing the tape drive, as well as the
most recent backup tape. As part of recreating the corporate database, we confirmed my
darkest suspicions--the replacement tape drive couldn't read their compressed backup
tapes.
All of the tape drive manufacturers are rating the capacity of their tape drives by their
compressed capacity. They claim that an 800Mb cartridge will hold 1.6 Gigabytes, thus
justifying the 1600 in the product name. Actually, 2-to-1 compression is quite possible on
many types of files, but some files don't compress at all, and on a typical collection of
files on a typical hard drive, 1.7-to-1 is realistic as average compression. That's 1360
Mb on a 800Mb tape. This minor exaggeration is not a major problem, however.
Problems arise when a compressed backup tape must actually be read and used to restore a
file system on a destroyed machine, or when a tape drive is burnt out and replaced, or
just plain upgraded. It turns out that there is no industry standard for tape compression,
and not much of a standard within the same manufacturer.
Think about this: Why did you buy that tape drive? Worst case, to have a complete
restoration of your destroyed hard drive. Well, if your PC is stolen or destroyed,
including your tape drive, the natural inclination is to buy a new tape drive of the
newest high-capacity type, of any current flavor that can read your old tape size.
That's not as easy as it sounds. Years might pass between when you bought your tape drive
and the dark day when you must use the tapes to rebuild your file system, and possibly
source code and customer database. In that time, the tape drive makers have done two
things. First, they've created wonderful new drives, faster and with larger capacities.
Second, they've started to farm out the creation of their backup software to another
company, or stopped farming it out altogether, or changed to another contractor that
promised better compression ratios.
The result is that although your new drive and the old drive are from the same
manufacturer, there is a much better than even chance that the new drive will be unable to
read a compressed tape from the old drive.
If your new tape drive is supplied by a clone maker, of course you will get whatever
they're selling this week, which will be the cheapest drive available of that size. If the
new drive is from a different manufacturer than the old drive, the chances of reading the
old tapes will be far worse than even.
I'll boil all these problems down to usable precautions:
1) Never compress your off-site backup tapes.
What's that? All your tapes are in a box sitting on top of the PC? Well, that's another
subject altogether.
2) Never password-protect your tape backups.
New tape software may not use the same encryption methods as your old tape software, so
your backup tapes will become paperweights. Instead, lock the tapes up for security.
3) Never compress tape backups of DATA.
Your backups of programs can be compressed, because if your computer is destroyed or
stolen, hardware differences in your replacement machine will force a software
reinstallation anyway.
By the way, if at all possible, keep all your data on a different drive letter than your
software. If d:\ contains only data, you can back it up four times as often as your c:\
drive, which contains only programs that are not major disasters to replace, and that
don't change as much or as often as data.
4) Your backups of critical files should ALWAYS be
created on a drive that you would replace if your machine were destroyed, and not on an
old-style drive (tape, disk, or removable hard drive cartridge) that you would replace
with the newest or greatest gadget, given the opportunity.
Make the assumption that if you need a backup, it will be because all your hardware has
been melted into a puddle by a lightning strike and the subsequent fire, and only current
hardware will be available to read your off-site backups.
Critical files include data files. And not much else. The conventional wisdom of backing
up config.sys, system.ini, system.dat, and so on, is handy if you have to restore a system
that has had a minor disaster land on it, like the disk heads performing root canal work
on a hard drive platter. In that scenario, only one piece of hardware has died, and you
will need those configuration files. But if what operated on the hard drive was a
hurricane-driven telephone pole, you will be replacing all your hardware, and old
configuration files will be of nearly no use at all.
5) If and when the worst happens, you may find that
despite all your best laid plans, and the commandments above, your new drive, from the
same manufacturer, can't run your old backup software or read your old tapes, your best
bet may be to find someone with the same drive as you used to have before the disaster,
and trade him a nice, new, high-capacity, but utterly useless drive, for his old, slow,
low-capacity, but wonderfully compatible drive. You both win.
In other words, RESIST THE TEMPTATION to upgrade your tape drive while you are busily
restoring a disaster-smashed system.
Better yet, make a buddy agreement now: You and your buddy buy identical tape drives now,
and promise each other that in the case of disaster, the buddy with the surviving computer
will remove his tape drive and lend it to the less fortunate buddy during the restoration
process.
6) When upgrading a tape drive on an existing system, or
buying your first tape drive, get a unit large enough that you can run a total system
backup on one tape, uncompressed.
That local customer of mine is back up and running. They lost some files, but only a
handful of recent files with paper copies handy. What saved them was a borrowed portable
tape drive, of about the same age as the original tape drive, which could read the old
compressed tapes. There were some gaps in the tapes that were not stolen along with the
computer, and those gaps were filled in by another tape, stored off-site. They're still in
business.
Will you fare as well?
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with any questions, comments or suggestions.