Backups
We have been receiving a lot of questions about whether backups arereally necessary. The answer is an unequivocal Yes!
Why Back Up?
If disks and people were perfect, there might be no reason to backup your data. But disks and disk sectors can go bad. When this happens, you will lose data. Windows NT can salvagedata from bad sectors if you are using fault-tolerant volumes, and you can recover from failed disk drives if youuse stripe sets with parity or mirror sets, but these only reduce the need for backups, they don't eliminate it.
Users can (and do) delete files and empty recycle bins. Users alsodelete files and then find they really need them. Without an undeleter, nothing but a backup (or an extremely skilledprogrammer) can recover a deleted file.
Files can also be modified with wrong data, such as an employee error,or a virus attack. Some applications are incompatible with others, and trying to run both may corrupt any or allfiles on your disk. A power surge or failure on an unprotected system can erase files or destroy a disk. A hardjolt, like from knocking over a PC, can cause the disk head to touch a platter, making the data unrecoverable.Even an undeleter can't help in these cases.
The point is that there are many ways you can lose data. Recoveringthe data may be expensive (or even impossible) and the time spent recovering and business lost while the systemis down will probably be even more expensive than the recovery. The fastest and only sure way of recovering datais from a backup. A proper backup system can even save your data if the whole building should burn down!
How Should Backups be Done?
No backup system can be right for all sites.
The simplest method is to just copy the entire disk to a tape periodically.This is called a "Full Backup". It requires no decisions and, if a single tape is large enough to holdall the data, it can be started and ignored until done, thus using very little operator time. But if you have manygigabytes of data, someone has to be there to swap tapes.
Another method is to make a full backup, then periodically back upjust those files which have changed since the last backup. These are called "Incremental Backups". Thereare software packages available which will do this automatically. The frequency of making incrementals would dependon the site. If your data changes slowly and can easily be re-entered, weekly incrementals may do. If you are addinglarge amounts of critical data all the time, you may need hourly backups. Daily is usually best.
An incremental backup is much faster than a full backup, and requiresfewer tapes (though tapes are so cheap these days that cost is of little concern), but finding the file or filesyou need to restore can be quite a chore. Again, some of the backup packages can handle this, quickly telling youwhich tape has the latest copy of the file. The author uses a form of this method at home, where each partitionis fully backed up to a tape, and incrementals are done to ZIP disks. This used to be done manually, by decidingwhich files were important to back up, but now this is done by a backup package that automatically selects thefiles to be backed up.
The method we consider generally best is to make a full backup everymonth. Then, each day, make an incremental of everything that changed that day, and each week make an incrementalof everything that changed that week. The "daily" tapes are re-used each week, and the "weekly"tapes are re-used each month. The monthly copies are kept forever. With this method, you can recover your choiceof:
- the most recent copy of a file
- the file as it was in any week of the current month
- the file as it was at the end of any month
You can modify this for your own needs, such as archive the weeklycopies, or only archive a copy quarterly or annually, or whatever. You may use this pattern for administrativefiles, but do a daily full backup of important data, such as programs in development, or orders taken.
Being Safe
The number of precautions you should take depends on the value ofyour data. The bottom line question is, "How does the expense of making backups compare with the potentialloss if I don't back up?" Tapes and tape drives are cheap and getting cheaper, and good software exists tocut the man-hours required way down, but just having a backup tape is no guarantee of data recoverability. Tapescan go bad or get lost or damaged, a backup program can skip a file or make a bad copy, and human error can resultin files being missed.
It is advisable to make two copies of each backup tape, andimmediately take one off-site. That's your disaster recovery copy. You might even want a third copy if the valueof the data warrants it. And always, always, verify your backup tapes, including the extra copies, to make surethey are readable and do contain the files you think they do.
Backups may never be needed, but with a proper set, you will neverlose critical data, and may save a lot of money - and your job!
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Executive Software Europe
