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 Your company's data may be its most
important asset. Imagine a large corporation with millions
of dollars of account receivable and account payable data
stored on their network. A fire causes massive data loss.
How much work would be required to re-create the lost
data? Without backups, the company may not survive the
disaster.
Choosing a backup media is important, but in this article
I'll focus on creating a plan for the type and frequency
of backups. Let's assume you will use some type of tape
backup media.
If your company has only a small amount of data, you may
be able to perform a full backup every night. You would
need two tapes which you would alternate in case the most
recent full backup turns out defective. At least you have
a previous one to go back to. You would store the tapes in
a secure off-site location to avoid being destroyed in the
same disaster that might destroy the original data.
When you perform a full backup, each file that gets baked
up has the "archive" attribute in its file
properties set to zero.
The archive attribute is a flag stored for each file that
has been created or modified. It indicates that the file
needs to be archived. Backup programs can reduce the size
of backups by saving only files which have been modified
since the previous backup. When the file is saved in a
full or incremental backup, the archive bit is set to one.
In Windows, you can view the archive attribute for any
file by right-clicking the file in Windows Explorer and
selecting "Properties" in the popup menu. In the
"Properties" dialog box, click on the
"Advanced..." button.
If your company has a large amount of data, it would be
too time consuming to perform a full backup every night.
Instead, you would perform a full backup only on Friday
nights, and perform a "differential" backup on
other weekday night. This backup plan would require six
tapes.
- A differential backup saves all files that have been
created or modified since the last FULL backup.
- Restoring with differential backups is easy. You need
just use two tapes. First restore the last full backup.
Then restore the last differential backup.
A differential backup doesn't reset the archive attribute.
The next time you perform a differential backup, it will
again save all the files that have changed since the last
full backup. So each night, the differential backup will
get progressively larger until the next you perform a full
backup.
If your company has a great amount of data that changes
every day, it would be too time consuming to perform a
differential backup every night. Instead, you might
perform a full backup on Friday nights, and an
"incremental" backup on other weekday night.
- An incremental backup saves only the files that have
changed since the last full or incremental backup.
An incremental backup checks the archive attribute to
determine if the file has changed and needs to be backed
up. Then it resets the archive attribute. An incremental
backup stays small because it includes only files that
have changed since the last full or incremental backup.
- Restoring with incremental backups is more time
consuming. First, restore the last full backup. Then
restore all the incremental backups made since the full
backup, in the order they were made. The only reason to
use an incremental backup scheme is if differential
backups become too large, requiring more than one tape.
Many company networks continue to operate on Saturday and
even Sunday. In this case, you would need to add the
required additional tapes to the plan.
If during the day your company creates a large volume of
data, or a small amount of data that is too critical to
wait for the standard night time backup, you can perform a
copy backup. To do this, you just copy the selected files
to a backup media.
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