Backing up and restoring operations can
take various forms. In this section, some common examples, including
normal, incremental, differential, and daily backup are discussed. The
first three backup procedures are the most widely used.
Normal Backup
Normal backup is also known as full backup. With normal
backup, all files on the disk are stored to tape, and the archive
bit for all files is set to off or cleared.
A full backup that is done each day requires only one tape to
restore the data, but it is impractical to run a full backup each
day because of the amount of dedicated time it requires. Neither
copy backup (discussed later) nor daily backup (discussed later)
resets the archive bit, and both are used for backup of selected
files.
Incremental Backup
Incremental backup procedure backs up all the files that have been
created or modified since the last backup. It is important to
remember two things about an incremental backup. First, it works
only in conjunction with a full (normal) backup. Second, any file
created or modified has its archive bit turned back on so that it
will be saved to tape during the next incremental backup.
To do an incremental backup, first do a full backup on Monday.
This resets all the archive bits on the files. On Tuesday, an
incremental backup would be performed to a separate tape. This
stores all the files modified on Tuesday to tape and resets their
archive bit. This process is repeated for all the other business
days of the week, each with a separate tape. This gives a complete
backup of all files modified during that week. On the following
Monday, the entire process starts over again.
- Advantage – This type of backup scheme requires
the least amount of time per day to do the backup, so it has the
least impact on the network resources in a networked environment.
- Disadvantage – The main disadvantage of
incremental backup is that if users need to restore the backup, it
requires that they first restore the full backup tape and then all
the incremental backup tapes in order. This takes a great deal of
time. In addition, if one of the tapes is bad, that information is
lost.
Differential Backup
This type of backup procedure backs up all the files that have
been created or modified since the last full backup. This sounds
the same as an incremental backup, but the difference is that even
though the file is saved to tape, the archive bit is not reset.
This means that each time a differential backup is done, all the
files modified or created since the last full backup will be
stored again.
To do a differential backup, first do a full backup on Monday
to reset all the archive bits on the files. On Tuesday a
differential backup would be performed to a separate tape. This
stores all the files modified on Tuesday to tape, but it does not
reset their archive bit. This process is repeated for all the
other business days of the week, each with the same tape. This
process also gives a complete backup of network data in a
networked environment.
- Advantages – The advantages of differential backup are
that it requires only two tapes to make and restore the backup, if
necessary.
- Disadvantages – The files that were backed up on
previous days are stored again, which takes a lot more of the
network resources per day. Also, if the differential backup tape is
damaged and the restore was performed on Friday, four days' worth
of data are lost and must be re-entered.
Copy Backup
This backup type backs up user-selected files to tape. This backup
also does not reset the archive bit to off.
Daily Backup
This type backs up only the files that are modified on the day of
the backup. This backup also does not reset the archive bit to
off.
