The easiest backup scheme is to simply schedule a normal backup every night. That way, all your data is backed up on a daily basis. So if the need arises, you
can restore files from a single tape or set of tapes. Restoring files is more com- plicated when other types of backups are involved.
As a result, I recommend that you do normal backups nightly if you have the tape capacity to do them unattended — that is, without having to swap tapes.
If you can’t do an unattended normal backup because the amount of data to be backed up is greater than the capacity of your tape drive or drives, you
may prefer to use other types of backups in combination with normal backups.
If you can’t get a normal backup on a single tape and you can’t afford a second tape drive, take a hard look at the data that’s being included in the backup
selection. I recently worked on a network that was having trouble backing up onto a single tape. When I examined the data that was being backed up,
I discovered about 5 GB of static data that was essentially an online archive of old projects. This data was necessary because network users needed it
for research purposes, but the data was read-only. Even though the data never changed, it was being backed up to tape every night, and the backups
required two tapes. After we removed this data from the cycle of nightly backups, the backups could squeeze onto a single tape again.
If you do remove static data from the nightly backup, make sure that you have a secure backup of the static data — on tape, CD-RW, or some other medium.
Copy backups
A copy backup is similar to a normal backup, except the archive bit is not reset as each file is copied. As a result, copy backups don’t disrupt the cycle
of normal, incremental, or differential backups.
Copy backups are usually not incorporated into regular, scheduled backups. Instead, you use a copy backup when you want to do an occasional one-shot
backup. For example, if you’re about to perform an operating-system upgrade, you should back up the server before proceeding. If you do a full backup, the
archive bits will be reset and your regular backups will be disrupted. However, if you do a copy backup, the archive bits of any modified files will remain
unchanged. As a result, your regular normal, incremental, or differential back- ups will be unaffected.
Note that if you don’t incorporate incremental or differential backups into your backup routine, the difference between a copy backup and a normal backup is
moot.
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Daily backups
A daily backup backs up just those files that have been changed the same day that the backup is performed. A daily backup examines the modification date
stored with each file’s directory entry to determine whether a file should be backed up.
Daily backups don’t reset the archive bit. I’m not a big fan of this option because of the small possibility that some files
may slip through the cracks. Someone may be working late one night and modify a file after the evening’s backups have completed, but before midnight.
Those files won’t be included in the following night’s backups. Incremental or differential backups, which rely on the archive bit rather than on the modifica-
tion date, are more reliable.
Incremental backups
An incremental backup backs up only those files that you’ve modified since the last time you did a backup. Incremental backups are a lot faster than full
backups because your network users probably modify only a small portion of the files on the server in any given day. As a result, if a full backup takes three
tapes, you can probably fit an entire week’s worth of incremental backups onto a single tape.
As an incremental backup copies each file, it resets the file’s archive bit. That way, the file is backed up before your next normal backup only if a user modi-
fies the file again.
Here are some thoughts about using incremental backups: ⻬ The easiest way to use incremental backups is to do a normal
backup every Monday and then do an incremental backup on Tuesday, Wednesday, Thursday, and Friday. This assumes, of course, that you
can do a full backup overnight on Monday. If your full backup takes more than 12 hours, you may want to do it on Friday instead so it can run over
the weekend.
⻬ When you use incremental backups, the complete backup consists of the full backup tapes and all the incremental backup tapes you’ve made since
you did the full backup. If the hard drive crashes and you have to restore the data onto a new drive, you first restore Monday’s normal backup, and
then you restore each of the subsequent incremental backups.
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⻬ Incremental backups complicate the task of restoring individual files because the most recent copy of the file may be on the full backup tape
or on any of the incremental backups. Fortunately, backup programs keep track of the location of the most recent version of each file, which
simplifies the process.
⻬ When you use incremental backups, you can choose whether you want to store each incremental backup on its own tape, or append each backup to
the end of an existing tape. In many cases, you can use a single tape for an entire week’s worth of incremental backups.
Differential backups
A differential backup is similar to an incremental backup, except that it doesn’t reset the archive bit as files are backed up. As a result, each differential backup
represents the difference between the last normal backup and the current state of the hard drive. To do a full restore from a differential backup, you
first restore the last normal backup, and then you restore the most recent dif- ferential backup.
For example, suppose you do a normal backup on Monday, differential backups on Tuesday, Wednesday, and Thursday, and your hard drive crashes Friday
morning. Friday afternoon you install a new hard drive. Then, to restore the data, you first restore the normal backup from Monday. Then, you restore the
differential backup from Thursday. The Tuesday and Wednesday differential backups aren’t needed.
The main difference between incremental and differential backups is that incre- mental backups result in smaller and faster backups, but differential backups
are easier to restore. If your users frequently ask you to restore individual files, you may want to consider differential backups.
Local versus Network Backups
When you back up network data, you have two basic approaches to running the backup software: You can perform a local backup, in which the backup
software runs on the file server itself and backs up data to a tape drive that’s installed in the server, or you can perform a network backup, in which you
use one network computer to back up data from another network computer. In a network backup, the data has to travel over the network to get to the
computer that’s running the backup.
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