Types of Backups


You can perform five different types of backups. Many backup schemes rely on full backups daily, but for some networks, it's more practical to use a scheme that relies on two or more of these backup types.

The differences among the fives types of backup involve a little technical detail known as the archive bit. The archive bit indicates whether a file has been modified since the last time it was backed up. The archive bit is a little flag that's stored along with the filename, creation date, and other directory information. Any time that a program modifies a file, the archive bit is set to the On position. That way, backup programs know that the file has been modified and needs to be backed up.

The differences among the various types of backup center around whether they use the archive bit to determine which files to back up, and whether they flip the archive bit to the Off position after they back up a file. Table 21-1 summarizes these differences, and they're explained in the following sections.

Table 21-1: How Backup Types Use the Archive Bit
Open table as spreadsheet

Backup Type

Selects Files Based on Archive Bit?

Resets Archive Bits after Backing Up?

Normal

No

Yes

Copy

No

No

Daily

No[*]

No

Incremental

Yes

Yes

Differential

Yes

No

[*]Selects files based on the Last Modified date.

Tip 

Backup programs allow you to select any combination of drives and folders to back up. As a result, you can customize the file selection for a backup operation to suit your needs. For example, you can set up one backup plan that backs up all a server's shared folders and drives plus its mail server stores but leaves out folders that rarely change, such as the operating system folders or installed program folders. You can then back up those folders on a less regular basis. The drives and folders that you select for a backup operation are collectively called the backup selection.

Tip 

The archive bit would've made a good Abbott and Costello routine. ("All right, I wanna know who modified the archive bit." "What." "Who?" "No, what." "Wait a minute … just tell me what's the name of the guy who modified the archive bit!" "Right.")

Normal backups

A normal backup, also called a full backup, is the most basic type of backup. In a normal backup, all files in the backup selection are backed up-regardless of whether the archive bit has been set. In other words, the files are backed up even if they haven't been modified since the last time they were backed up. When each file is backed up, its archive bit is reset, so backups that select files based on the archive bit setting won't back up the files.

When a normal backup finishes, none of the files in the backup selection will have their archive bits set. As a result, if you immediately follow a normal backup with an incremental backup or a differential backup, no files will be selected for backup by the incremental or differential backup because no files will have their archive bits set.

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 complicated when other types of backups are involved.

Tip 

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'll have to use other types of backups in combination with normal backups.

Tip 

If you can't get a normal backup on a single tape and you can't afford a second tape drive or a tape changer, 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 a large amount 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 were able to squeeze onto a single tape again.

If you remove static data from the nightly backup, make sure that you have a secure backup of the static data, either on tape, CD-RW, or some other media.

Copy backups

A copy backup is similar to a normal backup, except that the archive bit isn't reset when each file is copied. As a result, copy backups don't disrupt the cycle of normal and 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 are reset, and your regular backups are disrupted. However, if you do a copy backup, the archive bits of any modified files remain unchanged. As a result, your regular normal and incremental or differential backups are unaffected.

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.

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 the modification 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 on a single tape.

When an incremental backup copies each file, it resets the file's archive bit. That way, the file will be backed up again before your next normal backup only when a user modifies the file again.

Here are some thoughts about using incremental backups:

  • The easiest way to use incremental backups is to do

    • A normal backup every Monday.

      Tip 

      If your full backup takes more than 12 hours, you may want to do it on Friday so that it can run over the weekend.

    • An incremental backup on each other normal business day (for example, Tuesday, Wednesday, Thursday, and Friday).

  • When you use incremental backups, the complete backup consists of the full backup tapes and all the incremental backup tapes that 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.

  • 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.

    TECHNICAL STUFF 

    Backup programs keep track of the location of the most recent version of each file in order to simplify the process.

  • When you use incremental backups, you can choose whether you want to

    • Store each incremental backup on its own tape.

    • Append each backup to the end of an existing tape.

    Tip 

    Often, you can use a single tape for a week of incremental backups.

Differential backups

A differential backup is similar to an incremental backup except that it doesn't reset the archive bit when 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 differential backup.

For example, suppose that you do a normal backup on Monday and 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 differences between incremental and differential backups are that

  • Incremental backups result in smaller and faster backups.

  • Differential backups are easier to restore.

Tip 

If your users often ask you to restore individual files, consider differential backups.




Networking For Dummies
Networking For Dummies
ISBN: 0470534052
EAN: 2147483647
Year: 2004
Pages: 254
Authors: Doug Lowe

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net