Fundamentals of Backup

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• Determining a Backup Strategy :

After selecting the files to back up and specifying the backup destination, there is at least one more critical choice to make. Click Start Backup, then click Advanced, and the Advanced Backup Options dialog box appears, allowing you to specify the backup type. The backup type determines which of your selected files is in fact transferred to the destination media.

Each backup type relates in one way or another to an attribute maintained by every file: archive. The archive (A) attribute is a flag that is set when a file has been created or changed. To reduce the size and duration of backup jobs, most backup types will only transfer to media the files that have their archive attribute set. The most common source of confusion regarding the archive attribute arises from terminology. You will frequently hear, “The file is marked as backed up,” which really means that the archive attribute is cleard

after a particular backup job. The next job will not transfer that file to media. If the file is modified, however, the archive attribute will again be set, and the file will be transferred at the next backup.

• Normal Backups

All selected files and folders are backed up. The archive attribute is cleared. A Normal backup does not use the archive attribute to determine which files to back up; all selected items are transferred to the destination media. Every backup strategy begins with a Normal backup that essentially creates a baseline, capturing all files in the backup job.

• Normal and incremental backups

On Sunday a normal backup is performed, and on Monday through Friday incremental backups are performed. Incremental backups clear the archive attribute, which means that each backup includes only the files that changed since the previous backup. If data becomes corrupt on Fri-day, you need to restore the normal backup from Sunday and each of the incre-mental backups, from Monday through Friday. This strategy takes less time to back up but more time to restore.

 

 

Normal backups are the most time-consuming and require the most storage capacity of any backup type. However, because they generate a complete backup, normal back-ups are the most efficient type from which to restore a system. You do not need to restore multiple jobs. Normal backups clear the archive attribute from all selected files.

• Incremental Backups

Selected files with the archive attribute set are backed up. The archive attribute is cleared. Selected files with the archive flag are transferred to the destination media, and the flag is cleared. If you perform an incremental backup one day after a normal backup has been performed, the job will contain only the files that were created or changed during that day. Similarly, if you perform an incremental backup one day after another incremental backup, the job will contain only the files that were created or changed during that day.

Incremental backups are the fastest and smallest type of backup. However they are less efficient as a restore set, because you must restore the normal backup and then restore, in order of creation, each subsequent incremental backup.

• Differential Backups

Selected files with the archive attribute set are backed up. The archive attribute is not cleared. Because a differential backup uses the archive attribute, the job includes only files that have been created or changed since the last normal or incremental backup. A differential backup does not clear the archive attribute; therefore, if you perform differ-ential backups two days in a row, the second job will include all the files in the first backup, as well as any files that were created or changed during the second day. As a result, differential backups tend to be larger and more time-consuming than incremen-tal backups, but less so than normal backups.

Differential backups are significantly more efficient than incremental backups as a restore set, however. To fully restore a system you would restore the normal backup and the most recent differential backup.

• Copy Backups

All selected files and folders are backed up. Copy neither uses nor clears the archive attribute. Copy backups are not used for typical or scheduled backups. Instead, copy backups are useful to move data between systems or to create an archival copy of data at a point in time without disrupting standard backup procedures.

• Daily Backups

All selected files and folders that have changed during the day are backed up, based on the files’ modify date. The archive attribute is neither used nor cleared. If you want to back up all files and folders that change during the day without affecting a backup schedule, use a daily backup.

• Combining Backup Types

Although creating a normal backup every night ensures that a server can be restored from a single job the next day, a normal backup may take too much time to create, per-haps causing the overnight job to last well into the morning, thus disrupting perfor-mance during working hours. To create an optimal backup strategy, you must take into account the time and size of the backup job, as well as the time required to restore a system in the event of failure. Two common solutions are:

• Normal and differential backups

On Sunday a normal backup is performed, and on Monday through Friday nights, differential backups are performed. Differ-ential backups do not clear the archive attribute, which means that each backup includes all changes since Sunday. If data becomes corrupt on Friday, you only need to restore the normal backup from Sunday and the differential backup from Thursday. This strategy takes more time to back up, particularly if data changes frequently, but is easier and faster to restore, because the backup set is on fewer disks or tapes.