Database backup is the process of dumping data (from a database, a transaction log, or a file) into backup devices that the system creates and maintains. A backup device can be a disk file or a tape. The Database Engine provides both static and dynamic backups. Static backup means that during the backup process, the only active session supported by the system is the one that creates the backup. In other words, user processes are not allowed during backup. Dynamic backup means that a database backup can be performed without stopping the database server, removing users, or even closing the files. (The users will not even know that the backup process is in progress.)
The Database Engine provides four different backup methods:
- Full database backup
- Differential backup
- Transaction log backup
- File (or filegroup) backup
The following sections describe these backup methods.
Full Database Backup
A full database backup captures the state of the database at the time the backup is started. During the full database backup, the system copies the data as well as the schema of all tables of the database and the corresponding file structures. If the full database backup is executed dynamically, the database system records any activity that takes place during the backup. Therefore, even all uncommitted transactions in the transaction log are written to the backup media.
A differential backup creates a copy of only the parts of the database that have changed since the last full database backup. (As in a full database backup, any activity that takes place during a differential backup is backed up too.) The advantage of a differential backup is speed. It minimizes the time required to back up a database, because the amount of data to be backed up is considerably smaller than in the case of a full database backup. (Remember that a full database backup includes a copy of all database pages.)
Transaction Log Backup
A transaction log backup considers only the changes recorded in the log. This form of backup is therefore not based on physical parts (pages) of the database, but rather on logical operations—that is, changes executed using the
DELETE. Again, because the amount of data is smaller, this process can be performed significantly quicker than a full database backup and quicker than a differential backup.
Note – It does not make sense to back up a transaction log unless a full database backup has been performed at least once.
There are two main reasons to perform a transaction log backup: first, to store the data that has changed since the last transaction log backup or full database backup on a secure medium; second (and more importantly), to properly close the transaction log up to the beginning of the active portion of it. (The active portion of the transaction log contains all uncommitted transactions.)
Using a full database backup and a valid chain of all closed transaction logs, it is possible to propagate a database copy on a different computer. This database copy can then be used to replace the original database in case of a failure. (The same scenario can be established using a full database backup and the last differential backup.)
The Database Engine does not allow you to store the transaction log in the same file in which the database is stored. One reason for this is that if the file is damaged, the use of the transaction log to restore all changes since the last backup will not be possible.
Using a transaction log to record changes in the database is a common feature used by nearly all existing relational DBMSs. Nevertheless, situations may arise in which it becomes helpful to switch this feature off. For example, the execution of a heavy load can last for hours. Such a program runs much faster when the logging is switched off. On the other hand, switching off the logging process is dangerous, as it destroys the valid chain of transaction logs. To ensure successful database recovery, it is strongly recommended that you perform a full database backup after the successful end of the load.
One of the most common system failures occurs because the transaction log is filled up. Be aware that such a problem may cause a complete standstill of the system. If the storage used for the transaction log fills up to 100 percent, the system must stop all running transactions until the transaction log storage is freed again. This problem can be avoided only by making frequent backups of the transaction log: each time you close a portion of the actual transaction log and store it to a different storage media, that portion of the log becomes reusable, and the system thus regains disk space.
Note – A differential backup and a transaction log backup both minimize the time required to back up the database. But there is one significant difference between them: the transaction log backup contains all changes of a row that has been modified several times since the last backup, whereas a differential backup contains only the last modification of that row.
Some differences between log backups and differential backups are worth noting. The benefit of differential backups is that you save time in the restore process, because to recover a database completely, you need a full database backup and only the latest differential backup. If you use log backups for the same scenario, you have to apply a full database backup and all existing log backups to bring the database to a consistent state. A disadvantage of differential backups is that you cannot use them to recover data to a specific point in time, because they do not store intermediate changes to the database.
File or Filegroup Backup
File (or filegroup) backup allows you to back up specific database files (or filegroups) instead of the entire database. In this case, the Database Engine backs up only files you specify. Individual files (or filegroups) can be restored from a database backup, allowing recovery from a failure that affects only a small subset of the database files. You can use either a database backup or a filegroup backup to restore individual files or filegroups. This means that you can use database and transaction log backups as your backup procedure and still be able to restore individual files (or filegroups) from the database backup.
Note – File backup is also called file-level backup. This type of backup is recommended only when a database that should be backed up is very large and there is not enough time to perform a full database backup.