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Reading 2: Backup types and strategies Backup is not merely copying data from a computer to a storage device. There are many different types of backups and methods of implementing a backup system. The backup type chosen will depend on the amount of data to be backed up, what type of data it is and how quickly the data must be restored in the event of its loss. Archive bit During the formulation of a backup plan, decisions are made as to which files and systems are to be included in backup procedures. Apart from this, there are several methods that operating systems and backup software use to decide which files are to be included in a backup. Some file systems such as File Allocation Table (FAT) and New Technology File System (NTFS) use a special archive bit or attribute attached to every file on a computer for tracking whether it is to be included in a backup. This archive bit acts as a flag to backup software telling it whether or not to include the file in a backup operation. The archive bit is turned off (cleared) after some backup operations. This indicates to future backup operations that the file has been backed up. As soon as the file is modified or any new files are created the archive bit is turned on. This indicates to future backup operations that the file has been modified or is new and will need to be included in the backup operation. The archive bit system allows for certain backup types to only include new or modified files, hence speeding up the backup process and reducing the amount of storage space required for each backup. Some advanced backup systems are also capable of determining file changes at the block level. This allows only those parts of a file that 1 © TAFE NSW, 2007 have been changed since the previous backup to be included in the new backup, further increasing efficiency. Backup types There are numerous types of backups that can be used. It is important to note that the backup solution used could be a combination of several of these backup types. It is also important to note that not all operating systems and backup software use the same terminology to describe a type of backup. Beware of the confusion this can create and pay attention to the documentation specific to the operating system of the computers that are being backed up and the backup software being used. There are several common types of backup. Full backup A full backup creates a copy of all files on a system. This includes data files, the system state, open files etc. These files will be marked as backed up by resetting their archive bit. Similar to a full backup is a normal backup. This is a term commonly used in Windows operating systems and refers to a full backup, but only of files that a user has selected to be backed up. Copy backup A copy backup acts by copying all selected files, as does a full backup. However, a copy backup does not indicate that the files have been backed up by resetting the archive bit. This difference can be useful when a one-off backup needs to be performed without affecting other backup types that rely on the archive bit. Daily backup A daily backup copies all selected files that have been modified on the same day the backup operation is performed. Daily backups do not mark the files as having been backed up. 2 © TAFE NSW, 2007 Incremental backup Incremental backup only copies files that have been created or modified since the last full or incremental backup. These files will be marked as backed up by turning off their archive bit. Incremental backups are used in conjunction with full/normal backups. This allows a large full backup to be done, followed by much smaller incremental backups. The storage space, efficiency and high speed of this system make it a useful backup choice where large amounts of data need to be backed up. Restoring the data will require the restoration of the last full backup and every incremental backup since full backup. This can be time consuming and may not be appropriate for data where the cost of lost data over time is very high. Differential backup Differential backup works by copying any files that have been created or modified since the last full or incremental backup. A differential backup does not indicate that the files have been backed up by resetting the archive bit. All previous differential backups are ignored. As a result to restore data from a differential backup the last full backup and the last differential are all that is needed. The cost is that a differential backup is larger and therefore takes longer than an incremental backup. Other types of backup The above mentioned backup types are the most common methods employed by backup software packages. However, there are several other backup types that should be considered. Continuous backup Continuous backups are not snapshots in time of data as is the case with all of the previously mentioned backup types. The backup process is continuously occurring in real-time to create a mirror copy of the data. This type of backup system is usually employed in mission critical scenarios where data is being backed up to a remote site. The aim is to provide fault tolerance in the event of data loss at one of the sites. 3 © TAFE NSW, 2007 Closely related to continuous backup are Redundant Array of Inexpensive Disks (RAID) systems, such as disk mirroring. Disk imaging Disk imaging/cloning/ghosting is the process of creating a bit level copy of a hard drive partition. This produces a snapshot of the drive that can restore all data onto a new drive if the existing drive fails. Disk imaging is also widely used to maintain the large number of desktop computers that can be used in networks. A sample desktop computer can be configured to reflect the Standards Operating Environment (SOE) to be used for all desktops-or a large number of them. An image is taken of the sample desktop using disk imaging software. This image can then be pushed out to multiple desktop computers (multicasting). The image acts as a backup which can be quickly pushed out to fix a damaged computer system. However, this system relies on all user data being stored on servers as any locally stored files will be overwritten in the imaging process. Although disk imaging is a fast and thorough backup method, it usually requires the computer to be shutdown and booted into a special disk imaging environment to create the image. This is not appropriate for systems that need to be functioning continuously. Versioning systems At both the operating system level and application level there are version tracking systems that can form part of a backup strategy. These systems keep track of all changes made to a file and allow a user to roll back a file to a previous version. The disadvantage of versioning systems is that the file size is significantly larger and may take longer to open and save. Windows systems There are several useful tools and systems built into Windows operating systems. Perhaps the most obvious is the backup utility (ntbackup.exe) which offers the ability to perform and schedule basic backup operations. There are also several other useful inclusions. System restore Similar to file versioning is the concept of system restore. System restore is found in Windows XP and Vista (called Complete PC backup and restore) operating systems and can be thought of as 4 © TAFE NSW, 2007 versioning for operating systems. The operating system will track key changes (such as installing a new hardware driver) and creates ‘restore points’. The operating system can be rolled back to a selected restore point. System restore does have a resource overhead that consumes storage space and other system resources. Shadow copy A shadow copy is essentially a previous version of a file or folder at a specific point in time. Shadow copy is used in Windows XP, Vista and Server 2003 (or greater). It takes a snapshot of the data for backup and through advanced techniques ensures that even files that are open, such as databases are still cleanly copied. Shadow copies, like most backups, can be scheduled to occur multiple times per day if necessary. Automated System Recovery (ASR) The Automated System Recovery replaces the Emergency Repair Disk (ERD) used in Windows NT/2000. ASR helps to create a backup of vital system files and an ASR disk that can be used to restore the system in the event of a major system failure. 5 © TAFE NSW, 2007