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Reading 2: Backup types and
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
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
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
© 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.
© 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
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.
© 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
© 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.
© TAFE NSW, 2007