An incremental backup is one that provides a backup of files that have changed or are
new since the last incremental backup; it is one that backs up only the data that changed
since the last backup — be it a full or incremental backup.
An incremental backup is a backup of latest changes since the last backup so that when
a full recovery is needed the restoration process would need the last full backup plus
all the incremental backups until the point-in-time of the restoration.
Incremental backups are often desirable as they consume minimum storage space and are
quicker to perform than differential backups.
The purpose of an incremental backup is to preserve and protect data by creating copies
that are based on the differences in those data and thus minimize the amount of time
needed to perform the backup.
With incremental backups, successive copies of the data contain only that portion that
has changed since the preceding backup copy was made.
Variants Incremental
The most basic . A Unix example would be:
The use of rsync's --link-dest option is what makes this command an example of incremental
backup.
Multilevel incremental A more sophisticated incremental backup scheme
involves multiple numbered backup levels.
A full backup is level 0.
A level n backup will back up everything that has changed since the most recent level n-1
backup.
Suppose for instance that a level 0 backup was taken on a Sunday.
A level 1 backup taken on Monday would include only changes made since Sunday.
A level 2 backup taken on Tuesday would include only changes made since Monday.
A level 3 backup taken on Wednesday would include only changes made since Tuesday.
If a level 2 backup was taken on Thursday, it would include all changes made since Monday
because Monday was the most recent level n-1 backup.
Reverse incremental An incremental backup of the changes made
between two instances of a mirror is called a reverse incremental.
By applying a reverse incremental to a mirror, the result will be a previous version of the
mirror.
Incrementals forever This style is similar to the Synthetic backup
concept.
After an initial full backup, only the incremental backups are sent to a centralized backup server.
This server keeps track of all the incrementals and sends the proper data back to the server
during restores.
This can be implemented by sending each incremental directly to tape as it is taken and then refactoring
the tapes as necessary.
If enough disk space is available, an online mirror can be maintained along with previous
incremental changes so that the current or older versions of the systems being backed
up can be restored.
This is a suitable method in case of banking systems.
Block level incremental This method backs up only the blocks within
the file that changed.
This requires a higher level of integration between the sender and receiver.
Byte level incremental These backup technologies are similar to the
"block level incremental" backup method; however, the byte incremental backup method is based
on a binary variation of the files compared to the previous backup: while the block-based
technologies work with heavy changing units, the byte-based technologies work with the
minimum unit, saving space when reflecting a change on a file.
Another important difference is that they work independently on the file system.
At the moment, these are the technologies that achieve the highest relative compression
of the data, turning into a great advantage for the security copies carried out through
the Internet.
Other backup types Synthetic full backup
A Synthetic backup is an alternative method of creating full backups.
Instead of reading and backing up data directly from the disk, it will synthesize the data
from the previous full backup and the periodic incremental backups.
As only the incremental backups read data from the disk, these are the only files that
need to be transferred during Offsite Replication.
This greatly reduces the bandwidth needed for Offsite Replication.
Differential A differential backup is a cumulative backup
of all changes made since the last full or normal backup, i.e., the differences since
the last full backup.
The advantage to this is the quicker recovery time, requiring only a full backup and the
last differential backup to restore the system.
The disadvantage is that for each day elapsed since the last full backup, more data needs
to be backed up, especially if a significant proportion of the data has changed.
See also Backup rotation scheme
Continuous data protection Delta encoding
Dump - UNIX utility for multilevel incremental file system backups.
rsync - File synchronization algorithm and protocol.
References
Further reading