Information Technology Reference
In-Depth Information
called blocks (a
block
being a unit of storage typically equal to 4096
bytes—4K—on modern Macs). So, you'll sometimes see this feature
referred to as “block-level (or byte-level) incremental updates” or
words to that effect. With backup applications that use byte-level or
block-level delta encoding, if only 10 bytes of a file change, only those
10 bytes, or the block(s) containing those 10 bytes, are added to the
backup—a tiny amount of data.
The advantage of such an approach is that backups go much faster
after the initial run and take up far less storage space; this is
particularly important when backing up over the Internet. The
disadvantage is that restoring a file requires the backup software to
reconstruct it by putting together the pieces from all its incremental
backups. If even a single one of those incremental bits were to become
damaged or lost, you might be unable to restore the file.
Deduplication
You might have two or more identical copies of a certain file on your
disk. Some backup software notices this and puts only one copy in
your backup (along with a record that the file appears in multiple
places). That way, you save storage space and speed up your backups
considerably. Taking this concept further, many backup programs can
look within files for
portions
of files that are identical to
portions
of
other files and—thanks to delta encoding—copy only the unique parts
of the additional files. This process of preventing duplicate data (at
any level) from cluttering up your backups is called
deduplication
.
Deduplication applies only to versioned backups, not to duplicates
(you can see the contradiction in the name!), and is extremely useful.
Almost every online backup program offers deduplication, which
is great when you're paying by the gigabyte or when you're trying
to push data over a slow Internet connection. The result sometimes
seems impossible—how did hundreds of megabytes of data just upload
in a few seconds? That's deduplication magic at work. Several desktop
backup programs, including CrashPlan, QRecall, and Retrospect, also
offer deduplication—even from multiple computers.
