RAID mirrors as backups: Rather than using a mirror to protect data all
the time, some systems break, or disconnect, the mirrored disks so they
have a static, unchanging copy of the data to perform backups on. This
is done in coordination with database systems and the OS to make sure
that the data mirror is in a consistent state from an application point of
view. Once the backups are complete, the mirror set is reattached and
rebuilt to provide protection until the next backup process begins. The
benefit is that backups do not slow down normal data use, since they
affect only disks that are otherwise unused. The downside is that the
data is not protected during the backup operation, and the production
system runs much slower when the mirror is being rebuilt.
Many SAs use such mirroring capabilities to make an occasional
backup of an important disk, such as a server boot disk, in case of
drive failure, OS corruption, security compromise, or other issues. Since
any error or compromise would be faithfully mirrored onto the other
disk, the system is not run in true RAID 1 mirror mode. The mirror is
established and then broken so that updates will not occur to it. After
configuration changes, such as OS patches, are made and tested, the
mirror can be refreshed and then broken again to preserve the new
copy. This is better than restoring from a tape, because it is faster. It
is also more accurate, since some tape backup systems are unable to
properly restore boot blocks and other metadata.