Storage UPDATE--brought to you by the Windows & .NET Magazine Network
August 26, 2002

(contributed by Sheila Childs, schilds@legato.com)


Many new products on the market and new and evolving technologies
offer data-protection and disaster-recovery capabilities (especially
in the wake of last September 11). It's a good time for companies to
reassess their methodologies for data protection and disaster

The debate about whether tape is dead continues, kept alive by
improved storage-product functionality such as disk-to-disk backup. If
a company can implement disk-to-disk backup, for example, recovery
times are faster and the company has a reduced need for expensive and
less reliable hardware such as tape libraries, drives, and robotics.

And what about mirroring and snapshot technologies? Can and should
such technologies replace traditional tape backup as the method of
choice for data protection? Although these newer technologies have a
place in any good data-protection and disaster-recovery configuration,
companies shouldn't necessarily replace tape backup but rather
complement tape with other technologies to provide different recovery

Disk mirroring has been available since the early days of mainframe
computing. Mirroring is an effective way to protect data from certain
error conditions and events. In particular, mirroring protects against
online failure of storage devices--when one mirrored disk breaks, the
other disk can provide the application with data without interruption.
Mirrored copies of data are interchangeable, and access to mirrored
copies of data is transparent to the user. However, although mirroring
is effective in recovering from hardware failure, it's not a solution
for data recovery. Mirrored copies of data are just
that--mirrors--which implies that any corruption or user error
perpetrated on the original will be reflected on the mirrored copy. In
addition to these limitations, mirroring isn't a good solution for
copies of data that must reside at geographically distant locations.

Several vendors and customers advocate using snapshots to ensure quick
data recovery. Snapshot copies of data provide a fast way to make a
copy of data; however, snapshot technology generally isn't effective
for disaster-recovery purposes unless you combine it with backup. Two
examples of the different methodologies you can use to deliver data
recovery are software snapshots and split-mirror snapshots. Each
vendor generates software snapshots differently. In some cases,
snapshots don't actually make a copy of data but make a point-in-time
copy of pointers to the data. Subsequently, when new blocks are
written to existing files or databases, the new blocks are written to
locations that existing snapshots don't reference. Because the
snapshot copy references the location of the original blocks of data
on disk, applications such as backup can use this input without the
need to shut down critical applications.

Split-mirror snapshots consist of two or more mirrored copies of data
that you can split at any time to create a coherent point-in-time
snapshot. After you've performed a backup on the split-mirror copy,
you can bring the copies back together for resynchronization. Although
split-mirror snapshots provide excellent data protection, supporting
this configuration might cost more than traditional tape backup, and
resynchronizing copies after a split might involve significant

Snapshots and disk-to-disk backups are extremely useful for retrieving
individual files or file systems when you need quick recovery. These
technologies effectively provide near-online storage capabilities, and
if you have the disk space to support the technologies, you can get
extremely satisfactory recovery times.

However, you shouldn't use these methodologies alone to ensure data
protection because when it comes to disaster recovery, tape backup
continues to provide benefits that you'll find unavailable or
difficult to achieve with mirroring or snapshots. Only with a
secondary copy of data stored at a remote location can you truly
protect an IT environment in the event of a significant disaster--for
example, the destruction of a primary data center or its equipment.
You can transport backup tapes to any location and retrieve data
without regard to network connectivity or failed primary hardware. A
tape copy of data is portable. Traditional tape backups also let you
store and transport data securely if data is encrypted.

Of course, tape backup presents some problems too. It's complex, often
requiring complicated policies and expert technicians to maintain
them. Unlike with other data-protection technologies (e.g., split
mirror, snapshots), systems administrators must ensure that they've
backed up all data (all applications, all files) regularly. Even
though a good backup ensures a coherent copy of data, recovery can be
difficult and time-consuming.

Any method of data protection and disaster recovery has its pros and
cons. In general, depending on the IT funds available, it makes sense
to use a combination of technologies that provide quick recovery in
the event of data corruption or user error and provide complete
recovery in the event of a disaster.

Copyright 2002, Penton Media, Inc.

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Last Modified: March 9, 2008