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Tape vs. Disk: Another View...page 2
For example, to support
their CRM efforts, a major telecommunications company in the southern
U.S. wanted to provide their customer service representatives with almost
six years' worth of billing records.
After analysis, they
calculated that this requirement would eventually yield between a nine
and 11 terabyte data warehouse. Subsequent examination by the company
resulted in an important revelation: Billing inquiries by customers fell
to less than 16% of all queries after one year and then less than 8% past
15 months.
The illustration above
represents query volume to a phone company's customer service division
over time. As customer billing data ages, required access to it falls
dramatically after the first year. Though this aging data still needs
to be accessible, it can now be stored economically on a near-line tape
storage system.
Realizing that it
did not make performance or economic sense to retain data older than 12
months on disk, the company opted to migrate information based on age
to an alternative, near-line tape storage system. With this strategy,
the advantages were multifold:
- The company's main database application (in this case, IBM DB2) was
relieved of a heavy burden. It is no longer responsible for managing
multi-terabytes of archive data within its disk system.
- DB2 has access to all information because it can transparently access
the inactive, historical data on the near-line system.
- The combined system is more highly automated because, it eliminates
the need to manually mount tape cartridges.
Even though a disk-to-disk
system enables recovery from a hardware or site failure, periodic backups
of archive (unalterable) data to tape by the near-line system protect
against accidental deletes, software errors, and viruses that can corrupt
primary data managed by disk. If these types of problems should ever occur,
data can always be restored from previous backups.
Moving inactive data
to alternative, near-line media is just part of the story. What about
tape performance, capacity, cost savings, and the applicable software
interfaces? Here is where the disk versus tape debate gets interesting!
Since the early 90s,
tape has kept pace with disk in reducing the dollar cost per gigabyte
(GB) while improving access speed. At their inception, IBM cartridges
could store less than one GB. Today, LTO and S-DLT can store 100GB in
the same space. And with the advent of Sony's Super-AIT (Advanced Intelligent
Tape) next year, uncompressed capacity will rise to 500GB with a transfer
rate of 30MBps. Due in 2003, Tandberg's first-generation O-Mass offering
will have an uncompressed storage capacity of 600GB, with succeeding releases
rising up to an amazing 10 terabytes (TB) on a single cartridge. Transfer
rates on O-Mass' first generation cartridge is expected to be 64MBps,
accessing data in less than 3.5 seconds.(4)
Using the 80-20 rule
(80% of data access is targeted to only 20% of the data within a repository),
a hierarchical storage management system with a blend of media types can
provide an economical system for storing all enterprise information by
keeping the active 20% of data on disk and the remaining inactive 80%
of data only on tape. With a tape system at approximately 1/10 the cost
of an IDE disk system, this approach costs 28% of an all disk system
(20+80x1/10) /100. Of course this 80-20 rule can vary by application (for
example, 70-30 or even 90-10).
Tape hardware cost
and performance aside, there is still the near-line software management
issue. In an alternative storage system, Inmon states "there are
two places where software is needed-for the management of the data on
the alternate storage and for the management of the movement of data to
and from disk storage and alternate storage."(5) This software, which
Inmon refers to as a cross media storage manager (CMSM), is also responsible
for providing what he calls the most important function-query management.
Some present-day CMSM
software provides the ability to query data held on tape at the row-,
or detail-level, thus eliminating the need to move large files up the
hierarchy. Performance improves because only the required information
is transparently presented to the end-user. Other features of today's
CMSM software (e.g., FileTek's StorHouse®, OTG's DXDB) clearly answer
some of the reservations Mr. Lauffin has about tape maintenance. For example,
with current near-line management software, there is no need to manually
load and unload tape; monitoring tape bar code sequences is an automated
process; and even inspecting tape for wear is automated with alerts sent
to a remote site that predicts media failure before it happens.
While D2D storage
is extremely applicable for primary backup, especially with disk media
prices falling, there are still areas (such as large data repositories)
where tape is relevant, more cost effective, and technologically strong.
In fact, the marketplace agrees. According to Freeman Reports, tape drive
sales will expand by a 4% annual growth rate, climbing to 69,700 units
by 2006 with a value of approximately $1.9 billion.(6) As manufacturers
and software developers commit resources to advance tape technology further,
tape will remain a viable alternative to disk-only storage.
Article Copyright 2002. WestWorld Productions, Inc.
All rights reserved. Cannot be reproduced in any form without prior written
approval.
Back
4"Tape Still Alive and Kicking" by Marion Apicella
and Mark Jones, Infoworld.com-Storage Report, December 18, 2001
5"Storage Rant" by W.H. Inmon, BillInmon.com
LLC, Newsletter, Volume 3, Issue 1, January 4, 2002
6"High-Performance Tape Market Will Grow" Freeman
Reports Press Release, September 18, 2001
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