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I've recently inherited a data warehouse that uses disk mirroring, and we're faced with the need to add about a terabyte (mirrored) to make it through the end of 2000.

    Ask The Experts published in DMReview.com
September 1, 2000
  By Sid Adelman and Chuck Kelley and David Marco


I've recently inherited a data warehouse that uses disk mirroring, and we're faced with the need to add about a terabyte (mirrored) to make it through the end of 2000. With that type of projected growth, I am wrestling with the need to continue with disk mirroring. Is it a common practice for data warehouses to use disk mirroring?


Sid Adelman's Answer: I assume you have established service-level agreements for availability. The three situations where we see disk mirroring with data warehouses are:

  1. The need to have very high availability.
  2. An architecture patterned after your operational systems (that have a 99.99 percent SLA).
  3. An aggressive disk vendor.

Do you know how long it will take you to recover if you do not have disk mirroring? Do you know the impact of an outage? Will the cost of an outage outweigh the cost of disk mirroring?

Chuck Kelley's Answer: I generally think the reasons data warehouses are mirrored is because the organization has the corporate standard to mirror on disks. I would think that RAID-3, RAID-5 or RAID-53 would be acceptable for the data warehouse. The deciding factor would be the downtime requirements. In all the data warehouses that I have worked on, only one ended up with mirroring - and that was because the company would not relax the corporate standard.

David Marco's Answer: Disk mirroring is common in warehouses that cannot afford downtime. Typical growth is a newer data warehouse is around 150 percent a year. In my experience, established warehouses grow around 100 percent yearly.

Douglas Hackney's Answer: It is very common for a DW architecture to implement mirroring, especially for high performance data. You may want to look at what data types are being stored on which type of RAID. There may be opportunities where you could move some data types to RAID 5.


Check out DMReview.com's resource portals for additional related content, white papers, books and other resources.

Sid Adelman is a principal in Sid Adelman & Associates, an organization specializing in planning and implementing data warehouses, in data warehouse and BI assessments, and in establishing effective data architectures and strategies. He is a regular speaker at DW conferences. Adelman chairs the "Ask the Experts" column on www.dmreview.com. He is a frequent contributor to journals that focus on data warehousing. He co-authored Data Warehouse Project Management and is the principal author on Impossible Data Warehouse Situations with Solutions from the Experts. His new book, Data Strategy, is scheduled for publication this year. He can be reached at 818-783-9634 or sidadelman@aol.com.  Visit his Web site at www.sidadelman.com.

Chuck Kelley is a senior architect in the business intelligence practice for Hitachi Consulting (www.HitachiConsulting.com), a globally recognized leader in delivering value-based business and IT Solutions. Kelley is an internationally known expert in database and data warehousing technology. He has 30 years of experience in designing and implementing operational/production systems and data warehouses. Kelley has worked in some facet of the design and implementation phase of more than 50 data warehouses and data marts. He also teaches seminars, co-authored three books on data warehousing and has been published in many trade magazines on database technology, data warehousing and enterprise data strategies. He can be contacted at chuckkelley@hitachiconsulting.com.

David Marco is an internationally recognized expert in the fields of enterprise architecture, data warehousing and business intelligence and is the world's foremost authority on meta data. He is the author of Universal Meta Data Models (Wiley, 2004) and Building and Managing the Meta Data Repository: A Full Life-Cycle Guide (Wiley, 2000). Marco has taught at the University of Chicago and DePaul University, and in 2004 he was selected to the prestigious Crain's Chicago Business "Top 40 Under 40."  He is the founder and president of Enterprise Warehousing Solutions, Inc., a GSA schedule and Chicago-headquartered strategic partner and systems integrator dedicated to providing companies and large government agencies with best-in-class business intelligence solutions using data warehousing and meta data repository technologies. He may be reached at (866) EWS-1100 or via e-mail at DMarco@EWSolutions.com.

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