Q:  

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?

A:  

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.

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