The cost of tape vs. disk data backup

Here's how to determine the cost of disk vs. tape in your data backup environment.

  • The old adage of "time is money" couldn't be more accurate, particularly when determining the value of storing and recovering data. Backing up data can mean different things to IT and business users. To a business user, a backup may imply no loss of data, and nearly instantaneous data recovery. To an IT department, data backup may imply a tape at an offsite location.

It's important to understand how technology choices can impact business. This relationship can be represented by

a simple equation. On one side is the business opportunity or revenue potential, and on the other side the cost of providing the data protection service. The cost of providing the service must be less than the business opportunity.

Disk storage typically isn't less expensive than tape, but it can be the best way to deliver to meet a service-level agreement (SLA).

The business value of data

The value of data varies significantly among industries, the size of the company and even by an application within the same firm. The lost business opportunity due to a website outage for a small law firm may be inconsequential. The cost to a company such as Amazon, American Express, eBay or Visa may be on the order of several million dollars per hour.

To determine the value of data, answer the following questions:

 

  • What are the costs for lost data per hour for the applications that are being protected?
  • What are the costs for delayed processing (data not lost, although application isn't running for some time) on a per hour basis?

The SLA metrics must be specified by the business application owners and should include the recovery point objective (RPO) and the recovery time objective (RTO). The RPO is the amount of data that may be lost, and the RTO is the amount of time it takes to perform the recovery.

The cost of protecting data

Organizations often use only hard accounting costs because of to the relative ease in calculating these costs. However, operational costs shouldn't be overlooked.

Tape has historically been approximately eight or 10 times less expensive than disk storage to acquire and operate. With the recent deployments of high-capacity low-cost drives (such as SATA), the cost benefit ratios are now closer to 4:1 in favor of tape.

However, these hard cost calculations ignore the business improvements available by using disk, including regulatory compliance issues, faster time to recovery and other aspects. These hidden aspects have real value, but must be quantified in order to determine the appropriate place to store backup data.

Accounting costs

Acquisition costs (Capex): The capital or acquisition costs are the easiest to calculate, and are often the only "costs" used for comparisons, due to the very little work required. However, capital acquisition costs typically represent only 20% to 40% of the true cost of acquiring and operating equipment.

Operational costs (Opex): In a cost model, at least three operational costs should be considered:

 

  • Human costs to operate the equipment
  • Warranty costs for the equipment
  • Power and cooling costs

Creating a cost model

With the information gathered, it's possible to put these values into a simple spreadsheet to calculate the costs. The spreadsheet should capture the major aspects, which include the capital and operational costs, including power, cooling and maintenance. A typical operational time frame may be picked, such as three years to five years.

Provided below is a high-level summary of a detailed cost-comparison analysis. This analysis assumes 1 PB of storage and compares a tape-only approach to a disk or VTL-only approach for backup. A typical midrange tape library capable of supporting 1,000 cartridges was selected, and includes 24 LTO-4 tape drives and 800 cartridges. For the disk subsystem, a typical midrange system was chosen and includes the use of 1 TB SATA drives in a RAID 6 configuration.

The cost of disk vs. tape (1 PB for five years)

 

  Tape Disk
Acquisition costs $407,000 $1,620,000
Operational costs $205,500 $573,516
Warranty (Total five years) $184,200 $306,000
Total power and cooling cost $21,300 $267,516
Total Cost $612,500 $2,193,516

It's obvious that there are dramatic cost differences for using a pure disk or a pure tape architecture for storing backup data. However, most environments will use a variety of solutions for different applications.

For business applications that stipulate a RPO of less than one day, or a RTO of less than one hour, it's impractical -- if not impossible -- to meet these levels using tape as the primary means for storing recovery data. A combination of approaches is typically warranted, with some applications requiring remote replication, others being satisfied by a VTL and tape with disk-to-disk-to-tape solution, and still other applications satisfied with local backup to tape and off-site vaulting.

About the author: Russ Fellows is a Senior Analyst with Evaluator Group. He is responsible for leading research and analysis of product and market trends for NAS, virtual tape libraries and storage security. He is also the primary analyst for coverage of selected open-system arrays and virtualization products. Russ is a well regarded and successful industry professional with 20 years of high technology experience, including product design, product development, systems engineering, business strategy development, competitive analysis and portfolio management within both the vendor and end-user groups. 


 

This was first published in April 2008

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