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This study compares the costs of disk and tape storage over a 5-year period for a mid-sized business data center. Findings show that for long-term storage, disk costs are about 23 times higher than tape, with energy costs being 290 times higher for disk. Assumptions and cost models are provided for both disk and tape. Conclusions suggest that a combination of disk and tape is necessary, as disk is still needed for performance objectives while tape remains a viable option for archival storage.
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Disk and Tape Square Off Again Tape Remains King of Hill with LTO-4 Presented by Heba Saadeldeen
Goal • Decide whether the decreasing costs of disk subsystems and the increasing capacity of disk drives might have made the TCO for disk more attractive versus tape in the long-term storage of data. • Study Period : 5 years. • Size of enterprise: Data center of a mid-sized business
How do they compare the cost of both disk and tape storage ? • Store the 90 days worth of backup data on a disk (disk cache) to help meet the performance objectives defined in service level agreements. • We will then save the 13th weekly full backup as the archived quarterly backup and save it to a disk system and to a tape system – to compare the TCO costs of each long- term storage solution . • The cost comparison, over five years focuses on storing these “archived quarterly backups” on the two alternatives, disk and tape .
Findings • For long term storage, over the five year study period, the cost of disk is about 23 times that of the tape. • The cost of energy is 290 times that of tape
Assumptions • Tape assumptions • Disk assumptions • Energy assumptions
Tape Assumptions • Use LTO-4 • It was released in 2008. • Doubled capacity again to 800 GB. • Use standard compression ratio of 2:1 • Increase data transfer rate by 50% to be 120 MB/s • Tapes retire every 5 years. • Use N+1 tape drive to ensure device availability. N is the number of tapes required to complete backup.
Disk Assumptions • 750GB SATA disk drives • Disk retire every 3 year
Energy Assumptions • Use an average urban rate ($0.12/KWH). • Assume that the energy cost is constant throughout the entire period of study • A dollar to cool the environment for each dollar spent to run the data center's infrastructure.
Cost Model • Includes: • Equipment • Media • Energy • Floor space requirements
Energy Consumption for Disk • Assume that typical power usage for a normal system operation consumes 11,000 WPH. • Use conservative power rate = $0.12/KWH • Assume that the energy for cooling = energy for operation
Conclusion for Disks • The H/W cost dominates the total cost. • Energy and space consumes only 4% of the total cost. • This cost model does not include cost of administration or backup software.
Energy Consumption for Tape • The active tape library, without drives, consumes 185 Watts per hour • Each additional frame adds 110 watts • Each active LTO-4 drive, power supply, and associated infrastructure consumes about 50 watts while reading/writing, 12 watts when idle • The initial deployment of library with two drives will consume 1150KWH in the first year
Conclusions • All of one solution does not work. • Disk cache is still required to meet SLA. • Tape is not dead for archival storage. • Hardware cost dominates.
