Green hard drives are drives that reduce the amount of power they use through a variety of mechanisms, including unloading the heads during idle time to reduce aerodynamic drag. Further, the drives calculate the optimum seek speed to use just the amount of power necessary.
Western Digital is a major producer of green hard drives and estimates that its green drives can shave off US$10 per drive, per year in electricity costs. For example, its 1TB WD Caviar GreenPower hard drive uses about 5 watts less power than drives of the same size, which typically consume 13.5 watts.
Drives powered down, waiting to be accessed
Actively spinning drives
Actively spinning drives
Figure 3-6 In a MAID deployment, only a quarter of the disks are spinning at any one time.
the power used to run them as well as reducing the generation of heat, which in turn reduces cooling costs.
MAID differs from RAID in that it has increased storage density and is much less expensive, thus saving power and the need for cooling.
MAID comes with some compromises, however, such as increased latency, lower throughput, and much lower redundancy. Although a MAID solution can be slow—data access can take a few milliseconds up to 10 seconds—it is much faster than tape, which can take 60 seconds or longer to access data.
Also, because large hard drives are designed for constant spinning, continual shutdown and reactivation threatens their reliability. Drives that are designed for repeated spin-up/ spin-down cycles are much more expensive.
The MAID architecture really developed because of the introduction of SATA drives that are designed to be powered up and down. In a large deployment, MAID allows a dense packaging of drives, and typically only 25 percent of the disks are spinning at any given time, as illustrated in Figure 3-6. This helps with the problem of throughput.
The appeal of MAID is also apparent when you consider their use in large environments. SATA drives are rated at 400,000 hours of life. If a datacenter has 1000 drives powered on all the time, a drive would fail every 18 days. Clearly this is no good, so employing a MAID system quadruples the drive's life to 1.6 million hours.
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