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Technology upgrades or replacement activities include replacing on a proactive scheduled basis older disk drives with newer generation, faster disk drives that draw less power, if you need to retain your storage system for lease or other purposes. Another approach is to replace your existing storage system (controller and disk drives) using a newer model with better performance, less power consumption and increased capacity to meet your available power, cooling and application service requirements.
For those who have not heard it a million times already, exercise caution when aggregating storage capacity onto larger capacity disk drives so as not to cause an application performance problem.
Another variation of the storage system upgrade or replacement scenario is when you can replace two or more storage systems on both a performance and capacity basis with a storage system that can do the same amount of work (IOPS and bandwidth) using the same or fewer disk drives and thus less power. If two storage systems are required for availability, BC or DR purposes, then look for storage systems that can be scaled to meet your needs and use less power, including leveraging clustered block iSCSI or Fibre Channel and clustered NAS storage systems.
Some storage power and cooling stories focus on consolidation and using the aggregated capacity to reduce the total number of disk drives to reduce energy consumption and emissions. For dormant data, the approach of reducing components can be similar to migrating data off-line to tape or other mediums. However, for active workloads, another approach is to keep the disk drive count constant, increasing performance and capacity while reducing power consumption as in Figure 1 below.
For example, newer generation 4Gbps FC 146GB 15.5K disk drives draw less power and have twice the capacity with a slight performance increase over older generation 2Gbps 73GB disk drives. Assuming that your application requirements from a performance standpoint are fairly stable and you could leverage the extra capacity without incurring or causing a performance problem, then a simple disk drive swap might be a benefit for you. For example, if your applications only use about 30 percent of the performance of an existing storage system and you need to consolidate two like systems from different locations, yet you need to fit into a reduced available power footprint, switching from 73GB or even 146GB 15K disk drives to 300GB 15.5K drives could be an option.
Let's assume that new generation of 500GB, 750GB and 1TB disk drives are more energy efficient than previous versions, with a configured, operational average including packaging power overhead equal to or less than high-performance drives. For example, current generation Seagate high capacity SATA disk drives draw about on average one watt per 125GB, compared to about 80.65GB per watt in previous generations. Expect to see different power numbers for disk drives, since there are the manufacturers' (Seagate, HGST and Fujitsu, among others) specifications that include idle, seek, operational average or a maximum power draw. You can also expect to see different power consumption numbers from various vendors that include any packaging overhead, including disk interposers for dual-porting SATA disk drives as well as any overhead for enclosure power and cooling.
As an example, 1TB of usable solid state disk (SSD), that is, RAM- and not FLASH-based SSD, capable of delivering hundreds of MB/sec performance would occupy the footprint of a standard 19" rack or equipment cabinet while consuming about 3 kWh. Compare that with two other extremes, storage centric or disk I/O. Solid state disk can be a good fit for I/O intensive applications or workloads where you can reduce the number of disk drives and subsequent power consumption as part of a tiered data storage infrastructure.
For less active or dormant data, larger capacity disk drives can be used with new generation 750GB and 1TB disk drives, which are more energy efficient than previous 250GB or even 500GB disk drives while offering more capacity. Many storage vendors are supporting multiple power settings to vary the power consumption of the disk drive without having to spin disk drives completely down. An industry trend is for more storage systems to be able to intelligently use the built-in capabilities of modern disk drives to reduce power by retracing disk heads when not in use, to stepping down the RPM of the drives, to going into other low-power consumption modes.
In Figure 2, I'm using an annual energy cost of 18 cents per kWh, which might be higher than what you normally hear or read about, that accounts for energy surcharges, usage charges above a given kWh base, and other fees. If you have not done so lately, take a look at your electric bill sometime and make note of the base kWh rate plus all of the additional fees and pricing tiers depending on the number of kWh used per month. The CO2 emissions per ton are based on the kWh used to power the device plus an additional 50 percent to cover power required for cooling the equipment.