Analyst house IDC has calculated the cost of cooling and powering all the world's external storage.

IDC said in a new report that the total cost of providing power and cooling to spinning disk drives worldwide exceeded $1.3 billion (£654 million) in 2007.

More than 49 million hard disk drives (HDDs) were spinning in external storage arrays worldwide in 2007, and electricity cost seven cents (3.5 pence) per kilowatt-hour. The IT power and cooling burden is sure to become even more extreme, IDC says.

More than 14 exabytes (that's 14 billion gigabytes) of storage were sold to enterprises worldwide in the 10-year period ending in 2007. Nearly eight times that much - 110 exabytes - will ship in just the next five years, IDC predicts.

These numbers probably underestimate the total storage by a significant amount, as IDC's totals do not include servers or blades with fewer than three disk drives; drives used for test, research and development; or drives installed as supplements or replacements in existing storage arrays.

IDC credited HDD vendors with doing a "masterful job" keeping pace with growing storage requirements, but IT shops are still in a precarious position.

"IT managers are faced with a dilemma of how to meet terabyte storage-growth requirements of greater than 50 percent with storage devices that have capacities increasing at less than 40 percent," IDC analyst David Reinsel writes.

Enterprises can increase the number of drives and arrays, or use Serial Advanced Technology Attachment (SATA) drives, which store three or four times more data than drives optimised for performance, such as Fibre Channel and SCSI.

"Adopting lower 'tiers' of storage based on capacity-optimised drives is exactly what the industry did and continues to do," IDC writes. However, "the adoption of SATA drives into enterprise storage helped to soften but did not eliminate a number of serious issues for data centre managers, including limited square footage, limited budgets, and limited power and cooling supplies."

Vendors have improved capacity and price points dramatically, but quality measurements are improving only slowly in some areas. While the number of gigabytes per drive has risen greatly, speed measured in input/output operations per second (IOPS) per drive has improved slowly, "resulting in a decline of IOPS per gigabyte."

Such strategies as thin provisioning and data deduplication can help lower power costs, but ultimately customers will have to make tradeoffs between efficiency and performance, IDC notes.