The data centers will support Microsoft services such as its new search offering, Bing, and Azure, its cloud computing platform.

The Dublin facility, to open on Wednesday, will be the largest for Microsoft outside of the U.S. It covers 303,000 square feet and uses outside air to cool the facility, for power consumption savings.

The Chicago facility, scheduled to open July 20, will be more than twice as large, covering 700,000 square feet. Two-thirds of the center will be able to accommodate servers in containers. In some data centers, Microsoft has started using standard shipping containers loaded with 1,800 to 2,500 servers, because it can save on electricity by cooling just the containers rather than the whole facility.

The openings come after Microsoft announced earlier this year that it would put a planned Iowa data center on hold. It also delayed the openings of the Chicago and Dublin facilities.

At the time, the company optimistically described the Iowa postponement as a result of successful efforts to improve efficiency of data center operations elsewhere.

But in fact Microsoft may have put off construction after discovering that growth in hosted services has been lower than it may have expected. Revenue in Microsoft's online services group during the quarter ending March 31 dropped to US$721 million from $843 million in the same quarter last year.

Microsoft is not alone in reining back its data-center expansion plans during the recession. Google late last year decided to delay building a facility it planned in Oklahoma.

Microsoft has also lost a couple of well-known leaders in its data center group. In April, Michael Manos, the general manager of the data center services division, left to take a job at wholesale data-center provider Digital Realty Trust. Late last year, James Hamilton, another respected data center engineer, left Microsoft to join Amazon Web Services.