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While the leading cloud services have increased their reliability, Microsoft is lagging behind thanks to a major Azure outage in November.

Google's cloud platform has had about one hour's downtime over the last 12 months, and Amazon has had less than 20 minutes, but Microsoft has had more than 40 hours, which gives it a reliability of less than 99.5 percent, according to figures from web tracking company CloudHarmony. Rackspace meahwhile had 40 minutes of downtime.

Despite this, a survey of CIOs found that Azure has gained in popularity, ranking ahead of Google.

CloudHarmony runs servers on around 100 cloud platforms, reporting on the number of outages, as well as how long they last. It publishes aggregate figures, showing the current status of services, and their availability over the last days or months.

Cloudy prospects
Overall, the reliability of cloud services is good, with some pushing towards "five-nines" - 99.999 percent reliability. And storage services are generally more trustworthy than compute services, according to the CloudHarmony figures. 

However, one major outage can mar a year's good work, and Microsoft Azure suffered very badly in 2014. In November, to a botched update of the service downed the service for several hours. Microsoft later blamed this on human error. And it had previously suffered badly in August.

Microsoft has promised to improve its procedures in the wake of these problems, but many users will remember previous incidents; the service suffered avoidable problems because of a Leap Year error.

But reliability is not the end of the story. A survey of chief information officers (CIOs) appears to show that Azure is actually more popular than the more-reliable Google cloud service. Piper Jaffay polled an admittedly small (112) sample of CIOs. Thirty-five percent named Amazon as their preferred public cloud provider, while 21 percent favoured Microsoft Azure. Rackspace scored 16 percent, and Google was only favourite for seven percent of those questioned.

Despite their performance over the year, Microsoft had actually gained one percent, while Google had lost five percent.