By Michal Lev-Ram

HALF MOON BAY, Calif. — Tech executives understand the importance of reliability when delivering computing power over the Internet, a technology called “cloud computing.” The problem is, they haven’t yet achieved it.

The ups and downs of renting storage space – as opposed to owning and operating servers and a data center – was the subject of a panel at Fortune’s Brainstorm Tech conference Tuesday.

More technology startups are turning to cloud computing, which uses an on-demand system that allows them to pay for only the computing capacity they need. But as Fortune senior writer and moderator Michael Copeland pointed out, the “cloud” is not always reliable.

Last Sunday, Amazon’s data storage service, called S3, was down for nearly eight hours, causing outages for online companies that depend on the service for storing their data. Adam Selipsky, VP of product management and developer relations at Amazon (AMZN),  told audience members the crash was “completely unacceptable.”

“Any amount of downtime doesn’t work,” said Selipsky. “We have a lot of people maniacally focused on issues like this all the time, and our hope is that with that amount of focus we’re going to continue to get more and more reliable.”

Another panelist, Kevin Lynch, Adobe Systems’ (ADBE) chief technology officer, argued that the idea that everything should be in the “cloud” is “swinging the pendulum too far.” Adobe has been trying to push a new platform called AIR, which lets developers create web-based applications for desktops.

But Google’s (GOOG) VP of search products and user experience, Marissa Mayer, said she still thinks cloud computing is the best alternative. “Overall the cloud is a much better place to store your data,” Mayer told conference-goers.