The main shortcoming of Amazon's S3 online storage service - a cornerstone of its hot Web services platform - was that it didn't guarantee uptime through a service level agreement (SLA).

Companies used to such agreements were wary of storing crucial files with Amazon, limiting its appeal. The company hopes that will change with a new SLA, retroactive to Oct. 1, that services evangelist Jeff Barr announced on the Amazon Web Services blog:

We know that many of our customers, including a multitude of teams within Amazon, are using S3 in mission-critical ways and need a formal commitment from us in order to make commitments to their own users and customers.

The company will "commit to 99.9% uptime" and customers can apply for a 25 percent credit on their monthly bill if uptime falls below 99 percent. They can apply for 10 percent credit if uptime is between 99 percent and 99.9 percent.

Barr's advice:

As is the norm with agreements like this, there's some fine print and you should definitely read it yourself to learn more.

I wonder if the service assurance, combined with the cost and ease of use, will attract bigger companies to the service.

(I'm also waiting to see if Amazon blends S3 with its MP3 store somehow, giving consumers the option to buy and store music in an S3 vault that's accessible anywhere - and now most anytime - with a Web connection. The company wouldn't say if that's in the works when I asked while reporting Monday's column.)