Amazon has introduced a service level agreement (SLA) for its S3 online storage service to provide an uptime guarantee for customers.

Amazon S3 rents storage over the internet and charges fees based on the volume of transfers and size of the files stored.

Developers can use the service to quickly add storage capacity to their systems without having to purchase any hardware or software.

S3 is part of Amazon's broader web services bundle, which also includes its Elastic Compute Cloud rental servers and the Mechanical Turk.

"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," said Jeff Barr, services evangelist at Amazon, on a company blog.

The Amazon S3 Service Level Agreement guarantees an uptime of 99.9 per cent. Customers will be issued a 10 per cent credit if system availability falls below 99.9 per cent, and a 25 per cent credit if it falls below 99 per cent.

A downtime of 0.1 per cent per month represents a system crash for 45 minutes based on a 31-day month. Mission critical systems typically require 99.999 per cent uptime, or no more than 27 seconds downtime per month.

Business writer Nicholas Carr suggested that the Amazon S3 SLA illustrates the advantage of hosted services over in-house IT.

"Utilities will compete directly with one another on critical performance standards, like reliability and security, as well as pricing," Carr wrote on his Rough Type blog.

"That competition promises rapidly to drive up standards and push down prices to the benefit of the utilities' customers."

In house IT, by comparison, lacks a direct competitor to drive improvements.