As small and mid-size businesses (SMBs) evaluate cloud computing solutions, vendors often tout availability, or up-time, commitments. Understanding the metric, and how to compare vendor claims, is critical when selecting a solution.
Availability is generally expressed as a percentage — 99.9%, 99.99%, and so on. With 8760 hours in each year, 99.9% availability equates to 8.75 hours (0.1%) downtime each year, or an average of 45 minutes per month. Similarly, 99.99% availability equates to 45 minutes of downtime each year, or about 5 minutes per month.
Simple yes, but the devil is in the details. Most vendors caveat availability commitments by excluding schedule maintenance windows. For some vendors, this means regularly scheduled windows; for others this means any non-emergency maintenance activities with advanced notice.
When comparing availability claims, you need to account how availability is calculated.
For example, Google Apps has no scheduled maintenance windows. The Google cloud is designed to allow maintenance and updates without impacting user access to applications and data. Google’s 99.9% availability commitment means less than 8.75 total hours of downtime each year. In comparison, Company X running MS Exchange 2003 recently claimed availability of 99.99% as they had no unscheduled downtime in over a year. Looking more closely, they shutdown their email servers for about an hour each week to apply Microsoft security patches, install software updates, and run deep malware scans on the server.
Company X has total downtime of 52 hours per year. In a fair comparison, user availability is only 99.4%.
One of the advantages of Software-as-a-Service solutions, like Google Apps, is that SaaS solutions are design to provide better availability while reducing system administration and support costs. For Company X, moving to Google Apps would reduce the amount of time spent maintaining and supporting their email service — saving money and letting their IT staff focus on activities with greater benefit to the business.